AVR1000b: Getting Started with Writing C-Code for AVR® MCUs

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AVR1000b: Getting Started with Writing C-Code for AVR

MCUs

Introduction

Authors: Cristina Ionescu, Cristian Săbiuţă, Microchip Technology Inc.

This technical brief provides the recommended steps to successfully program the AVR

microcontrollers (MCUs) and

to define coding guidelines to help writing more readable and reusable code.

High-level programming languages have become a necessity due to the imposed short development time and high-

quality requirements. They make it easier to maintain and reuse code due to better portability and readability than the

low-level instructions specific for each microcontroller architecture.

Programming language alone does not ensure high readability and reusability, but good coding style does. Therefore,

the AVR MCU peripherals, header files and drivers are designed according to this presumption.

The most widely used high-level language for AVR microcontrollers is C, so this document will focus on C

programming. To ensure compatibility with most AVR C compilers, the code examples in this document are written

using ANSI C coding standard.

This document contains code examples developed with the Atmel Studio Integrated Development Environment (IDE).

Most code examples are compatible with other IDEs, presented in Section 5: Further Steps.

Technical Brief

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Table of Contents

Introduction.....................................................................................................................................................1

1. Data Sheet Module Structure and Naming Conventions........................................................................ 4

1.1. How to Find the Data Sheet......................................................................................................... 4

1.2. Pin Description............................................................................................................................. 4

1.3. Modules Description.....................................................................................................................6

1.4. Naming Conventions.................................................................................................................... 7

1.5. Configuration Change Protection (CCP) Registers...................................................................... 9

1.6. Fuses..........................................................................................................................................10

2. Module Representation in Header Files................................................................................................ 11

2.1. Module Location in Memory....................................................................................................... 11

2.2. Module Structures...................................................................................................................... 11

2.3. Bit Masks, Bit Group Masks and Group Configuration Masks................................................... 13

3. Writing Bare Metal C-Code for AVR

....................................................................................................16

3.1. Set, Clear and Read Register Bits............................................................................................. 16

3.2. Register Initialization.................................................................................................................. 18

3.3. Change Register Bit Field Configurations.................................................................................. 21

3.4. Advantages of Using Bit Masks and Group Configuration Masks..............................................22

3.5. Writing to Configuration Change Protection (CCP) Registers....................................................23

3.6. Configuring Fuses...................................................................................................................... 24

3.7. Function Calls Using Module Pointers....................................................................................... 25

4. Application Example Showing Alternative Ways of Writing Code......................................................... 27

4.1. Register Names..........................................................................................................................27

4.2. Bit Positions................................................................................................................................27

4.3. Virtual Ports................................................................................................................................27

4.4. PORT Example.......................................................................................................................... 28

4.5. ADC Example.............................................................................................................................29

5. Further Steps........................................................................................................................................ 31

5.1. Application Notes and Technical Briefs Description................................................................... 31

5.2. Relevant Videos for Bare Metal AVR

Development................................................................. 31

5.3. MPLAB

XC8 Compiler..............................................................................................................31

5.4. IDE (MPLAB

X, Atmel Studio, IAR) – Getting Started..............................................................31

6. Conclusion............................................................................................................................................ 33

7. References............................................................................................................................................34

8. Revision History.................................................................................................................................... 35

The Microchip Website.................................................................................................................................36

Product Change Notification Service............................................................................................................36

Customer Support........................................................................................................................................ 36

Microchip Devices Code Protection Feature................................................................................................ 36

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Legal Notice................................................................................................................................................. 36

Trademarks.................................................................................................................................................. 37

Quality Management System....................................................................................................................... 37

Worldwide Sales and Service.......................................................................................................................38

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1. Data Sheet Module Structure and Naming Conventions

The first step in writing C-code for a microcontroller is to know and understand what type of information can be found

in the data sheet of the device used for programming. The data sheet contains information about the features, the

memories, the core and the peripheral modules of the microcontroller, the functional description of the peripheral

modules, the peripherals base addresses, the names and addresses of the registers, and other functional and

electrical characteristics.

1.1 How to Find the Data Sheet

Any documentation related to Microchip products can be found at:

• Microchip Data Sheets

The device data sheets, for the device families of interest in this document, can be found at:

• ATtiny416 Overview

• ATtiny212 Overview

• ATtiny3217 Overview

• ATtiny1634 Overview

• ATmega4809 Overview

• ATmega808 Overview

• AVR128DA28 Overview

1.2 Pin Description

The pin description can be found in any device data sheet. The pinout is contained in the Pinout, Pin Configurations

section, or any other name convention, depending on the device. The pinout of the ATmega809/1609/3209/4809 48-

pin devices is presented in Figure 1-1.

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Figure 1-1. ATmega809/1609/3209/4809 48-Pin Pinout

(1)

GND

VDD

PA5

PA6

PA7

PC0

PC1

PC4

PC5

PD2

PD3

PD6

PD7

PB0

PD0

PD1

PC3

PA2

PA3

PB1

PB2

PB3

PE1

PE2

PE0

PE3

PC2

GND

VDD

PF2

PF3

PF0 (TOSC1)

PF1 (TOSC2)

PF4

PD5

PC6

PC7

UPDI

PF5

PF6

PA1

PA4

PD4

PB4

PB5

GND

AVDD

PA0 (EXTCLK)

GPIO on VDD power domain

GPIO on AVDD power domain

Clock, crystal

Programming, debug

Input supply

Ground

TWI

Analog functions

Digital functions only

Power Functionality

The configurable functionalities for each I/O pin are described in the I/O Multiplexing and Considerations section, or

the Alternate Port Functions subsection of the I/O Ports section, depending on the device. If an evaluation board is

used, such as AVR128DA48 Curiosity Nano, the user needs to know how the microcontroller’s pins are allocated on

the specific board. The information is available on the AVR128DA48 Curiosity Nano webpage, in the AVR128DA48

Curiosity Nano Schematics document. Other documents that describe the AVR128DA48 Curiosity Nano board and

microcontroller characteristics are available on the same webpage.

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1.3 Modules Description

An AVR microcontroller is comprised of several building blocks: AVR CPU, SRAM, Flash, EEPROM and several

peripheral modules called module types. Throughout this document, all peripheral modules will be referred to as

modules.

Newer AVR microcontroller families can have one or more instances of a given module type. All instances have the

same features and functions. Some module types are a subset of others and inherit some of their features. The

inherited features are fully compatible with the respective module type. For example, the subset module for a timer

can have fewer compare and capture channels than a full timer module.

Figure 1-2. Modules Types, Instances, Registers and Bits

MODULE0

MODULEn

Bit 0

Module Type

Bit 7

Bit 0Bit 7

A module type can be the Universal Synchronous-Asynchronous Receiver Transmitter (USART), while the module

instance is, e.g., USART0, where the 0 suffix indicates the instance is “USART number 0”. For simplicity, a module

instance will be referred to as a module throughout this document, unless there is a need to differentiate.

Each module has a fixed base address in the I/O memory map, and all registers contained in the module have fixed

offset addresses relative to the module base address. This way, each register will not only have an absolute address

in the I/O memory space, but also a relative address defined by its offset. The register offset addresses are equal for

all instances of a module type, simplifying the task of writing drivers that can be used for all modules of a specific

type. The peripheral module address map can be found in the data sheet and shows the base address for each

peripheral.

Each module has several registers that contain control or status bits. All modules of a given type contain the same

set (or subset) of registers, and all these registers contain the same set (or subset) of control and status bits.

Table 1-1 presents the base address of some of the ATtiny804/1604 peripherals.

Table 1-1. Peripheral Module Address Map (section)

(1)

Base Address Name Description

0x0000 VPORTA Virtual Port A

0x0004 VPORTB Virtual Port B

0x001C GPIO General Purpose I/O registers

0x0030 CPU CPU

0x0040 RSTCTRL Reset Controller

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...........continued

Base Address Name Description

0x0050 SLPCTRL Sleep Controller

0x0060 CLKCTRL Clock Controller

... ... ...

Every module has a dedicated section presenting the features that the module has, a functional description of the

module, and the specific signals and guidelines on how to configure a certain mode of operation with all the

terminology explained. At the end of a module section, there is a register description subsection that contains the

scope of every register, the initial value, and if it is readable or writable. It also provides the position of every

configurable/accessible bit of a register.

All the registers, their addresses offsets, and the bit names and positions are described in the Register Summary

section for each module. The register summary for the ADC module is presented in Figure 1-3.

Figure 1-3. ADC Register Summary

(1)

Offset Name Bit Pos.

0x00 CTRLA 7:0 RUNSTBY RESSEL FREERUN ENABLE

0x01 CTRLB 7:0 SAMPNUM[2:0]

0x02 CTRLC 7:0 SAMPCAP REFSEL[1:0] PRESC[2:0]

0x03 CTRLD 7:0 INITDLY[2:0] ASDV SAMPDLY[3:0]

0x04 CTRLE 7:0 WINCM[2:0]

0x05 SAMPCTRL 7:0 SAMPLEN[4:0]

0x06 MUXPOS 7:0 MUXPOS[4:0]

0x07 Reserved

0x08 COMMAND 7:0 STCONV

0x09 EVCTRL 7:0 STARTEI

0x0A INTCTRL 7:0 WCOMP RESRDY

0x0B INTFLAGS 7:0 WCOMP RESRDY

0x0C DBGCTRL 7:0 DBGRUN

0x0D TEMP 7:0 TEMP[7:0]

0x0E

...

0x0F

Reserved

0x10 RES

7:0 RES[7:0]

15:8 RES[15:8]

0x12 WINLT

7:0 WINLT[7:0]

15:8 WINLT[15:8]

0x14 WINHT

7:0 WINHT[7:0]

15:8 WINHT[15:8]

0x16 CALIB 7:0 DUTYCYC

1.4 Naming Conventions

This section describes the register and bit naming conventions that can be found in the device data sheet and in the

header file used to develop any application.

1.4.1 Register Naming Conventions

The registers are divided into control, status, and data registers, and the names of the registers reflect this. A general

purpose control register of a module is named CTRL. If multiple general purpose control registers exist in the same

module, they will be differentiated by a suffix character. In this case, the control registers will be named CTRLA,

CTRLB, CTRLC, and so on. This also applies to status registers.

For registers that have a specific function, the name reflects their functionality. For example, a control register that

controls the interrupts of a module is named INTCTRL.

Since, for the microcontrollers presented in this document, the data bus width is 8-bit, larger registers are

implemented using several 8-bit registers. For a 16-bit register, the high and low bytes are accessed by appending H

and L respectively to the register name. For example, the 16-bit Analog-to-Digital Result register is named RES. The

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two bytes are named RESL (RES-Low, the Least Significant Byte of the register) and RESH (RES-High, the Most

Significant Byte of the register). Another way to identify multiple registers with the same name is to add a number

suffix; for example, the ADDR register in NVMCTRL is a 24-bit register, for the AVR DA family. The three bytes that

can be accessed using the number suffix are ADDR0, ADDR1 and ADDR2.

Most C compilers offer automatic handling of access to multibyte registers. In that case, the name RES, without H or

L suffix, can be used to perform a 16-bit access to the ADC Result register. This is also the case for 32-bit registers.

Additionally, the registers that contain the SET/CLR suffix allow the user to set and clear bits in those registers

without doing a Read-Modify-Write operation since it is implemented in hardware. These registers come in pairs.

Writing a logic ‘1’ to a bit in the CLR register will clear the corresponding bit in both registers, while writing a logic ‘1’

to a bit in the SET register will set the corresponding bits in both registers. For example, in the PORT module, writing

a logic ‘1’ to a bit in the Data Direction Set (DIRSET) register will clear the corresponding bit in the Data Direction

(DIR) and Data Direction Clear (DIRCLR) registers. Both registers will return the same value when read. If both

registers are written simultaneously, the write to the CLR register will take precedence.

1.4.2 Bit and Bit Field Naming Conventions

Most of the following examples are provided for the Analog-to-Digital Converter (ADC) module.

Register bits can have an individual function or be part of a bit field that has a joint function. An individual bit can be a

bit that enables a module, e.g., the ENABLE bit of the ADC module in the CTRLA register. A bit field can consist of

two or more bits that jointly select a specific configuration of the module they belong to. A bit field offers up to 2

selections, where n is the number of bits in the bit field.

The position of the ENABLE bit is presented in Figure 1-4, and the PRESC bit field position is presented in Figure

1-5.

1.4.2.1 Bit Naming Conventions

The bit names found in the register diagrams are abbreviations of the full bit name, which describes the functionality

that bit configures. For example, the RUNSTBY bit allows the user to enable the peripheral to Run in Standby sleep

mode. The user can enable the ADC by configuring the ENABLE bit.

Figure 1-4. Bit Naming Conventions for the ADC Control A Register

(1)

Name: CTRLA

Offset: 0x00

Reset: 0x00

Property: -

Bit 7 6 5 4 3 2 1 0

RUNSTBY RESSEL FREERUN ENABLE

Access

R/W R/W R/W R/W

Reset 0 0 0 0

1.4.2.2 Bit Field Naming Conventions

The bits from the same bit field can be referred to using a numerical suffix appended with the number of the bit

position relative to the bit field. For example, the Most Significant bit (MSb) from the ADC Control C prescaler bit field

will be PRESC2. This naming convention is not specified in the data sheet, but it will be described later in this

document, and it is used in the header files to handle the individual register bits from a bit field.

Figure 1-5. The PRESC Configuration Bit Field in the Control C Register

(1)

Name: CTRLC

Offset: 0x02

Reset: 0x00

Property: -

Bit 7 6 5 4 3 2 1 0

SAMPCAP REFSEL[1:0] PRESC[2:0]

Access

R R/W R/W R/W R R/W R/W R/W

Reset 0 0 0 0 0 0 0 0

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The PRESC bits offer the selection presented in Figure 1-6, for each value of the bit field.

Figure 1-6. Prescaler Configuration for the ADC Clock Signal Frequency

(1)

Bits 2:0 – PRESC[2:0] Prescaler

These bits define the division factor from the peripheral clock (CLK_PER) to the ADC clock (CLK_ADC).

Value Name Description

0x0

DIV2 CLK_PER divided by 2

0x1

DIV4 CLK_PER divided by 4

0x2

DIV8 CLK_PER divided by 8

0x3

DIV16 CLK_PER divided by 16

0x4

DIV32 CLK_PER divided by 32

0x5

DIV64 CLK_PER divided by 64

0x6

DIV128 CLK_PER divided by 128

0x7

DIV256 CLK_PER divided by 256

1.5 Configuration Change Protection (CCP) Registers

CCP registers are used to protect system-critical I/O register settings from accidental modification, as well as Flash

self-programming from accidental execution. Writing to a register under CCP is possible only after writing a specific

signature/key to the CCP register, which is part of the CPU module. The values of the signatures can be found in the

Configuration Change Protection subsection of the device data sheet.

There are two types of writes to a protected register, each with its key:

• protected I/O registers (the key/signature is IOREG)

• protected self-programming (the key/signature is SPM)

Some of the registers that are under the Configuration Change Protection are listed in the table below.

CLKCTRL.MCLKCTRLA Clock Controller – Main Clock

Control A

IOREG It allows the user to select the clock source

for the main clock and to output the clock

CLKCTRL.MCLKLOCK Clock Controller – Main Clock

Lock

IOREG It allows the user to lock the Main Clock

Control registers

RSTCTRL.SWRR Reset Controller – Software

Reset Enable

IOREG It allows the user to apply a software Reset to

the device

IVSEL in CPUINT.CTRLA CPU Interrupt Controller – the

Interrupt Vector Select bit from

the CTRLA register

IOREG It allows the user to place the interrupt vector

at the start of the application section of Flash

or the start of the boot section of Flash

NVMCTRL.CTRLA Nonvolatile Memory Controller

– the CTRLA register

SPM It allows the user to issue one of the next

commands:

• Write page buffer to memory

• Erase page

• Erase and write page

• Page buffer clear, etc.

Changes to the protected I/O registers or bits, or execution of protected instructions are only possible after the CPU

writes one of these signatures to the CCP register. The signatures are listed in the description of the CCP

(CPU.CCP) register.

A code example is provided in 3.5 Writing to Configuration Change Protection (CCP) Registers.

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1.6 Fuses

The fuses hold the device configuration and are part of the nonvolatile memory. They are available from device

power-up. Their values are maintained through a chip erase. The fuses can be read by the CPU or the programming

interface (e.g., UPDI), but can only be programmed or cleared through the program/debug interface. The

configuration values stored in the fuses are copied to their respective target registers at the end of the start-up

sequence so that the fuse values can be used by the CPU.

The Fuse Summary table can be found in the Memories → Fuses (FUSE) subsection from the device data sheet. An

example extracted from the ATmega4808/4809 data sheet is presented below.

Figure 1-7. FUSE Register Summary

Offset Name Bit Pos.

0x00 WDTCFG 7:0 WINDOW[3:0] PERIOD[3:0]

0x01 BODCFG 7:0 LVL[2:0] SAMPFREQ ACTIVE[1:0] SLEEP[1:0]

0x02 OSCCFG 7:0 OSCLOCK FREQSEL[1:0]

0x03

...

0x04

Reserved

0x05 SYSCFG0 7:0 CRCSRC[1:0] RSTPINCFG EESAVE

0x06 SYSCFG1 7:0 SUT[2:0]

0x07 APPEND 7:0 APPEND[7:0]

0x08 BOOTEND 7:0 BOOTEND[7:0]

0x09 Reserved

0x0A LOCKBIT 7:0 LOCKBIT[7:0]

A code example is provided in 3.6 Configuring Fuses.

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2. Module Representation in Header Files

A dedicated header file is available for each AVR device. The target device needs to be specified in the project

settings (for any used IDE – MPLAB

X, Atmel Studio, or IAR EWAVR), and the header file will be automatically

included in the main file of the created project. The header file is included as shown in the code below.

#include <avr/io.h>

All the needed register and structure definitions can be found in the header file. The macros and structures definitions

which are already defined in the device-specific header file can be used, instead of using a register’s address.

This is useful in devices that contain the same module, and the header file definitions for that module are the same.

2.1 Module Location in Memory

All registers for a given peripheral module are placed in one continuous memory block. Registers that belong to

different modules are not mixed up, which makes it possible to organize all peripheral modules in C structures, where

the instance macro is defined as shown in the code below. The definitions of all peripheral modules are found in the

device header files available for these AVR devices. The address for the modules is specified in ANSI C to make it

compatible with most available C compilers.

#define PORTMUX (*(PORTMUX_t *) 0x0200) /* Port Multiplexer */

#define PORTA (*(PORT_t *) 0x0400) /* I/O Ports */

#define PORTB (*(PORT_t *) 0x0420) /* I/O Ports */

#define PORTC (*(PORT_t *) 0x0440) /* I/O Ports */

The module instance definition uses a dereferenced pointer to the absolute address in the memory, coinciding with

the module instance base address. The module pointers are defined in the header files; therefore, it is not necessary

to add these definitions in the source code.

For example, the base address of the PORTA module is 0x0400. The module with all its registers and reserved bytes

has an available memory space from 0x0400 to 0x0420, which means 32 bytes in decimal. Therefore, the PORT_t

contains 32 allocated bytes for all its registers (or reserved bytes).

2.2 Module Structures

2.2.1 Example - ADC

The ADC_t structure type is defined in the header file, as presented in the code below. It contains all the module’s

registers in the data sheet specified order, as they are organized in the memory.

/* Analog-to-Digital Converter */

typedef struct ADC_struct

{

register8_t CTRLA; /* Control A */

register8_t CTRLB; /* Control B */

register8_t CTRLC; /* Control C */

register8_t CTRLD; /* Control D */

register8_t CTRLE; /* Control E */

register8_t SAMPCTRL; /* Sample Control */

register8_t MUXPOS; /* Positive MUX input */

register8_t reserved_1[1];

register8_t COMMAND; /* Command */

register8_t EVCTRL; /* Event Control */

register8_t INTCTRL; /* Interrupt Control */

register8_t INTFLAGS; /* Interrupt Flags */

register8_t DBGCTRL; /* Debug Control */

register8_t TEMP; /* Temporary Data */

register8_t reserved_2[2];

_WORDREGISTER(RES); /* ADC Accumulator Result */

_WORDREGISTER(WINLT); /* Window comparator low threshold */

_WORDREGISTER(WINHT); /* Window comparator high threshold */

register8_t CALIB; /* Calibration */

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register8_t reserved_3[1];

} ADC_t;

A macro for an instance of a module is then defined in the header file using that structure type, as presented in the

code below.

#define ADC0 (*(ADC_t *) 0x0600) /* Analog-to-Digital Converter */

Therefore, a particular module register, e.g., the CTRLA register, can be addressed as ADC0.CTRLA.

2.2.2 Example - PORT

The PORT_t structure type is defined in the header file, as presented in the code below.

/* I/O Ports */

typedef struct PORT_struct

{

register8_t DIR; /* Data Direction */

register8_t DIRSET; /* Data Direction Set */

register8_t DIRCLR; /* Data Direction Clear */

register8_t DIRTGL; /* Data Direction Toggle */

register8_t OUT; /* Output Value */

register8_t OUTSET; /* Output Value Set */

register8_t OUTCLR; /* Output Value Clear */

register8_t OUTTGL; /* Output Value Toggle */

register8_t IN; /* Input Value */

register8_t INTFLAGS; /* Interrupt Flags */

register8_t reserved_1[6];

register8_t PIN0CTRL; /* Pin 0 Control */

register8_t PIN1CTRL; /* Pin 1 Control */

register8_t PIN2CTRL; /* Pin 2 Control */

register8_t PIN3CTRL; /* Pin 3 Control */

register8_t PIN4CTRL; /* Pin 4 Control */

register8_t PIN5CTRL; /* Pin 5 Control */

register8_t PIN6CTRL; /* Pin 6 Control */

register8_t PIN7CTRL; /* Pin 7 Control */

register8_t reserved_2[8];

} PORT_t;

2.2.3 Multibyte Registers in Module Structures

Some registers are used in conjunction with other registers to represent 16- or 32-bit values. For example, the ADC

result (RES), the Window Comparator Low Threshold WINLT, and the Window Comparator High Threshold WINTH

are 16-bit registers for the ATmega4809 device, declared using the _WORDREGISTER macro. The macro is presented

in the listing below, along with the _DWORDREGISTER macro used to declare 32-bit registers. These macros are

already defined in the header file.

The _WORDREGISTER macro is used to extend the name of a register to access its lower byte and its higher byte, by

adding the L or the H suffix. The _DWORDREGISTER macro is also providing access to all bytes of a multibyte register,

by adding a number suffix. Both _WORDREGISTER and _DWORDREGISTER definitions are presented in the code

below.

#ifdef _WORDREGISTER

#undef _WORDREGISTER

#endif

#define _WORDREGISTER(regname) \

__extension__ union \

{ \

register16_t regname; \

struct \

{ \

register8_t regname ## L; \

register8_t regname ## H; \

}; \

}

#ifdef _DWORDREGISTER

#undef _DWORDREGISTER

#endif

#define _DWORDREGISTER(regname) \

__extension__ union \

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{ \

register32_t regname; \

struct \

{ \

register8_t regname ## 0; \

register8_t regname ## 1; \

register8_t regname ## 2; \

register8_t regname ## 3; \

}; \

}

2.3 Bit Masks, Bit Group Masks and Group Configuration Masks

The user can access any register using the structure configuration of the modules. The ADC can be fully configured

using the steps found in the data sheet, in the module’s description specific section. For example, the recommended

steps to initialize the ADC are presented in ADC - Analog-to-Digital Converter → Functional Description →

Initialization. Register bits can be manipulated using bit masks or bit position masks, which are defined in the header

file. The predefined masks are either related to individual bits, in which case they are called bit masks, or to a group

of bits (bit field), in which case they are called a bit group mask, or a group mask. For example, the ADC0 module

can be enabled and configured to start a conversion cycle by using bit masks.

2.3.1 Bit Masks and Bit Position Masks

A bit mask is used both when setting and clearing individual bits. A bit group mask is mainly used when clearing

multiple bits in a bit field. For example, the bit fields, bit names, bit positions, and bit masks of the CTRLD register of

ADC0 are shown in Table 2-1.

Table 2-1. Bit Fields, Bit Names, Bit Positions and Bit Masks

Bit Field INITDLY[2:0] - SAMPDLY[3:0]

Bit Name INITDLY2 INITDLY1 INITDLY0 ASDV SAMPDLY3 SAMPDLY2 SAMPDLY1 SAMPDLY0

Bit Position 7 6 5 4 3 2 1 0

Bit Mask 0x80 0x40 0x20 0x10 0x08 0x04 0x02 0x01

Since the bit names need to be unique for the compiler to handle them, all bits are prefixed with the module type they

belong to. In many cases, the module type name is abbreviated. For all bit definitions related to the Timer/Counter

Type A module, the bit names are prefixed by TCA_.

To differentiate between bit masks and bit positions, a suffix is also appended. For a bit mask, the suffix is _bm, and

for a bit position it is _bp. The name of the bit mask for the INITDLY0 bit is thus ADC_INITDLY0_bm, and the name of

the bit position is ADC_INITDLY0_bp. Additionally, the header file provides definitions for group positions. The suffix

for a group position is _gp, and the name of the INITDLY group position mask, for example, is ADC_INITDLY_gp. The

code below shows the definitions of the INITDLY bit masks, bit positions, and group positions, as they are available in

the device header file.

#define ADC_INITDLY_gp 5 /* Initial Delay Selection group position */

#define ADC_INITDLY0_bm (1<<5) /* Initial Delay Selection bit 0 mask */

#define ADC_INITDLY0_bp 5 /* Initial Delay Selection bit 0 position */

#define ADC_INITDLY1_bm (1<<6) /* Initial Delay Selection bit 1 mask */

#define ADC_INITDLY1_bp 6 /* Initial Delay Selection bit 1 position */

#define ADC_INITDLY2_bm (1<<7) /* Initial Delay Selection bit 2 mask */

#define ADC_INITDLY2_bp 7 /* Initial Delay Selection bit 2 position */

A naming convention example for the INITDLY0 bit mask is presented in Figure 2-1.

Figure 2-1. Naming Convention of Bit Masks

ADC_INITDLY0_bm

Moduleprefix Bitname Bitmasksuffix

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2.3.2 Bit Field Masks (Group Masks)

Many functions such as clock selection for timers, prescaler selection for converters, or filter selection for the

configurable custom logic are configured by a field of bits, referred to as a bit field or a bit group. The value of the bits

in a group selects a specific configuration.

The masks for configuring a bit field are referred to as a bit group masks, or group configuration masks.

When changing bits in a bit field, it is not enough to set the bits for the desired configuration; it is also required to

clear the bits from the old configuration before assigning a new value. To make this easy, a bit group mask is defined.

The group mask uses the same name as the bits in the bit field and is suffixed _gm.

The code below shows how the ADC prescaler group mask is defined in the header file.

#define ADC_PRESC_gm 0x07 /* Clock the prescaler group mask */

The naming convention is presented in Figure 2-2.

Figure 2-2. Naming Convention of Group Masks

ADC_PRESC_gm

Moduleprefix Bitfieldname

Groupmask

suffix

The bit group mask is primarily intended to clear the old configuration of a bit field before writing a new value. The

code below shows how this can be done. The code will clear the PRESC bit field in the CTRLC register of the ADC0

module. This construction does not set a configuration. It only sets all the prescaler configuration bits. This is an

advantage because there is no need to use all the bit masks to reset a specific configuration; they only need a group

mask for this operation. The group mask will typically be used in conjunction with a group configuration mask to clear

a particular configuration.

ADC0.CTRLC &= ~(ADC_PRESC_gm); /* Clearing the prescaler bit field using a group mask */

2.3.3 Group Configuration Masks and Enumerators

It is often required to consult the data sheet to check the bit pattern to be used when setting a bit field to the desired

configuration. This also applies when reading or debugging code. To increase the readability and to minimize the

possibility of setting bits in bit fields incorrectly, several group configuration masks are defined in the header file. Each

group configuration mask selects a configuration for a specific group mask.

The name of a group configuration mask is a concatenation of the module type, the bit field name, a description of

the configuration and a suffix, _gc, indicating that this is a group configuration. An example of the ADC prescaler

configuration is presented in Figure 2-3.

Figure 2-3. Naming Convention of Group Configuration Masks

ADC_PRESC_DIV4_gc

Module

prefix

Bit field

name

Configuration

name

Group

configuration suffix

The group configuration presented in Figure 2-3 sets the prescaler from the peripheral clock (CLK_PER) to the ADC

clock with the division factor 4.

The ADC prescaler bit field consists of three bits that define the division factor. The possible configuration names are

DIV2, DIV4, DIV8, DIV16, DIV32, DIV64, DIV128, and DIV256. These names make writing and maintaining code

very easy, as it requires very little effort to understand what configuration the specific mask selects. Table 2-2 shows

the available configurations for this bit field.

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Table 2-2. PRESC Bits and Corresponding Bit Group Configurations

PRESC2 PRESC1 PRESC0 Division Factor Group Configuration Mask

0 0 0 CLK_PER divided by 2 ADC_PRESC_DIV2_gc

0 0 1 CLK_PER divided by 4 ADC_PRESC_DIV4_gc

0 1 0 CLK_PER divided by 8 ADC_PRESC_DIV8_gc

0 1 1 CLK_PER divided by 16 ADC_PRESC_DIV16_gc

1 0 0 CLK_PER divided by 32 ADC_PRESC_DIV32_gc

1 0 1 CLK_PER divided by 64 ADC_PRESC_DIV64_gc

1 1 0 CLK_PER divided by 128 ADC_PRESC_DIV128_gc

1 1 1 CLK_PER divided by 256 ADC_PRESC_DIV256_gc

To change a bit field to a new configuration, the bit group configuration is typically used in conjunction with the bit

group mask, to ensure that the old configuration is cleared first.

Unlike bit masks and group masks, the bit group configuration masks are defined using C enumerations. One

enumeration is defined for each bit field. The enumeration for the USART CMODE bit field is shown in the code

below.

typedef enum USART_CMODE_enum

{

USART_CMODE_ASYNCHRONOUS_gc = (0x00<<6), /* Asynchronous mode */

USART_CMODE_SYNCHRONOUS_gc = (0x01<<6), /* Synchronous mode */

USART_CMODE_IRCOM_gc = (0x02<<6), /* Infrared communication */

USART_CMODE_MSPI_gc = (0x03<<6), /* Master SPI mode */

} USART_CMODE_t;

The name of the enumeration is a concatenation of the module type (USART), the bit field (CMODE), and a suffix

(_enum).

The naming convention is presented in Figure 2-4.

Figure 2-4. Naming Convention of Enumerations

USART_CMODE_enum

Moduleprefix Bitfieldname

Enumeration

suffix

Each of the enumeration constants behaves much like a normal constant when used on its own. However, using an

enumeration type definition has the advantage of creating a new data type. The name of the data type, for this

example, is USART_CMODE_t. A USART_CMODE_t variable can be used directly as an integer, but assigning an

integer to an enumeration type will trigger a compiler warning. This can be used to the programmer’s advantage.

For example, if a function that sets the communication mode for a USART module accepts the communication mode

as an integer type (for example, unsigned char), any legal or illegal value can be passed to the function. If the

function instead accepts a parameter of type USART_CMODE_t, only the four predefined constants in the

USART_CMODE_t enumeration type can be passed to the function. Passing anything else will result in a compiler

warning.

Note: It is important to notice that the constants in the code listing are already shifted to their bit position. The

enumeration constants are the actual values to be written in the register, and no additional shifting is needed.

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3. Writing Bare Metal C-Code for AVR

The following subsections focus on how to write C-code for AVR. The examples describe how to make the code

readable and portable between different AVR devices. The examples can also be used as a guideline on how to write

code that is easy to verify and maintain.

The microcontroller modules are located in dedicated and continuous blocks in the memory space and can be seen

as encapsulated units. The modules are encapsulated in C structures, in which all module registers are contained.

This document introduces a naming convention and register access methods that are compliant with the AVR header

files, providing readability and portability to the codes written in the C-code.

3.1 Set, Clear and Read Register Bits

Setting and clearing register bits are fundamental operations used in embedded programming and represent a

technique any application is based on.

The Read-Modify-Write operations are a class of atomic operations. They read a memory location and

simultaneously write a new value to it, either with a completely new value or a part of the previous value.

As it has wide applicability, reading the value of a bit is mainly used in conditional expressions (e.g., if statement)

and as a condition in loop expressions (e.g., while statement). A common use case of this technique is polling on an

interrupt flag, which means reading the value of the bit and executing a set of instructions if the bit is set/clear.

Note: For further details on binary arithmetic, refer to the Bitwise Operators.

3.1.1 Set, Clear and Read Register Bits Using Bit Masks

The register bits can be set, cleared and read using the bit masks provided by the header file.

Access the registers using the structure declaration

To set a specific bit in a register, the recommended coding style is to use the structure instance declared in the

header file and bit masks macro definitions. Using the binary OR operator with the bit mask will ensure that the other

bit settings made inside that register will remain the same, unaffected by the operation.

ADC0.CTRLA |= ADC_ENABLE_bm; /* Enable the ADC peripheral */

To clear a bit from a register, the binary AND operator will be applied between the register and the negated value of

the bit mask. This operation also keeps the other bit settings unchanged.

ADC0.CTRLA &= ~ADC_ENABLE_bm; /* Disable the ADC peripheral */

The ADC_ENABLE_bm bit mask is defined in the header file, as presented below.

#define ADC_ENABLE_bm 0x01 /* ADC Enable bit mask */

Reading a bit value is necessary for various applications. For example, the ADC RESRDY flag is set by hardware

when an ADC result is ready. The code listing below shows how to test if this bit is set and execute some instructions

in this case.

if(ADC0.INTFLAGS & ADC_RESRDY_bm) /* Check if the ADC result is ready */

{

/* Insert some instructions here */

}

To test if a bit is clear, and execute instructions or wait while it remains clear, the following code can be used. In this

case, the instructions inside this loop will be executed until the ADC result is ready.

while(!(ADC0.INTFLAGS & ADC_RESRDY_bm))

{

/* Insert some instructions here */

}

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Access the registers using the macro definition

Alternatively, the registers can be accessed also using the macro definitions from the header file. The example below

shows how to set a bit using these definitions.

ADC0_CTRLA |= ADC_ENABLE_bm; /* Enable the ADC peripheral */

To clear a bit using macro definitions, the following code example can be used.

ADC0_CTRLA &= ~ADC_ENABLE_bm; /* Disable the ADC peripheral */

The code listing below shows how to test if this bit is set and to execute some instructions in this case.

if(ADC0_INTFLAGS & ADC_RESRDY_bm) /* Check if the ADC result is ready */

{

/* Insert some instructions here */

}

To test if a bit is clear, and execute instructions while it remains clear, the following code can be used. In this case,

the instructions inside this loop will be executed until the ADC result is ready.

while(!(ADC0_INTFLAGS & ADC_RESRDY_bm))

{

/* Insert some instructions here */

}

3.1.2 Set, Clear and Read Register Bits Using Bit Positions

An alternative way to set, clear and read register bits is by using bit positions, also provided by the device header file.

Access the registers using the structure declaration

To set a bit using a bit position macro and access the registers using the structure declaration, the following code

example can be used.

ADC0.CTRLA |= (1 << ADC_ENABLE_bp); /* Enable the ADC peripheral */

To clear the bit using a bit position macro, the following example is provided.

ADC0.CTRLA &= ~(1 << ADC_ENABLE_bp); /* Disable the ADC peripheral */

To test if a bit is set, the following code can be used.

if(ADC0.INTFLAGS & (1 << ADC_RESRDY_bp)) /* Check if the ADC result is ready */

{

/* Insert some instructions here */

}

To test if a bit is clear, and execute instructions while it remains clear, the following code can be used. In this case,

the instructions inside this loop will be executed until the ADC result is ready.

while(!(ADC0.INTFLAGS & (1 << ADC_RESRDY_bp)))

{

/* Insert some instructions here */

}

Access the registers using the macro definition

To set a bit using a bit position macro and access the registers using the macro definitions, the following code

example can be used.

ADC0_CTRLA |= (1 << ADC_ENABLE_bp); /* Enable the ADC peripheral */

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To clear the bit using a bit position macro, the following example is provided.

ADC0_CTRLA &= ~(1 << ADC_ENABLE_bp); /* Disable the ADC peripheral */

To test if a bit is set, the following code can be used.

if(ADC0_INTFLAGS & (1 << ADC_RESRDY_bp)) /* Check if the ADC result is ready */

{

/* Insert some instructions here */

}

To test if a bit is clear, and execute instructions while it remains clear, the following code can be used. In this case,

the instructions inside this loop will be executed until the ADC result is ready.

while(!(ADC0_INTFLAGS & (1 << ADC_RESRDY_bp)))

{

/* Insert some instructions here */

}

3.2 Register Initialization

To initialize a register, the user has to find the desired configuration by consulting the device data sheet. Then, the

known Reset state (default).

For most AVR registers, the Reset value for all bits and bit fields is ‘0’. The Reset value of a register can be found, as

shown in Figure 3-1, along with all the register’s bits configurations desired for this example.

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Figure 3-1. ADC Control A Register - Reset Value and Bit Settings

Name: CTRLA

Offset: 0x00

Reset: 0x00

Property: -

Bit 7 6 5 4 3 2 1 0

RUNSTDBY CONVMODE LEFTADJ RESSEL[1:0] FREERUN ENABLE

Access

R/W R/W R/W R/W R/W R/W R/W

Reset 0 0 0 0 0 0 0

Bit 7 – RUNSTDBY Run in Standby

This bit determines whether the ADC still runs during Standby

Value Description

ADC will not run in Standby leep mode. An ongoing conversion will finish before the ADC enters

ADC will run in Standby leep mode.

Bit 5 – CONVMODE Conversion Mode

This bit defines if the ADC is working in Single-Ended or Dif

ferential mode.

Value Name Description

0x0

SINGLEENDED The ADC is operating in Single-Ended mode where only the positive input is used.

The ADC result is presented as an unsigned value.

Bit 4 – LEFTADJ Left Adjust Result

riting a ‘1’ to this bit will enable left adjustment of the ADC result.

Bits 3:2 – RESSEL[1:0] Resolution Selection

This bit field selects the ADC resolution. When changing the resolution from 12-bit to 10-bit, the conversion time is

reduced from 13.5 CLK_ADC cycles to 11.5 CLK_ADC cycles.

Value Description

0x00

12-bit resolution

Other

Reserved

Bit 1 – FREERUN Free-Running

riting a ‘1’ to this bit will enable the Free-Running mode for the ADC. The first conversion is started by writing a ‘1’

to the Start Conversion (STCONV) bit in the Command (ADCn.COMMAND) register.

Bit 0 – ENABLE ADC Enable

Value Description

ADC is disabled

sleep mode.

0x1

DIFF The ADC is operating in Differential mode where both positive and negative inputs

are used. The ADC result is presented as a signed value.

10-bit resolution

0x01

ADC is enabled

Read-Modify-Write operations are not needed when working with bit masks or bit positions if the Reset value of the

The desired configuration can be, for example:

• ADC is enabled – the ENABLE bit will be ‘1’

• ADC is operating in Differential mode – the CONVMODE bit will be ‘1’

• ADC will run with 10-bit resolution – the RESSEL bit field value will be 0x00 (default)

This configuration can be implemented by writing the resulting value directly to the register, using either the binary,

hexadecimal, or decimal value, as presented below.

ADC0.CTRLA = 0b00100001; /* binary */

ADC0.CTRLA = 0x21; /* hexadecimal */

ADC0.CTRLA = 33; /* decimal */

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However, to improve the readability (and potentially the portability) of the code, it is recommended to use the device

defines, which are shown in the upcoming sections.

Note: For AVR registers, the Reset value for most bits and bit fields is ‘0’, but there are exceptions. For example,

the USART0 Control C register has a couple of bits with the Reset value ‘1’. In this case, the user must explicitly set

the desired configuration without relying on the fact that the Reset values of the bits are usually ‘0’.

Figure 3-2. USART Control C Register – Reset Value

Name: CTRLC

Offset: 0x07

Reset: 0x03

Property: -

This register description is valid for all modes except the Master SPI mode. When the USART Communication Mode

for the correct description.

Bit 7 6 5 4 3 2 1 0

CMODE[1:0] PMODE[1:0] SBMODE CHSIZE[2:0]

Access

R/W R/W R/W R/W R/W R/W R/W R/W

Reset 0 0 0 0 0 0 1 1

(CMODE) bits in this register are written to ‘MSPI’, see CTRLC - Master SPI mode

The USART0 Control C register has a Reset value of 0x03. In this case, a Read-Modify-Write operation is needed to

initialize one of the other bits or bit fields without changing the value of the CHSIZE bit field. The register is presented

in Figure 3-2.

3.2.1 Register Initialization Using Bit Masks and Group Configuration Masks

This subsection provides the recommended way to configure the ADC CTRLA register using bit masks and group

configuration masks.

ADC0.CTRLA = ADC_ENABLE_bm /* Enable the ADC */

| ADC_CONVMODE_bm /* Select Differential Conversion mode */

| ADC_RESSEL_10BIT_gc; /* 10-bit conversion */

Note the absence of the bitwise OR (‘|’) on the register side of the assignment. In most cases, device and peripheral

routines are written in this way.

The ADC_RESSEL_enum enumeration contains the group configuration mask presented below.

ADC_RESSEL_10BIT_gc = (0x01<<2), /* 10-bit mode */

CAUTION

The above initialization of the register must be done in a single line of C-code. Writing as follows, the

group configuration in the second line will clear the bit set in the first line.

ADC0.CTRLA = ADC_ENABLE_bm;

ADC0.CTRLA = ADC_CONVMODE_bm;

ADC0.CTRLA = ADC_RESSEL_10BIT_gc;

The correct way of writing this code using three code lines is presented below.

ADC0.CTRLA = ADC_ENABLE_bm;

ADC0.CTRLA |= ADC_CONVMODE_bm;

ADC0.CTRLA |= ADC_RESSEL_10BIT_gc;

Note: Bit masks can only set bits in a single line of code, so any configurations which require bits to be set to ‘0’ can

be left out since they are correctly configured by their Reset value.

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3.2.2 Register Initialization Using Bit Positions

The same configuration, as presented above, can be done by using the bit position macros, as follows.

ADC0.CTRLA = (1 << ADC_ENABLE_bp) /* Enable the ADC */

| (1 << ADC_CONVMODE_bp) /* Select Differential Conversion mode */

| (1 << ADC_RESSEL0_bp) /* 10-bit conversion */

| (0 << ADC_RESSEL1_bp);

Note: The (0 << ADC_RESSEL0_bp) line is added simply for readability, but it can be removed.

ADC0.CTRLA = (1 << ADC_ENABLE_bp) /* Enable the ADC */

| (1 << ADC_CONVMODE_bp) /* Select Differential Conversion mode */

| (1 << ADC_RESSEL0_bp); /* 10-bit conversion */

Other position masks that can be used to configure bit fields are group position masks. They can be used as

presented below. The desired configuration value must be shifted with the bit field position (group position).

ADC0.CTRLA = (1 << ADC_ENABLE_bp) /* Enable the ADC */

| (1 << ADC_CONVMODE_bp) /* Select Differential Conversion mode */

| (0x01 << ADC_RESSEL_gp); /* 10-bit conversion */

3.3 Change Register Bit Field Configurations

This section covers considerations when updating a register bit field, using various header file defines. The RXMODE

bit field will be used as an example, where an update is made compared to the initialization presented below.

USART0.CTRLB = USART_RXEN_bm /* Receiver Enable */

| USART_TXEN_bm /* Transmitter Enable */

| USART_RXMODE_LINAUTO_gc; /* LIN Constrained Auto-Baud mode */

The following subsections will provide code examples to change the Receiver Mode (RXMODE) configuration to

Generic Auto-Baud (GENAUTO) mode. The available configurations for this bit field, as they are in the device data

sheet, are presented in the table below.

Figure 3-3. USART0 Receiver Mode Configurations

Value Name Description

0x00

NORMAL Normal USART mode, standard transmission speed

0x01

CLK2X Normal USART mode, double transmission speed

0x02

GENAUTO Generic Auto-Baud mode

0x03

LINAUTO LIN Constrained Auto-Baud mode

3.3.1 Change Register Bit Field Configurations Using Group and Group Configuration Masks

When updating only a bit field in a register, a Read-Modify-Write operation must be used. Therefore, to change the

configuration of a register bit field, it is recommended to first clear the old configuration and then set a new one.

It is recommended using a bit group configuration mask to set a configuration in a bit field.

One way to change a bit field to a new configuration is presented in the code listing below. To be sure that the desired

configuration is obtained, the user must clear the old configuration first, using the group mask.

/* Changing a bit group configuration */

USART0.CTRLB &= (~USART_RXMODE_gm); /* Clear the old configuration */

/* Set the new configuration using the group configuration mask */

USART0.CTRLB |= USART_RXMODE_GENAUTO_gc;

Note: The first line will put the USART0 RXMODE, for a short time, into a specific state: 0x00.

A group mask macro is defined in the header file, as presented below.

#define USART_RXMODE_gm 0x06 /* Receiver mode group mask */

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The USART_RXMODE_enum enumeration contains the group configuration mask presented below.

USART_RXMODE_GENAUTO_gc = (0x02<<1), /* Generic Auto-Baud mode */

CAUTION

Even though it may seem easier to split the code into two separate code lines, one to clear the register

and another one to set the desired configuration, it is recommended to use a single line to do this, as

presented in the code listing below.

/* Changing a bit group configuration */

USART0.CTRLB = (USART0.CTRLB & ~USART_RXMODE_gm) | USART_RXMODE_GENAUTO_gc;

These steps must be implemented in a single line to avoid putting the microcontroller in an unintended state.

The CTRLB register is declared this way:

register8_t CTRLB;

The register8_t data type is a volatile defined data type.

typedef volatile uint8_t register8_t;

Since the register is defined as volatile, two different code lines will trigger the reading and writing in the CTRLB

unintended state. The reason is that when an interrupt is triggered between the two lines of code, the context will be

changed, and the register can remain in an undefined state.

3.3.2 Change Register Bit Field Configurations Using Bit Masks

The example below shows how to update a register bit field using bit masks to set the new configuration. The current

configuration is cleared, and the new configuration is set, in a single line of code.

USART0.CTRLB = (USART0.CTRLB & ~(USART_RXMODE0_bm | USART_RXMODE1_bm))

| USART_RXMODE1_bm; /* Generic Auto-Baud mode */

3.3.3 Change Register Bit Field Configurations Using Bit Positions

The example below shows how to update a register bit field using bit positions to set the new configuration. The

current configuration is cleared, and the new configuration is set, in a single line of code.

USART0.CTRLB = (USART0.CTRLB & ~((1 << USART_RXMODE0_bp) | (1 << USART_RXMODE1_bp)))

| (0 << USART_RXMODE0_bp)

| (1 << USART_RXMODE1_bp);

Note: The (0 << USART_RXMODE0_bp) line is added simply for readability, but it can be removed.

USART0.CTRLB = (USART0.CTRLB & ~((1 << USART_RXMODE0_bp) | (1 << USART_RXMODE1_bp)))

| (1 << USART_RXMODE1_bp);

3.4 Advantages of Using Bit Masks and Group Configuration Masks

The advantages of using the recommended coding style are:

• Obtaining a more readable code. By using the group configuration mask, the user will know for sure what

configuration is being set – the configuration name being contained in the mask name.

• Obtaining a more compact code. The group mask will help the user clear all the bits in a bit field, without the

need of using a bit mask for each bit. The configuration mask can also be used to configure the entire bit field.

• The code is easier to maintain. For example, to change a bit field configuration, the user will only have to

change the group configuration mask name instead of changing each bit value using bit masks or bit positions.

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3.5 Writing to Configuration Change Protection (CCP) Registers

Attempting to write to a protected register without following the appropriate CCP unlock sequence leaves the

protected register unchanged. The sequence, specified in the device data sheet, typically involves writing a signature

to the CPU.CCP register and then, within four instructions, writing the desired value to the protected register.

The CCP signatures are provided by the device header file, as presented below.

/* CCP signature select */

typedef enum CCP_enum

{

CCP_SPM_gc = (0x9D<<0), /* SPM instruction protection */

CCP_IOREG_gc = (0xD8<<0), /* I/O register protection */

} CCP_t;

The following example shows how to write to a register under CCP using an IOREG signature. The main clock

prescaler division will be set to 16.

CCP = CCP_IOREG_gc; /* Write the needed signature to CCP*/

CLKCTRL.MCLKCTRLB = CLKCTRL_PDIV_16X_gc; /* Set the prescaler division to 16 */

The following example shows how to write to a register under CCP using the SPM signature.

CPU.CCP = CCP_SPM_gc; /* Write the needed signature to CCP*/

NVMCTRL.CTRLA = NVMCTRL_CMD_FLPER_gc; /* Flash Page Erase Enable*/

CAUTION

These are not the recommended ways of writing to registers under CCP. To write to registers protected by

CCP, after writing the signature to the CCP register, the software must write the desired data to the

protected register within four instructions. To meet the timing requirements, writes to CCP registers are

often handled by assembly code.

Thus, the recommended way of writing to a protected I/O register is by using the ccp_write_io function. To use

this function, the following header file must be included.

#include <avr/cpufunc.h> /* Required header file */

The following code example provides the same functionality as the example presented above, but using the

ccp_write_io function.

/* Set the prescaler division to 16 */

ccp_write_io((void *) & (CLKCTRL.MCLKCTRLB), (CLKCTRL_PDIV_16X_gc));

Alternatively, a macro is defined to write a value to a CCP register. This is protected through the XMEGA

CCP

mechanism, which implements the timed sequence that is required for CCP.

Note: Since the CCP registers were introduced with the XMEGA family of AVR devices, the header file that contains

_PROTECTED_WRITE and _PROTECTED_WRITE_SPM macros is xmega.h, which must be included as shown below.

#include <avr/xmega.h> /* Required header file */

These macros must be used to write to the desired registers, as presented in the code examples below.

/* Select the 32 kHz internal Ultra-Low Power oscillator */

_PROTECTED_WRITE(CLKCTRL.MCLKCTRLA, CLKCTRL.MCLKCTRLA | CLKCTRL_CLKSEL_OSCULP32K_gc);

/* Write page command */

_PROTECTED_WRITE_SPM(NVMCTRL.CTRLA, NVMCTRL_CMD_PAGEWRITE_gc);

Note: Both the XC8 Compiler (MPLAB X IDE) and GCC (Atmel Studio 7) support the usage of these macros.

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3.6 Configuring Fuses

The fuses are preprogrammed but the fuse registers can be altered by the user. A fuse can be changed only by

programming it. However, some fuse values are loaded into registers and these register values can be changed

during run-time. The fuses can be configured by writing C-code, as described in the following subsections.

3.6.1 Configuring Fuses Using XC8 Configuration Bits

To configure the fuses with the MPLAB X IDE, configuration pragmas can be used. More information about the

compiler and the configuration bits of the desired device can be found by accessing the Compiler Help – the blue

question mark from the MPLAB X IDE project dashboard, as presented in Figure 3-4.

Figure 3-4. Accessing Compiler Help

The configuration settings for all supported devices are presented at Configuration Settings Reference → 8-bit AVR

MCUs, as shown in Figure 3-5.

Figure 3-5. Compiler Help → 8-bit Language Tools Readme and Reference

The configuration pragmas can be used as presented below.

#pragma config <setting> = <named value | literal constant>

The following example shows how to disable the Watchdog Timer and the CRC and to configure the start-up time to

be 8 ms, using configuration pragmas.

/* Disable Watchdog Timer */

#pragma config PERIOD = PERIOD_OFF

/* Disable CRC and set the Reset Pin Configuration to GPIO mode */

#pragma config CRCSRC = CRCSRC_NOCRC, RSTPINCFG = RSTPINCFG_GPIO

/* Start-up Time select: 8 ms */

#pragma config SUT = SUT_8MS

3.6.2 Configuring Fuses Using AVR

LibC

To configure the fuses using Atmel Studio, the fuse.h header file must be included, as presented below.

#include <avr/fuse.h> /* Required header file */

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The following example shows how to disable the Watchdog Timer using the Watchdog Configuration (WDTCFG) fuse

(SYSCFG0) register, and configure the start-up time to 8 ms using the System Configuration 1 (SYSCFG1) register.

FUSES = {

/* Disable Watchdog Timer */

.WDTCFG = PERIOD_OFF_gc,

/* Disable CRC and set the Reset Pin Configuration to GPIO mode */

.SYSCFG0 = CRCSRC_NOCRC_gc | RSTPINCFG_GPIO_gc,

/* Start-up Time select: 8 ms */

.SYSCFG1 = SUT_8MS_gc

};

CAUTION

If not initialized by the user, the fuses will be initialized with default values (‘0’) when using AVR LibC. If

there are fuses not initialized which must be different from ‘0’, the device may not work as expected.

3.7 Function Calls Using Module Pointers

When writing drivers for module types that have multiple instances, the fact that all instances have the same register

memory map can be utilized to make the driver reusable for all instances of the module type. If the driver takes a

pointer argument pointing to the relevant module instance, the driver can be used for all modules of this type. This

represents a great advantage when considering portability. Moreover, the written code may be portable between

devices of the same family. Details on the compatibility between devices from the same family are provided in the

data sheet’s series Overview section, and some differences are shown in Figure 3-6.

Figure 3-6. megaAVR

0-series Overview

(1)

8 KB

Pins

ATmega1608

40 48

ATmega3208

ATmega808

ATmega3209

ATmega1609

16 KB

32 KB

Flash

48 KB

ATmega4808 ATmega4809

ATmega809

In a device with multiple timers/counters, the functions to initialize and access these modules can be shared by all

module instances, instead of replicating the same lines of code for every instance. Even though there is a small

overhead in passing the module pointer to the functions, the total code size will be reduced because the code is

reused for all instances of each module type. Moreover, development time, maintenance cost, and portability can be

greatly improved by using this approach.

The code below shows a function that uses a module pointer to select a clock source for any timer/counter module.

void TC_ConfigClockSource (volatile TCB_t *tc, TCB_CLKSEL_t clockSelection)

{

tc->CTRLA = (tc->CTRLA & ~TCB_CLKSEL_gm) | clockSelection;

}

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The function takes two arguments: A module pointer of type TCB_t and a group configuration type TCB_CLKSEL_t.

The code in the function uses the timer/counter module pointer to access the CTRLA register and set a new clock

selection for the timer/counter module with the address provided through the tc parameter. The code below shows

how the function described above is used to set different configurations for different timer/counter instances.

/* Configure the TCB0 clock selection as CLKDIV2*/

TCB_ConfigClockSource (&TCB0, TCB_CLKSEL_CLKDIV2_gc);

/* Configure the TCB0 clock selection as CLKDIV1*/

TCB_ConfigClockSource (&TCB0, TCB_CLKSEL_CLKDIV1_gc);

/* Configure the TCB1 clock selection as CLKDIV2*/

TCB_ConfigClockSource (&TCB1, TCB_CLKSEL_CLKDIV2_gc);

/* Configure the TCB2 clock selection as CLKDIV2*/

TCB_ConfigClockSource (&TCB2, TCB_CLKSEL_CLKDIV2_gc);

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4. Application Example Showing Alternative Ways of Writing Code

For convenience and for maintaining compatibility with older versions of AVR coding styles, it is still possible to use a

programming style that does not involve structures. This section briefly describes alternative ways of accessing

registers and using bit names.

4.1 Register Names

It is possible to access any register without using the module structures. To refer to a register directly, concatenate

the module instance name, then add an underscore and the register name. The same naming convention is used

when programming in assembly.

For example, to access the CTRLA register of Timer/Counter type B, instance 0, the name TCB0_CTRLA can be

used, instead of the access through the structures.

4.2 Bit Positions

It is possible to use bit masks to set or clear bits. A bit’s position within a register is defined using the same name as

the bit mask, but a different suffix. The code below shows how the bit position can be used to configure a register.

PORTB_OUT |= (1 << PIN0_bp); /* Set PORTB_OUT, bit0 */

The bit position definitions are included for compatibility reasons. They are also needed when programming in

assembly for instructions that use a bit number.

4.3 Virtual Ports

Virtual port (VPORT) registers allow some PORT registers to be mapped virtually in the bit accessible I/O memory

space. When this is done, writing to the VPORT register will be the same as writing to the real PORT register. This

enables the use of I/O memory specific instructions, such as bit manipulation instructions (SBI/CBI), on a PORT

limited to four or six, resulting in a smaller number of available virtual ports than the number of PORT modules. The

advantages of using virtual ports are a smaller and less complex program and less Flash memory used.

A compatibility diagram is presented in Figure 4-1, which explains what coding styles are available for specific AVR

product families.

Figure 4-1. AVR

Family Register Configuration Options

Bit position masks

All AVR

megaAVR

0/1

tinyAVR

and AVR

Hardware Read-Modify-Write

VPORT registers

AVR XMEGA

Bit masks and group configuration masks

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4.4 PORT Example

This subsection contains an example on how to configure a PORT to turn on an LED when pressing a user button. To

identify which pins of the microcontroller are routed to the user LED and to the user button, the used board schematic

is necessary. For the AVR128DA48 Curiosity Nano board used for this example, the LED is connected to the sixth pin

of PORTC, PC6, and the button is connected to the seventh pin of PORTC, PC7. To make sure this is the correct

configuration of the pins, the user must check the board schematic.

The code below shows how to write code for turning on and off an LED, using bit position macros.

Example 4-1. Turn LED On when Button Pressed Using Bit Positions

/* Direction configuration of the pins */

/* Read-Modify-Write: Software */

PORTC.DIR |= (1 << PIN6_bp); /* Output for the user LED */

PORTC.DIR &= ~(1 << PIN7_bp); /* Input for the user button */

PORTC.PIN7CTRL |= (1 << PORT_PULLUPEN_bp); /* The pull-up configuration */

while (1)

{

if(PORTC.IN & (1 << PIN7_bp)) /* Check the button state */

{

/* The button is released */

PORTC.OUT |= (1 << PIN6_bp); /* Turn off the LED */

}

else

{

/* The button is pressed */

PORTC.OUT &= ~(1 << PIN6_bp); /* Turn on the LED */

}

Note: This is the coding style used by Atmel START for peripheral configuration.

The code below provides the same functionality but using the bit masks.

Example 4-2. Turn LED On when Button Pressed Using Bit Masks

/* Direction configuration of the pins */

/* Read-Modify-Write: Software */

PORTC.DIR |= PIN6_bm; /* Output for the user LED */

PORTC.DIR &= ~ PIN7_bm; /* Input for the user button */

PORTC.PIN7CTRL |= PORT_PULLUPEN_bm; /* The pull-up configuration */

while (1)

{

if(PORTC.IN & PIN7_bm) /* Check the button state */

{

* The button is released */

PORTC.OUT |= PIN6_bm; /* Turn off the LED */

}

else

{

/* The button is pressed */

PORTC.OUT &= ~(PIN6_bm); /* Turn on the LED */

}

Note: This is the coding style used by MCC when generating code for AVR.

Also, the SET and CLR registers can be used to set and clear the pin value without Read-Modify-Write operations, as

shown in the code below. This allows performing the setting and the clearing of the desired value using an atomic

instruction, instead of reading the register value first. The main advantage is that this action cannot be interrupted.

Only one instruction is executed instead of three (Read-Modify-Write).

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Example 4-3. Turn LED On when Button Pressed Using SET and CLR Registers

PORTC.DIRSET = PIN6_bm; /* Output for the user LED */

PORTC.DIRCLR = PIN7_bm; /* Input for the user button */

PORTC.PIN7CTRL |= PORT_PULLUPEN_bm; /* The pull-up configuration */

while (1)

{

/* Read-Modify-Write: Hardware */

if(PORTC.IN & PIN7_bm) /* Check the button state */

{

/* The button is released */

PORTC.OUTSET = PIN6_bm; /* Turn off the LED */

}

else

{

/* The button is pressed */

PORTC.OUTCLR = PIN6_bm; /* Turn on the LED */

}

Another way to configure the direction and set/clear the value of the output pin is by using virtual ports, as presented

in the code below.

Example 4-4. Turn LED On when Button Pressed Using Virtual Ports

/* Direction configuration of the pins */

/* Read-Modify-Write: VPORT registers */

VPORTC.DIR |= PIN6_bm; /* Output for the user LED */

VPORTC.DIR &= ~PIN7_bm; /* Input for the user button */

PORTC.PIN7CTRL |= PORT_PULLUPEN_bm; /* The pull-up configuration */

while (1)

{

if(VPORTC.IN & PIN7_bm) /* Check the button state */

{

/* The button is released */

VPORTC.OUT |= PIN6_bm; /* Turn off the LED */

}

else

{

/* The button is pressed */

VPORTC.OUT &= ~PIN6_bm; /* Turn on the LED */

}

4.5 ADC Example

This section presents a simple configuration example for the ADC0 module on an AVR128DA48 device from the AVR

DA series of devices. The information needed to program this controller can be found in the device data sheet, as

mentioned in Section 1: Data Sheet Structure and Naming Conventions.

To fully configure the ADC, the user can follow the recommended initialization steps that can be found in the

Functional Description subsection from the ADC – Analog-to-Digital Converter section of the data sheet.

After the ADC prescaler is configured, the ADC module is enabled, and a conversion is started. In the code below,

the configurations are made using the bit position macros.

Example 4-5. Configure ADC Using Bit Positions

/* ADC register configuration using bit position macros */

ADC0.CTRLC |= (1 << ADC_PRESC0_bp) | (1 << ADC_PRESC1_bp); /* Configuring

the prescaler bits. Configuration: DIV8 */

ADC0.CTRLA |= (1<< ADC_ENABLE_bp); /* Enabling the ADC */

ADC0.COMMAND |= (1<< ADC_STCONV_bp); /* Starting a conversion */

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Alternatively, the ADC can be configured using the bit masks, as in the code below.

Example 4-6. Configure ADC Using Bit Masks

/* ADC register configuration using bit masks macros */

ADC0.CTRLC |= ADC_PRESC0_bm | ADC_PRESC1_bm; /* Configuring

the prescaler bits. Configuration: DIV8 */

ADC0.CTRLA |= ADC_ENABLE_bm; /* Enabling the ADC */

ADC0.COMMAND = ADC_STCONV_bm; /* Starting a conversion */

To change the prescaler configuration, a group configuration mask can be used. The original configuration must be

cleared first, as shown in the code below.

Example 4-7. Change the ADC Prescaler Configuration Using a Group Configuration Mask

/* Changing an ADC prescaler configuration, ensuring to clear the original

configuration */

ADC0.CTRLC = (ADC0.CTRLC & ~ADC_PRESC_gm) | ADC_PRESC_DIV4_gc;

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5. Further Steps

This section has the purpose to direct the user to the IDE installation guides and instructions, and the available

application notes.

5.1 Application Notes and Technical Briefs Description

This series of technical briefs utilizes the same principles recommended in this document, covering all peripherals for

the megaAVR

0-series family of AVR microcontrollers. Each technical brief starts with a summary of the use cases

covered. Each use case is then developed, showing how to use the data sheet to configure the peripheral in the

required configuration.

For example, the Getting Started with ADC technical brief provides an overview of the peripheral and a description on

how to use the peripheral in several use cases in different operation modes: single conversion, free-running, sample

accumulator, etc.

• TB3209 - Getting Started with ADC

• TB3211 - Getting Started with AC

• TB3213 - Getting Started with RTC

• TB3214 - Getting Started with TCB

• TB3215 - Getting Started with SPI

• TB3216 - Getting Started with USART

• TB3217 - Getting Started with TCA

• TB3218 - Getting Started with CCL

• TB3229 - Getting Started with GPIO

5.2 Relevant Videos for Bare Metal AVR

Development

The following videos are particularly relevant for this technical brief.

• Getting Started with Atmel Studio 7 - Episode 10 - I/O View & Bare Metal Programming References

• Getting Started - AVR

in MPLAB

X - Context Data Sheet Help & AVR Interrupts

The following is a 28-part video series, which builds up functionality using the data sheet and device header files as

primary programming references.

• Getting Started with AVR

5.3 MPLAB

XC8 Compiler

The XC compilers are comprehensive solutions for the software development of any suitable project. The MPLAB

XC8 compiler supports all 8-bit PIC

and AVR microcontrollers

(2)

, and it is available as free, unrestricted-use

download. A pro license is also available. By using a pro license, the user will obtain a more efficient code.

Additionally, a certified XC8 Functional Safety license is now available.

When combined with the MPLAB X IDE, the front-end provides editing errors and breakpoints, that match

corresponding lines in the source code, and single-stepping through C source code to inspect variables and

structures at critical points.

More information on Microchip’s MPLAB X IDE can be found on the user guides page, by searching for “MPLAB X

IDE User’s Guide”.

5.4 IDE (MPLAB

X, Atmel Studio, IAR) – Getting Started

To program AVR microcontrollers, either the MPLAB X, Atmel Studio, or IAR Embedded Workbench IDEs can be

used.

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DS90003262B-page 31

MPLAB X Integrated Development Environment (IDE) is an expandable and highly configurable software program. It

incorporates powerful tools to help the user discover, configure, develop, debug and qualify embedded designs for

most of Microchip’s microcontrollers and digital signal controllers. MPLAB X IDE offers support for AVR MCUs.

All the information needed to be familiar with Atmel Studio, including hands-on and video tutorials, is provided in this

user’s guide: Getting Started with Atmel Studio 7. Additionally, information on all the project configurations needed to

develop a project (fuse programming, oscillator calibration, interface setting, etc) can be found in the Atmel Studio 7

user’s guide. To find and develop more examples for any board, configure drivers, find example projects, and easily

configure system clock settings, the online code configuration tool Atmel START can be used. For more details, refer

to the Atmel START User Guide.

The IAR Embedded Workbench for AVR with all the features is described in the AN3419 - Getting Started with IAR

Embedded Workbench for AVR application note.

The microcontroller families tinyAVR

0/1-series, megaAVR 0-series, and AVR DA have also a series of Getting

Started with dedicated guides available online, with information on how to create a project and what starter kit to use.

Examples of these guides are listed below:

• Getting Started with megaAVR

0-series

• Getting Started with tinyAVR

1-series Microcontrollers

• Getting Started with tinyAVR

0-series

• Getting Started with the AVR DA Family

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DS90003262B-page 32

6. Conclusion

This document introduces the user to a preferred coding style for programming the AVR microcontrollers. After going

through this document, the user knows what type of information the data sheet is providing, and also the macro

definitions, the variable declarations, and the data type definitions provided by the header files. The goals are to use

an easily maintainable, portable and readable coding style, to get familiar with the naming conventions for the AVR

registers and bits, and to get ready for the further steps in developing a project using these microcontrollers.

This document covers information on specific data sheets, information description, naming conventions, how to write

C-code for AVR microcontrollers, alternative ways of writing the code, and, finally, the next steps in developing a

project.

Using the suggested methods to write C-code is not mandatory, but the advantages presented here can be

considered. The larger the project and the more features the device has, the bigger the advantage.

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DS90003262B-page 33

7. References

1. ATmega4808/4809 Data Sheet

2. MPLAB

XC Compilers

3. AVR Libc Library Reference

4. AVR

Devices in MPLAB

XC8

5. MPLAB

XC8 C Compiler User’s Guide for AVR

MCU

6. Fundamentals of the C Programming Language

7. Fundamentals of C Programming - Enumerations

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Technical Brief

DS90003262B-page 34

8. Revision History

Doc. Rev. Date Comments

B 06/2020 The Summary label on the front page has been removed

A 05/2020 Initial document release

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Technical Brief

DS90003262B-page 35

The Microchip Website

Microchip provides online support via our website at www.microchip.com/. This website is used to make files and

information easily available to customers. Some of the content available includes:

• Product Support – Data sheets and errata, application notes and sample programs, design resources, user’s

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• General Technical Support – Frequently Asked Questions (FAQs), technical support requests, online

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Microchip’s product change notification service helps keep customers current on Microchip products. Subscribers will

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To register, go to www.microchip.com/pcn and follow the registration instructions.

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Users of Microchip products can receive assistance through several channels:

• Distributor or Representative

• Local Sales Office

• Embedded Solutions Engineer (ESE)

• Technical Support

Customers should contact their distributor, representative or ESE for support. Local sales offices are also available to

help customers. A listing of sales offices and locations is included in this document.

Technical support is available through the website at: www.microchip.com/support

Microchip Devices Code Protection Feature

Note the following details of the code protection feature on Microchip devices:

• Microchip products meet the specification contained in their particular Microchip Data Sheet.

• Microchip believes that its family of products is one of the most secure families of its kind on the market today,

when used in the intended manner and under normal conditions.

• There are dishonest and possibly illegal methods used to breach the code protection feature. All of these

methods, to our knowledge, require using the Microchip products in a manner outside the operating

specifications contained in Microchip’s Data Sheets. Most likely, the person doing so is engaged in theft of

intellectual property.

• Microchip is willing to work with the customer who is concerned about the integrity of their code.

• Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code

protection does not mean that we are guaranteeing the product as “unbreakable.”

Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection

features of our products. Attempts to break Microchip’s code protection feature may be a violation of the Digital

Millennium Copyright Act. If such acts allow unauthorized access to your software or other copyrighted work, you

may have a right to sue for relief under that Act.

Legal Notice

Information contained in this publication regarding device applications and the like is provided only for your

convenience and may be superseded by updates. It is your responsibility to ensure that your application meets with

TB3262

Technical Brief

DS90003262B-page 36

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All other trademarks mentioned herein are property of their respective companies.

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TB3262

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DS90003262B-page 37

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Technical Brief

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