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AVR is a family of microcontrollers (MCUs) developed by Microchip Technology (former AVR). AVRs are especially common in hobbyist and educational embedded applications, popularized by Arduino project. This page deals with 8-bit series of these MCUs.

## Toolchain

Install avr-gcc to get toolchain and GNU compiler.

## Programmers

To flash compiled firmware to the AVR chip you will need programmer and software to rule it. Most popular programmers are USBasp, AVRISP mkII, Atmel-ICE and STK500. There also exists a simple DIY-programmer which works with LPT port. avrdude supports them all great.

### udev rules

To be able to run avrdude without superuser rights, a Udev rule needs to be created to change ownership of the usb device to another group (the commonly used group for this is `uucp`):

    /etc/udev/rules.d/99-avrprogrammer.rules

    # Set group for a programmer matching with vendor and product IDs
     SUBSYSTEM=="usb", ATTRS{idVendor}=="your Vendor ID", ATTRS{idProduct}=="your Product ID", GROUP="uucp", MODE="0666"

Replace the italic parts with your programmer's vendor and product IDs (these can be found with the lsusb command from usbutils) and add the `uucp` group to your user's supplementary groups.

## Usage

To compile C program for AVR chip (let us consider ATmega8A running at 8 MHz as example) you can use `avr-gcc` directly. You should only specify target MCU (full list of supported MCUs could be found with avr-gcc(1) § mmcu=) and its working frequency:

    $ avr-gcc -DF_CPU=8000000UL -mmcu=atmega8a -std=gnu99 main.c -o main.elf

avrdude is smart enough to work with the resulting ELF file but you can convert it explicitly to Intel HEX:

    $ avr-objcopy -O ihex -j .text -j .data main.elf main.hex

Then run avrdude and specify flash ROM as destination for firmware burning (in this example AVRISP mkII is used and clock speed is lowered to the 125 kHz to be on safe side):

    $ avrdude -p atmega8 -c avrispmkII -B 125kHz -U flash:w:main.hex

That's all. Among other things avrdude can work with EEPROM memory, fuse and lock bits. For example, to set up low and high fuses to the 0x9F and 0xD1 respectively use the following incantation:

    $ avrdude -p atmega8 -c avrispmkII -B 125kHz -U lfuse:w:0x9F:m -U hfuse:w:0xD1:m

Just remember that ISP programming speed should not exceed 1/8 of MCU's working frequency. A lot of new chips comes with 1 MHz speed settings so using 125 kHz as starting value should be fine enough.

## Tips and tricks

### Optimization

Because AVRs come with tight flash ROM size and relatively weak CPUs you can consider some optimizations to improve space usage and overall performance of your device. It is common practice to enable GCC optimization level `-Os` and turn on some extra features: `-funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums`. To exclude unnecessary library references perform garbage collection: `-ffunction-sections -fdata-sections -Wl,--gc-sections`.

### Sample Makefile

Managing huge project could be tedious and Makefile workflow is the most efficient way to deal with it. Here are sample Makefile based on AVRfreaks[dead link 2023-04-22 ⓘ] version:

    CC = avr-gcc
    OBJCOPY = avr-objcopy
    SIZE = avr-size
    NM = avr-nm
    AVRDUDE = avrdude
    REMOVE = rm -f

    MCU = atmega8a
    F_CPU = 8000000

    LFUSE = 0x9f
    HFUSE = 0xd1

    TARGET = firmware
    SRC = main.c lcd.c twi.c
    OBJ = $(SRC:.c=.o)
    LST = $(SRC:.c=.lst)

    FORMAT = ihex

    OPTLEVEL = s

    CDEFS =

    CFLAGS = -DF_CPU=$(F_CPU)UL
    CFLAGS += $(CDEFS)
    CFLAGS += -O$(OPTLEVEL)
    CFLAGS += -mmcu=$(MCU)
    CFLAGS += -std=gnu99
    CFLAGS += -funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums
    CFLAGS += -ffunction-sections -fdata-sections
    CFLAGS += -Wall -Wstrict-prototypes
    CFLAGS += -Wa,-adhlns=$(<:.c=.lst)

    LDFLAGS = -Wl,--gc-sections
    LDFLAGS += -Wl,--print-gc-sections

    AVRDUDE_MCU = atmega8
    AVRDUDE_PROGRAMMER = avrispmkII
    AVRDUDE_SPEED = -B 1MHz

    AVRDUDE_FLAGS = -p $(AVRDUDE_MCU)
    AVRDUDE_FLAGS += -c $(AVRDUDE_PROGRAMMER)
    AVRDUDE_FLAGS += $(AVRDUDE_SPEED)

    MSG_LINKING = Linking:
    MSG_COMPILING = Compiling:
    MSG_FLASH = Preparing HEX file:

    all: gccversion $(TARGET).elf $(TARGET).hex size

    .SECONDARY: $(TARGET).elf
    .PRECIOUS: $(OBJ)

    %.hex: %.elf
            @echo
            @echo $(MSG_FLASH) $@
            $(OBJCOPY) -O $(FORMAT) -j .text -j .data $< $@

    %.elf: $(OBJ)
            @echo
            @echo $(MSG_LINKING) $@
            $(CC) -mmcu=$(MCU) $(LDFLAGS) $^ --output $(@F)

    %.o : %.c
            @echo $(MSG_COMPILING) $<
            $(CC) $(CFLAGS) -c $< -o $(@F)

    gccversion:
            @$(CC) --version

    size: $(TARGET).elf
            @echo
            $(SIZE) -C --mcu=$(AVRDUDE_MCU) $(TARGET).elf

    analyze: $(TARGET).elf
            $(NM) -S --size-sort -t decimal $(TARGET).elf

    isp: $(TARGET).hex
            $(AVRDUDE) $(AVRDUDE_FLAGS) -U flash:w:$(TARGET).hex

    fuses:
            $(AVRDUDE) $(AVRDUDE_FLAGS) -U lfuse:w:$(LFUSE):m -U hfuse:w:$(HFUSE):m

    release: fuses isp

    clean:
            $(REMOVE) $(TARGET).hex $(TARGET).elf $(OBJ) $(LST) *~

### Calculating control register values

To speed up development of your projects you can use avrcalcAUR utility which helps to calculate different parameters for control registers regarding timers, frequencies etc.

## See also

- Arduino
- https://www.microchip.com/design-centers/8-bit/avr-mcus

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- Development

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