btorgis/avr-assembly

adventures programming AVR boards in assembly language

Assembly with arduino board

Get ready

• Install avrdude and get avra or gavrasm assembler
• Connect the Arduino board to the usbtiny programmer as shown in the picture below.
• Compile the program gavrasm hello.asm
• Upload the hex file sudo avrdude -p m328p -P usb -c usbtiny -U flash:w:hello.hex

Arduino pinout

Hacking the microcontroller

Reading info from the microcontroller

You can obtain info about the microcontroller with sudo avrdude -p m328p -P usb -c usbtiny -v. Like the memory detail:

                                  Block Poll               Page                       Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  MaxW   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
           eeprom        65    20     4    0 no       1024    4      0  3600  3600 0xff 0xff
           flash         65     6   128    0 yes     32768  128    256  4500  4500 0xff 0xff
           lfuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           hfuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           efuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           lock           0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           calibration    0     0     0    0 no          1    0      0     0     0 0x00 0x00
           signature      0     0     0    0 no          3    0      0     0     0 0x00 0x00

The current fuse values:

avrdude: safemode: hfuse reads as D6
avrdude: safemode: efuse reads as FD
avrdude: safemode: Fuses OK (E:FD, H:D6, L:FF)

Download the flash content

sudo avrdude -p m328p -P usb -c usbtiny -U flash:r:program.dump:i. Note that program.dump must exist. i stands for intel hex format. Check avrdude for more info.

Backup/restore the eeprom content

The eeprom is a non-volatile memory (it does not require energy to keep its content). Hence you can use it to store data that will remain even when the microcontroller is powered off. The data can be written from the code or uploaded externally.

When a program is flashed the eeprom is also deleted, loosing any data you might have stored. Hence you might want to backup that data first.

sudo avrdude -p m328p -P usb -c usbtiny -U eeprom:r:memory.eep:i. Note that the memory.eep file must exist beforehand.

You might want to restore the eeprom data once the program has been flashed.

sudo avrdude -p m328p -P usb -c usbtiny -U eeprom:w:memory.eep:i.

But you must agree that the above procedure is annoying. You can prevent the eeprom memory being erased at the chip erase cycle by clearing the EESAVE bit (number 3) in hfuse.

Disassembling a program

Disassembling is the process of converting a .hex file back to assembly language. I use vavrdisasm. Usage: vavrdisasm --assembly --no-opcodes hello.hex

.org 0x0000
A_0000: ldi     R16, 0x20
A_0002: out     $04, R16
A_0004: out     $05, R16
A_0006: rjmp    A_0006  ; 0x6

Compare it with the original hello.asm

;hello.asm
;  turns on an LED which is connected to PB5 (digital out 13)

.include "./m328Pdef.inc"

        ldi     r16,0b00100000
        out     DDRB,r16
        out     PORTB,r16
Start:
        rjmp    Start

As you can see 0x20 = 0b100000.

Looking at the datasheet DDRB has 0x04 address in the data memory map (AVR has Harvard architecture so data and program memories are separated. $04 refers to the offset address in the I/O section of the memory (for the out instruction).