How to program an Arduino Uno in Assembly#

2023 Feb 20

This tutorial shows you how to program an Arduino Uno R3 in AVR Assembly. The assembly program that you flash onto the Uno just blinks the built-in LED (pin 13) on and off.

Kudos to mhitza for documenting a workflow that actually works.

Assumptions#

If you don’t have the following equipment or experience, you may have a tough time completing this tutorial.

Equipment#

Arduino Uno R3
Debian-based Linux laptop that supports apt
USB-A to USB-B cable

Experience#

Proficiency in Linux CLI stuff like cd, git and ~
Proficiency in programming a high-level language like Python
Familiarity with builing binaries from source

Setup#

Create the repository#

mkdir ~/uno
touch ~/uno/blink.asm
touch ~/uno/Makefile

Install `avra`#

sudo apt install avra

avra (AVR Assembler) will be your assembler.

Install `avrdude`#

sudo apt install avrdude

avrdude (AVR Downloader Uploader) will be your tool for flashing your assembled program onto your Uno.

Create the assembly program#

Copy-paste the following code into each respective file.

`~/uno/blink.asm`#

; Modified from https://gist.github.com/mhitza/8a4608f4dfdec20d3879
; which is copyrighted by Marius Ghita and licensed under Apache-2.0.

.device ATmega328P
.equ PORTB = 0x05
.equ DDRB  = 0x04

.org 0x0000
    jmp setup

setup:
    sbi DDRB, 5
    clr r16
    jmp main

main:
    tst r16
    brne off
    rjmp on

on:
    sbi PORTB, 5
    ldi r16, 1
    rjmp delay

off:
    cbi PORTB, 5
    clr r16
    rjmp delay

delay:
    ldi  r18, 41
    ldi  r19, 150
    ldi  r20, 128

loop:
    dec  r20
    brne loop
    dec  r19
    brne loop
    dec  r18
    brne loop
    rjmp main

I’m an assembly n00b. I can only verify that this program is correct in the sense that I can see the LED on my Uno blinking on and off. Also when I left the Uno running overnight nothing blew up.

Resources#

Resources for figuring out how the blink.asm code works:

Programming Arduino Uno (ATmega328P) in assembly]
AVR Instruction Set Manual explains the instructions (e.g. jmp, sbi, etc.)
AVR Assembler Reference explains AVR Assembly syntax and keywords (e.g. .device, .equ, etc.)
m328Pdef.inc is the source for hardware definitions (e.g. PORTB, DDRB, etc.)
ATmega328P datasheet provides details on how exactly the instructions need to be constructed in order to work with the Uno’s MCU, the ATmega328P
ATmega328P
AVR Delay Loop Calculator is the source of the delay and loop code
AVR-Assembly-Tutorial is a comprehensive, approachable, and delightfully old school walkthrough of AVR Assembly programming

`~/uno/Makefile`#

# Modified from https://gist.github.com/mhitza/8a4608f4dfdec20d3879
# which is copyrighted by Marius Ghita and licensed under Apache-2.0.

%.hex: %.asm
     ~/uno/avra -fI $<
     if [ -f "*.eep.hex" ]; then rm *.eep.hex; fi
     if [ -f "*.obj" ]; then rm *.obj; fi
     if [ -f "*.cof" ]; then rm *.cof; fi

all: $(patsubst %.asm,%.hex,$(wildcard *.asm))

upload: ${program}.hex
     sudo avrdude -c arduino -p m328p -P /dev/ttyACM0 -b 115200 -U flash:w:$<

.PHONY: all upload

Assemble and flash the program#

cd ~/uno
# Assumes that the Uno is connected to your Linux computer via the USB-A
# to USB-B cable and is available at /dev/ttyACM0.
make program=blink upload

You should see the built-in LED labeled L blink on for about 1 second and then off for about 1 second.

Conclusion#

The insect overlords are surely watching your (assembly) career with great interest now.