ISR definition causing boot loop?

Question

I am trying to compile and upload code to an Arduino Uno from the command-line, however I seem to be experiencing an odd issue.

When I compile this code:

#include <avr/interrupt.h>
#include <stdbool.h>
ISR(TIMER0_OVF_vect) {
}
int main() {
 while (true);
}

With these commands:

$ avr-gcc -mmcu=atmega328p -DF_CPU=16000000 -std=c17 -O3 -Wall -Wextra -c boot_loop.c -o boot_loop.o
$ avr-gcc boot_loop.o boot_loop.elf
$ avr-objcopy -j .text -j .data -O ihex boot_loop.elf boot_loop.hex
$ avrdude -p atmega328p -c arduino -P /dev/ttyUSB0 -U flash:w:boot_loop.hex

My Arduino ends up in a boot loop, flashing the LED 3 times every second or so (similarly to when you press the reset button or the device powers up).

I've tried it without an empty ISR and also with different interrupt vectors (e.g. TIMER0_COMPA_vect) but to no avail. Completely removing the ISR definition stops the boot loop.

• sa_leinad: just add a nop

Answer 1

Your problem is not with your code. It's with how you're linking your elf file.

You haven't specified the chip to link for.

$ avr-gcc -mmcu=atmega328p boot_loop.o -o boot_loop.elf

• Jack Wilsdon: That's the ticket! Can't believe I missed that, thanks!
• Majenko: You know why I spotted it so readily? Because I did the exact same thing once :)