lezyne-rear-light-firmware/main.c

#include <avr/io.h>
#include <util/delay.h>

#define PA3_SET_MASK 0x08

int main(void)
{
    VPORTA.DIR = 0x0;

    // Configure PA3 as output (Pin 7 on ATTINY202-SSN)
    VPORTA.DIR |= PA3_SET_MASK;

    while (1)
    {
        VPORTA.OUT ^= PA3_SET_MASK; // toggle PA3
        _delay_ms(500);
    }
}