lezyne-rear-light-firmware/main.c

#define F_CPU 5000000UL // 5 MHz
#include <avr/io.h>
#include <util/delay.h>

int main(void) {
    // Configure PA3 as output (Pin 7 on ATTINY202-SSN)
    VPORTA.DIR |= 8;
    
    while (1) {
        VPORTA.OUT ^= 8;  // toggle PA3
        _delay_ms(500);
    }
}