Project: Blink an LED with Timer Interrupts

Your first complete firmware.

Wire an LED and a resistor to one pin and ground. Configure the pin as an output, set a timer to fire every 500 ms, and toggle the pin inside its handler.

Compared with a delay loop, the CPU is free between blinks to do other work or sleep.

Check the result with a stopwatch: ten blinks should take about ten seconds.

volatile uint8_t flag = 0;
void timer_isr(void) { flag = 1; }   /* hypothetical, fires every 500 ms */

int main(void) {
    pin_output(3);
    timer_start_ms(500);
    for (;;) { if (flag) { flag = 0; pin_toggle(3); } }
}