Timer 1 uses a secondary xtal oscillator at 32,768kHz as clock source. Every 2 seconds the timer overflows. Output pulse from the geiger tube is connected to the external interupt pin. When there’s a high to low transition a count is registered. Every 2 seconds the number of counts is stored into a circular buffer and the thing spits the counts per minute value out of the serial port every 2 seconds. At every count it also generates 2ms worth of 4kHz cycles with the PWM module to drive a buzzer and make a “clack” sound and makes a LED flash for the same amount of time.