The TOSC inputs can be configured as normal I/O pins, or as input pins for a low frequency oscillator that can clock the asynchronous timer. The STK503 has a 32kHz crystal that can be connected to these pins.
Normal port pins - the TOSC pins are connected to the PORTG connector (set the switch to the position indicated by the text PORTG
32kHz crystal (set the switch to the position nearest the 32kHz crystal)