Figure 1. TOSC Switch
On the ATmega169 the TOSC1 and TOSC2 lines are shared with XTAL1 and XTAL2. The TOSC switch selects whether the 32 kHz crystal on the STK502, or the XT1/XT2 signals from STK500 should be connected to these pins on the device.
The Figure 2 shows a simplified block schematic on how this is implemented.
Figure 2. TOSC Block Schematic
