22.10 Timer/Counter Prescaler

The clock source for Timer/Counter2 is named clk_T2S. clk_T2S is by default connected to the main system clock clk_I/O. By setting the AS2 bit in ASSR, Timer/Counter2 is asynchronously clocked from the TOSC1 pin. This enables use of Timer/Counter2 as a Real Time Counter (RTC). When AS2 is set, pins TOSC1 and TOSC2 are disconnected from Port B. A crystal can then be connected between the TOSC1 and TOSC2 pins to serve as an independent clock source for Timer/Counter2. The Oscillator is optimized for use with a 32.768kHz crystal. Applying an external clock source to TOSC1 is not recommended.

For Timer/Counter2, the possible prescaled selections are: clk_T2S/8, clk_T2S/32, clk_T2S/64, clk_T2S/128, clk_T2S/256, and clk_T2S/1024. Additionally, clk_T2S as well as 0 (stop) may be selected. Setting the PSR2 bit in SFIOR resets the prescaler. This allows the user to operate with a predictable prescaler.