8-bit AVR Microcontrollers

Timer/Counter Prescaler

Figure 1. Prescaler for TC2

The clock source for TC2 is named clkT2S. It is by default connected to the main system I/O clock clkI/O. By writing a '1' to the Asynchronous TC2 bit in the Asynchronous Status Register (ASSR.AS2), TC2 is asynchronously clocked from the TOSC1 pin. This enables the use of TC2 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 TC2. The oscillator is optimized for use with a 32.768 kHz crystal.

For TC2, the possible prescaled selections are: clkT2S/8, clkT2S/32, clkT2S/64, clkT2S/128, clkT2S/256, and clkT2S/1024. Additionally, clkT2S, as well as 0 (stop), may be selected. The prescaler is reset by writing a '1' to the Prescaler Reset TC2 bit in the General TC2 Control Register (GTCCR.PSRASY). This allows the user to operate with a defined prescaler.