8-Bit AVR® Real Time Counter (RTC)

Last modified by Microchip on 2023/11/09 09:02

Real Time Counter (RTC)

AVR® devices have a Timer/Counter Type 2 (TC2) general-purpose, dual-channel, 8-bit timer/counter module. This timer/counter allows clocking from an external 32 kHz watch crystal, independent of the I/O. This allows the timer to run as a relatively accurate Real Time Counter (RTC).

Timer Counter Type 2 diagram

Clock Source

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 an RTC.

When AS2 is set, pins TOSC1 and TOSC2 are disconnected from the I/O Port. 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.

Additional Information