Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 158
17.10 Timer/Counter Prescaler
Figure 17-12. Prescaler for Timer/Counter2
The clock source for Timer/Counter2 is named clk
T2S
. clk
T2S
is by default connected to the main system I/O
clock clk
IO
. 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 C. 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. By setting the EXCLK bit in the ASSR a 32kHz external clock can be applied. See ”ASSR –
Asynchronous Status Register” on page 163 for details.
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 PSRASY bit in GTCCR
resets the prescaler. This allows the user to operate with a predictable prescaler.
10-BIT T/C PRESCALER
TIMER/COUNTER2 CLOCK SOURCE
clk
I/O
clk
T2S
TOSC1
AS2
CS20
CS21
CS22
clk
T2S
/8
clk
T2S
/64
clk
T2S
/128
clk
T2S
/1024
clk
T2S
/256
clk
T2S
/32
0
PSRASY
Clear
clk
T2