Datasheet

190
ATmega640/1280/1281/2560/2561
2549K–AVR–01/07
Timer/Counter Prescaler Figure 77. 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 asyn-
chronously 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 32.768 kHz crystal. By setting the EXCLK bit in the ASSR, a 32 KHz external
clock can be applied. See “ASSR – Asynchronous Status Register” on page 196 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