Datasheet

152
8018P–AVR–08/10
ATmega169P
17.9 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 32.768 kHz crystal. If
applying an external clock on TOSC1, the EXCLK bit in ASSR must be set.
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 GTCCR resets the prescaler. This allows the user to operate with a pre-
dictable 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
PSR2
Clear
clk
T2