Datasheet

111
ATmega8A [DATASHEET]
8159E–AVR–02/2013
processor can read the timer value causing the setting of the Interrupt Flag. The Output Compare Pin is
changed on the timer clock and is not synchronized to the processor clock.
18.10 Timer/Counter Prescaler
Figure 18-12. 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
I/O
. 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 dis-
connected from Port B. 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. Apply-
ing an external clock source to TOSC1 is not recommended.
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 SFIOR resets the pres-
caler. 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
PSR2
Clear
clk
T2