Datasheet

149
7810C–AVR–10/12
Atmel ATmega328P [Preliminary]
During asynchronous operation, the synchronization of the Interrupt Flags for the asynchronous
timer takes 3 processor cycles plus one timer cycle. The timer is therefore advanced by at least
one before the 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.
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 32.768 kHz crystal.
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