155
7530J–AVR–03/12
Atmel ATmega48/88/168 Automotive
15.10 Timer/Counter Prescaler
Figure 15-12. Prescaler for Timer/Counter2
The clock source for Timer/Counter2 is named clk
T2S. clkT2S 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. Apply-
ing an external clock source to TOSC1 is not recommended.
For Timer/Counter2, the possible prescaled selections are: clk
T2S/8, clkT2S/32, clkT2S/64,
clk
T2S/128, clkT2S/256, and clkT2S/1024. Additionally, clkT2S 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.
15.10.1
General Timer/Counter Control Register – GTCCR
Bit 1 – PSRASY: Prescaler Reset Timer/Counter2
When this bit is one, the Timer/Counter2 prescaler will be reset. This bit is normally cleared
immediately by hardware. If the bit is written when Timer/Counter2 is operating in asynchronous
mode, the bit will remain one until the prescaler has been reset. The bit will not be cleared by
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
Bit
7
6
5
4
3
2
1
0
TSM
–
PSRASY
PSRSYNC
GTCCR
Read/Write
R/W
R
R/W
Initial Value
0