ATMEGA8_08 Datasheet, PDF (123/308 Pages) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

Timer/Counter
Prescaler
ATmega8(L)
Figure 56. Prescaler for Timer/Counter2
clk_I/O
TOSC1
clk_T2S
Clear
10-BIT T/C PRESCALER
AS2
PSR2
0
CS20
CS21
CS22
Special Function IO
Register – SFIOR
TIMER/COUNTER2 CLOCK SOURCE
clk_T2
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 disconnected 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 32.768 kHz crystal. Apply-
ing an external clock source to TOSC1 is not recommended.
For Timer/Counter2, the possible prescaled selections are: clk_T2S/8, 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 prescaler. This allows the user to operate with a pre-
dictable prescaler.
Bit
7
6
5
4
3
2
1
0
–
–
–
–
ACME
PUD
PSR2 PSR10 SFIOR
Read/Write
R
R
R
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
• Bit 1 – PSR2: Prescaler Reset Timer/Counter2
When this bit is written to one, the Timer/Counter2 prescaler will be reset. The bit will be cleared
by hardware after the operation is performed. Writing a zero to this bit will have no effect. This bit
will always be read as zero if Timer/Counter2 is clocked by the internal CPU clock. If this bit is
written when Timer/Counter2 is operating in Asynchronous mode, the bit will remain one until
the prescaler has been reset.
2486T–AVR–05/08
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