ATmega8(L)
Timer/Counter Prescaler Figure 56. Prescaler for Timer/Counter2
clkI/O
clkT2S
10-BIT T/C PRESCALER
Clear
TOSC1
AS2
PSR2
0
CS20
CS21
CS22
TIMER/COUNTER2 CLOCK SOURCE
clkT2
The clock source for Timer/Counter2 is named clkT2S. clkT2S is by default connected to
the main system I/O clock clkI/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. Applying an external clock source to TOSC1 is not
recommended.
For Timer/Counter2, the possible prescaled selections are: clkT2S/8, clkT2S/32, clkT2S/64,
clkT2S/128, clkT2S/256, and clkT2S/1024. Additionally, clkT2S 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 predictable prescaler.
Special Function IO Register –
SFIOR
Bit
7
–
6
–
5
–
4
–
3
ACME
R/W
0
2
1
PSR2
R/W
0
0
PSR10
R/W
0
PUD
R/W
0
SFIOR
Read/Write
Initial Value
R
0
R
0
R
0
R
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.
121
2486M–AVR–12/03
ATMEL [ ATMEL ]
