Datasheet

117
ATmega8(L)
2486M–AVR–12/03
• Bit 2:0 – CS22:0: Clock Select
The three clock select bits select the clock source to be used by the Timer/Counter, see
Table 46.
Timer/Counter Register –
TCNT2
The Timer/Counter Register gives direct access, both for read and write operations, to
the Timer/Counter unit 8-bit counter. Writing to the TCNT2 Register blocks (removes)
the Compare Match on the following timer clock. Modifying the counter (TCNT2) while
the counter is running, introduces a risk of missing a Compare Match between TCNT2
and the OCR2 Register.
Output Compare Register –
OCR2
The Output Compare Register contains an 8-bit value that is continuously compared
with the counter value (TCNT2). A match can be used to generate an Output Compare
interrupt, or to generate a waveform output on the OC2 pin.
Asynchronous Operation
of the Timer/Counter
Asynchronous Status
Register – ASSR
• Bit 3 – AS2: Asynchronous Timer/Counter2
When AS2 is written to zero, Timer/Counter 2 is clocked from the I/O clock, clk
I/O
. When
AS2 is written to one, Timer/Counter 2 is clocked from a crystal Oscillator connected to
the Timer Oscillator 1 (TOSC1) pin. When the value of AS2 is changed, the contents of
TCNT2, OCR2, and TCCR2 might be corrupted.
Table 46. Clock Select Bit Description
CS22 CS21 CS20 Description
0 0 0 No clock source (Timer/Counter stopped).
0 0 1 clk
T2S
/(No prescaling)
0 1 0 clk
T2S
/8 (From prescaler)
0 1 1 clk
T2S
/32 (From prescaler)
1 0 0 clk
T2S
/64 (From prescaler)
1 0 1 clk
T2S
/128 (From prescaler)
1 1 0 clk
T
2
S
/256 (From prescaler)
1 1 1 clk
T
2
S
/1024 (From prescaler)
Bit 7 6 5 4 3 2 1 0
TCNT2[7:0] TCNT2
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Bit 7 6 5 4 3 2 1 0
OCR2[7:0] OCR2
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Bit 7 6 5 4 3 2 1 0
– – – – AS2 TCN2UB OCR2UB TCR2UB ASSR
Read/Write R R R R R/W R R R
Initial Value 0 0 0 0 0 0 0 0