ATMEGA8L ATMEL [ATMEL Corporation], ATMEGA8L Datasheet - Page 117

• Manufacturer Part Number: ATMEGA8L
• Description: 8-bit AVR with 8K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Timer/Counter Register –
TCNT2
Output Compare Register –
OCR2
Asynchronous Operation
of the Timer/Counter
Asynchronous Status
Register – ASSR
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.
Table 46. Clock Select Bit Description
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.
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.
• Bit 3 – AS2: Asynchronous Timer/Counter2
When AS2 is written to zero, Timer/Counter 2 is clocked from the I/O clock, clk
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.
Bit
Read/Write
Initial Value
Bit
Read/Write
Initial Value
Bit
Read/Write
Initial Value
CS22
0
0
0
0
1
1
1
1
CS21
R/W
R/W
R
7
–
0
0
0
1
1
0
0
1
1
7
0
7
0
R
6
–
0
R/W
R/W
6
0
6
0
CS20
0
1
0
1
0
1
0
1
R
5
0
–
R/W
R/W
5
0
5
0
Description
No clock source (Timer/Counter stopped).
clk
clk
clk
clk
clk
clk
clk
T2S
T2S
T2S
T2S
T2S
T
T
2
2
R
S
S
4
–
0
/256 (From prescaler)
/1024 (From prescaler)
/(No prescaling)
/8 (From prescaler)
/32 (From prescaler)
/64 (From prescaler)
/128 (From prescaler)
R/W
R/W
4
0
4
0
TCNT2[7:0]
OCR2[7:0]
AS2
R/W
3
0
R/W
R/W
3
0
3
0
TCN2UB
R
2
0
R/W
R/W
2
0
2
0
OCR2UB
R
1
0
ATmega8(L)
R/W
R/W
1
0
1
0
TCR2UB
R
R/W
R/W
0
0
0
0
0
0
I/O
TCNT2
ASSR
. When
OCR2
117