155
2552K–AVR–04/11
ATmega329/3290/649/6490
Bit 2:0 – CS22:0: Clock Select
The three Clock Select bits select the clock source to be used by the Timer/Counter, see
Table17.11.2
TCNT2 – Timer/Counter Register
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 OCR2A Register.
17.11.3
OCR2A – Output Compare Register A
The Output Compare Register A 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 OC2A pin.
17.11.4
ASSR – Asynchronous Status Register
Bit 4 – EXCLK: Enable External Clock Input
When EXCLK is written to one, and asynchronous clock is selected, the external clock input buf-
fer is enabled and an external clock can be input on Timer Oscillator 1 (TOSC1) pin instead of a
32kHz crystal. Writing to EXCLK should be done before asynchronous operation is selected.
Note that the crystal Oscillator will only run when this bit is zero.
Table 17-6.
Clock Select Bit Description
CS22
CS21
CS20
Description
000
No clock source (Timer/Counter stopped).
001
clkT2S/(No prescaling)
010
clkT2S/8 (From prescaler)
011
clkT2S/32 (From prescaler)
100
clkT2S/64 (From prescaler)
101
clkT2S/128 (From prescaler)
110
clkT2S/256 (From prescaler)
111
clkT2S/1024 (From prescaler)
Bit
765
4321
0
TCNT2[7:0]
TCNT2
Read/Write
R/W
Initial Value
000
0000
0
Bit
765
4321
0
OCR2A[7:0]
OCR2A
Read/Write
R/W
Initial Value
000
0000
0
Bit
765
4
3
2
1
0
–
EXCLK
AS2
TCN2UB
OCR2UB
TCR2UB
ASSR
Read/Write
R
R/W
R
Initial Value
0