ATMEGA8_14 Datasheet, PDF (116/331 Pages) ATMEL Corporation – High-performance, Low-power Atmel

ATmega8(L)
Timer/Counter
Register – TCNT2
Output Compare
Register – OCR2
Table 45 shows the COM21:0 bit functionality when the WGM21:0 bits are set to phase correct
PWM mode.
Table 45. Compare Output Mode, Phase Correct PWM Mode⁽¹⁾
COM21 COM20 Description
0
0
Normal port operation, OC2 disconnected
0
1
Reserved
1
0
Clear OC2 on Compare Match when up-counting. Set OC2 on Compare
Match when downcounting
1
1
Set OC2 on Compare Match when up-counting. Clear OC2 on Compare
Match when downcounting
Note:
1. A special case occurs when OCR2 equals TOP and COM21 is set. In this case, the Compare
Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on page
111 for more details
• 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
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_T2S/256 (From prescaler)
1
1
1
clk_T2S/1024 (From prescaler)
Bit
Read/Write
Initial Value
7
6
5
4
3
2
1
0
TCNT2[7:0]
TCNT2
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
0
0
0
0
0
0
0
0
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.
Bit
Read/Write
Initial Value
7
6
5
4
3
2
1
0
OCR2[7:0]
OCR2
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
0
0
0
0
0
0
0
0
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.
2486AA–AVR–02/2013
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