45
ATmega161(L)
1228B–09/01
cleared according to the settings of the COMn1/COMn0 bits in the Timer/Counter Con-
trol Registers TCCR0 or TCCR2.
If CTC0/CTC2 is set and PWM mode is selected, the Timer/Counters will wrap and start
counting from $00 after reaching $FF. The PB0(OC0/PWM0) or PB1(OC2/PWM2) pin
will be set or cleared according to the settings of COMn1/COMn0 on a Timer/Counter
overflow or when the counter value matches the contents of the Output Compare Regis-
ter. Refer to Table 12 for details.
Note:
1. n = 0 or 2
Note that in PWM mode, the value to be written to the Output Compare Register is first
transferred to a temporary location and then latched into the OCR when the
Timer/Counter reaches $FF. This prevents the occurrence of odd-length PWM pulses
(glitches) in the event of an unsynchronized OCR0 or OCR2 write. See Figure 34 and
Figure 35 for examples.
Figure 34.
Effects of Unsynchronized OCR Latching in Up/Down Mode
Table 12.
Compare Mode Select in PWM Mode
CTCn
COMn1
COMn0
Effect on Compare Pin
Frequency
0
0
0
Not connected
0
0
1
Not connected
0
1
0
Cleared on compare match, up-counting. Set on
compare match, down-counting (non-inverted
PWM).
f
TCK0/2
/510
0
1
1
Cleared on compare match, down-counting. Set
on compare match, up-counting (inverted PWM).
f
TCK0/2
/510
1
0
0
Not connected
1
0
1
Not connected
1
1
0
Cleared on compare match, set on overflow
f
TCK0/2
/256
1
1
1
Set on compare match, cleared on overflow
f
TCK0/2
/256
PWM Output OCn
PWM Output OCn
Unsynchronized OCn Latch
Synchronized OCn Latch
Compare Value changes
Counter Value
Compare Value
Glitch
Counter Value
Compare Value
Compare Value changes