
117
4378C–AVR–09/08
AT90PWM1
I/O pin it is connected to. However, note that the Data Direction Register (DDR) bit correspond-
ing to the OCnA or OCnB pin must be set in order to enable the output driver.
When the OCnA or OCnB is connected to the pin, the function of the COMnx1:0 bits is depen-
dent of the WGMn3:0 bits setting.
Table 29 shows the COMnx1:0 bit functionality when the
WGMn3:0 bits are set to a Normal or a CTC mode (non-PWM).
Table 30 shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to the fast PWM
mode.
Note:
1. A special case occurs when OCRnA/OCRnB equals TOP and COMnA1/COMnB1 is set. In
this case the compare match is ignored, but the set or clear is done at TOP.
See “Fast PWMTable 31 shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to the phase cor-
rect or the phase and frequency correct, PWM mode.
Table 29. Compare Output Mode, non-PWM
COMnA1/COMnB1
COMnA0/COMnB0
Description
0
Normal port operation, OCnA/OCnB disconnected.
0
1
Toggle OCnA/OCnB on Compare Match.
10
Clear OCnA/OCnB on Compare Match (Set output to
low level).
11
Set OCnA/OCnB on Compare Match (Set output to
high level).
Table 30. Compare Output Mode, Fast PWM
COMnA1/COMnB1
COMnA0/COMnB0
Description
0
Normal port operation, OCnA/OCnB disconnected.
01
WGMn3:0 = 14 or 15: Toggle OC1A on Compare
Match, OC1B disconnected (normal port operation).
For all other WGM1 settings, normal port operation,
OC1A/OC1B disconnected.
10
Clear OCnA/OCnB on Compare Match, set
OCnA/OCnB at TOP
11
Set OCnA/OCnB on Compare Match, clear
OCnA/OCnB at TOP
Table 31. Compare Output Mode, Phase Correct and Phase and Frequency Correct PWM
(1) COMnA1/COMnB1
COMnA0/COMnB0
Description
0
Normal port operation, OCnA/OCnB disconnected.
01
WGMn3:0 = 8, 9 10 or 11: Toggle OCnA on Compare
Match, OCnB disconnected (normal port operation).
For all other WGM1 settings, normal port operation,
OC1A/OC1B disconnected.
10
Clear OCnA/OCnB on Compare Match when up-
counting. Set OCnA/OCnB on Compare Match when
downcounting.
11
Set OCnA/OCnB on Compare Match when up-
counting. Clear OCnA/OCnB on Compare Match
when downcounting.