108
ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
Figure 15-5. Compare Match Output unit, schematic.
The general I/O port function is overridden by the Output Compare (OCnx) from the Waveform Generator if either
of the COMnx1:0 bits are set. However, the OCnx pin direction (input or output) is still controlled by the Data Direc-
tion Register (DDR) for the port pin. The Data Direction Register bit for the OCnx pin (DDR_OCnx) must be set as
output before the OCnx value is visible on the pin. The port override function is generally independent of the Wave-
The design of the Output Compare pin logic allows initialization of the OCnx state before the output is enabled.
The COMnx1:0 bits have no effect on the Input Capture unit.
15.8.1
Compare Output Mode and Waveform Generating
The Waveform Generator uses the COMnx1:0 bits differently in normal, CTC, and PWM modes. For all modes,
setting the COMnx1:0 = 0 tells the Waveform Generator that no action on the OCnx Register is to be performed on
A change of the COMnx1:0 bits state will have effect at the first compare match after the bits are written. For non-
PWM modes, the action can be forced to have immediate effect by using the FOCnx strobe bits.
15.9
Modes of operation
The mode of operation, that is, the behavior of the Timer/Counter and the Output Compare pins, is defined by the
combination of the Waveform Generating mode (WGMn3:0) and Compare Output mode (COMnx1:0) bits. The
Compare Output mode bits do not affect the counting sequence, while the Waveform Generating mode bits do.
The COMnx1:0 bits control whether the PWM output generated should be inverted or not (inverted or non-inverted
PWM). For non-PWM modes the COMnx1:0 bits control whether the output should be set, cleared or toggle at a
PORT
DDR
DQ
OCnx
pin
OCnx
DQ
Waveform
generator
COMnx1
COMnx0
0
1
D
ATA
B
U
S
FOCnx
clk
I/O