
93
ATmega64A [DATASHEET]
8160D–AVR–02/2013
Figure 15-4. Compare Match Output Unit, Schematic
The general I/O port function is overridden by the Output Compare (OC0) from the Waveform Generator if either of
the COM01:0 bits are set. However, the OC0 pin direction (input or output) is still controlled by the Data Direction
Register (DDR) for the port pin. The Data Direction Register bit for the OC0 pin (DDR_OC0) must be set as output
before the OC0 value is visible on the pin. The port override function is independent of the Waveform Generation
mode.
The design of the Output Compare pin logic allows initialization of the OC0 state before the output is enabled. Note
15.6.1
Compare Output Mode and Waveform Generation
The Waveform Generator uses the COM01:0 bits differently in Normal, CTC, and PWM modes. For all modes, set-
ting the COM01:0 = 0 tells the Waveform Generator that no action on the OC0 Register is to be performed on the
A change of the COM01:0 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 FOC0 strobe bits.
15.7
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 Generation mode (WGM01:0) and Compare Output mode (COM01:0) bits. The
Compare Output mode bits do not affect the counting sequence, while the Waveform Generation mode bits do.
The COM01:0 bits control whether the PWM output generated should be inverted or not (inverted or non-inverted
PWM). For non-PWM modes the COM01:0 bits control whether the output should be set, cleared, or toggled at a
PORT
DDR
DQ
OCn
Pin
OCn
DQ
Waveform
Generator
COMn1
COMn0
0
1
D
ATA
B
U
S
FOCn
clk
I/O