Datasheet

8021G–AVR–03/11

ATmega329P/3290P

Figure 14-4. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Output Compare (OC0A) from the Waveform

Generator if either of the COM0A1:0 bits are set. However, the OC0A pin direction (input or out-

put) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction

ible 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 OC0A state before the

output is enabled. Note that some COM0A1:0 bit settings are reserved for certain modes of

operation. See ”Register Description” on page 155.

14.6.1 Compare Output Mode and Waveform Generation

The Waveform Generator uses the COM0A1:0 bits differently in Normal, CTC, and PWM

modes. For all modes, setting the COM0A1:0 = 0 tells the Waveform Generator that no action on

the OC0A Register is to be performed on the next compare match. For compare output actions

in the non-PWM modes refer to Table 17-3 on page 156. For fast PWM mode, refer to Table 17-

4 on page 156, and for phase correct PWM refer to Table 17-5 on page 156.

A change of the COM0A1:0 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

FOC0A strobe bits.

14.7 Modes of Operation

The mode of operation, i.e., 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 (COM0A1:0) bits. The Compare Output mode bits do not affect the counting sequence,

while the Waveform Generation mode bits do. The COM0A1:0 bits control whether the PWM

output generated should be inverted or not (inverted or non-inverted PWM). For non-PWM

modes the COM0A1:0 bits control whether the output should be set, cleared, or toggled at a

compare match (See ”Compare Match Output Unit” on page 98).

For detailed timing information refer to Figure 1, Figure 2, Figure 3 and Figure 4 in ”Timer/Coun-

ter Timing Diagrams” on page 104.

PORT

DDR

OCn

OCnx

Waveform

Generator

COMnx1

COMnx0

DATA BU S

FOCn

clk

I/O