AT90PWM3 datasheet(113/361 Pages) ATMEL

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4317J–AVR–08/10

AT90PWM2/3/2B/3B

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 out-

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

ble on the pin. The port override function is generally independent of the Waveform Generation

mode, but there are some exceptions. Refer to Table 15-2, Table 15-3 and Table 15-4 for

details.

The design of the Output Compare pin logic allows initialization of the OCnx state before the out-

put is enabled. Note that some COMnx1:0 bit settings are reserved for certain modes of

operation. See “16-bit Timer/Counter Register Description” on page 122.

The COMnx1:0 bits have no effect on the Input Capture unit.

15.7.1

Compare Output Mode and Waveform Generation

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 the next compare match. For compare output actions in the

non-PWM modes refer to Table 15-2 on page 123. For fast PWM mode refer to Table 15-3 on

page 123, and for phase correct and phase and frequency correct PWM refer to Table 15-4 on

page 123.

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.

15.8

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 (WGMn3:0) and Compare Output

mode (COMnx1:0) bits. The Compare Output mode bits do not affect the counting sequence,

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

FOCnx

clk

I/O

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