AT90PWM2 datasheet(115/365 Pages) ATMEL

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115

AT90PWM2/3

4317B–AVR–02/05

Figure 46. 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 Direction 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 Waveform Generation mode, but there are some exceptions. Refer

to Table 47, Table 48 and Table 49 for details.

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

the output 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 125.

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

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 com-

pare output actions in the non-PWM modes refer to Table 47 on page 125. For fast

PWM mode refer to Table 48 on page 125, and for phase correct and phase and fre-

quency correct PWM refer to Table 49 on page 126.

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.

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, while the Waveform Generation 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 out-

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

FOCnx

clk

I/O

AT90PWM2 Datasheet(PDF) 115 Page - ATMEL Corporation

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