ATmega88 datasheet(139/340 Pages) ATMEL

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139

7530H–AVR–02/09

ATmega48/88/168 Automotive

Figure 15-4. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Output Compare (OC2x) from the Waveform

Generator if either of the COM2x1:0 bits are set. However, the OC2x 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 independent of the Waveform Generation mode.

The design of the Output Compare pin logic allows initialization of the OC2x state before the out-

put is enabled. Note that some COM2x1:0 bit settings are reserved for certain modes of

operation. See “8-bit Timer/Counter Register Description” on page 146.

15.5.1

Compare Output Mode and Waveform Generation

The Waveform Generator uses the COM2x1:0 bits differently in normal, CTC, and PWM modes.

For all modes, setting the COM2x1:0 = 0 tells the Waveform Generator that no action on the

OC2x Register is to be performed on the next compare match. For compare output actions in the

non-PWM modes refer to Table 15-4 on page 147. For fast PWM mode, refer to Table 15-5 on

page 147, and for phase correct PWM refer to Table 15-6 on page 148.

A change of the COM2x1:0 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

FOC2x strobe bits.

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

FOCnx

clk

I/O

ATmega88 Datasheet(PDF) 139 Page - ATMEL Corporation