ATmega165 datasheet(124/336 Pages) ATMEL

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124

ATmega165/V

2573G–AVR–07/09

Compare Match Output

Unit

The Compare Output mode (COM2A1:0) bits have two functions. The Waveform Gener-

ator uses the COM2A1:0 bits for defining the Output Compare (OC2A) state at the next

compare match. Also, the COM2A1:0 bits control the OC2A pin output source. Figure 56

shows a simplified schematic of the logic affected by the COM2A1:0 bit setting. The I/O

Registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of the

general I/O Port Control Registers (DDR and PORT) that are affected by the COM2A1:0

bits are shown. When referring to the OC2A state, the reference is for the internal OC2A

Figure 56. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Output Compare (OC2A) from the

Waveform Generator if either of the COM2A1:0 bits are set. However, the OC2A 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 OC2A pin (DDR_OC2A) must be set as

output before the OC2A value is visible on the pin. The port override function is indepen-

dent of the Waveform Generation mode.

The design of the Output Compare pin logic allows initialization of the OC2A state

before the output is enabled. Note that some COM2A1:0 bit settings are reserved for

certain modes of operation. See “8-bit Timer/Counter Register Description” on page

131.

Compare Output Mode and

Waveform Generation

The Waveform Generator uses the COM2A1:0 bits differently in normal, CTC, and PWM

modes. For all modes, setting the COM2A1:0 = 0 tells the Waveform Generator that no

action on the OC2A Register is to be performed on the next compare match. For com-

pare output actions in the non-PWM modes refer to Table 55 on page 132. For fast

PWM mode, refer to Table 56 on page 132, and for phase correct PWM refer to Table

57 on page 132.

A change of the COM2A1:0 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 FOC2A strobe bits.

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

FOCnx

clk

I/O

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