ATMEGA640 datasheet(144/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

Figure 17-5. 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 output) is still controlled by the Data Direc-

tion 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 Wave-

form Generation mode, but there are some exceptions. Refer to Table 17-3 on page 155, Table 17-4 on page 155

and Table 17-5 on page 155 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 “Register Description” on

page 154.

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

17.8.1

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 17-3 on page 155. For

fast PWM mode refer to Table 17-4 on page 155, and for phase correct and phase and frequency correct PWM

refer to Table 17-5 on page 155.

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.

17.9 Modes of Operation

The mode of operation, that is, 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 output should be set, cleared or toggle at a

compare match. See “Compare Match Output Unit” on page 143.

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

FOCnx

clk

I/O

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