ATMEGA640 datasheet(185/449 Pages) ATMEL

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185

ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

OCR2x to be initialized to the same value as TCNT2 without triggering an interrupt when

the Timer/Counter clock is enabled.

Using the Output Compare

Unit

Since writing TCNT2 in any mode of operation will block all compare matches for one

timer clock cycle, there are risks involved when changing TCNT2 when using the Output

Compare channel, independently of whether the Timer/Counter is running or not. If the

value written to TCNT2 equals the OCR2x value, the compare match will be missed,

resulting in incorrect waveform generation. Similarly, do not write the TCNT2 value

equal to BOTTOM when the counter is downcounting.

The setup of the OC2x should be performed before setting the Data Direction Register

for the port pin to output. The easiest way of setting the OC2x value is to use the Force

Output Compare (FOC2x) strobe bit in Normal mode. The OC2x Register keeps its

value even when changing between Waveform Generation modes.

Be aware that the COM2x1:0 bits are not double buffered together with the compare

value. Changing the COM2x1:0 bits will take effect immediately.

Compare Match Output

Unit

The Compare Output mode (COM2x1:0) bits have two functions. The Waveform Gener-

ator uses the COM2x1:0 bits for defining the Output Compare (OC2x) state at the next

compare match. Also, the COM2x1:0 bits control the OC2x pin output source. Figure 72

shows a simplified schematic of the logic affected by the COM2x1:0 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 COM2x1:0

bits are shown. When referring to the OC2x state, the reference is for the internal OC2x

Figure 72. 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 output) is still controlled by the Data Direction Register (DDR) for the

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

FOCnx

clk

I/O

ATMEGA640 Datasheet(PDF) 185 Page - ATMEL Corporation

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