ATmega64A datasheet(150/392 Pages) ATMEL

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8160C–AVR–07/09

ATmega64A

form generation. Similarly, do not write the TCNT2 value equal to BOTTOM when the counter is

downcounting.

The setup of the OC2 should be performed before setting the Data Direction Register for the port

pin to output. The easiest way of setting the OC2 value is to use the Force Output Compare

(FOC2) strobe bits in Normal mode. The OC2 Register keeps its value even when changing

between Waveform Generation modes.

Be aware that the COM21:0 bits are not double buffered together with the compare value.

Changing the COM21:0 bits will take effect immediately.

17.6

Compare Match Output Unit

The Compare Output mode (COM21:0) bits have two functions. The Waveform Generator uses

the COM21:0 bits for defining the Output Compare (OC2) state at the next Compare Match.

Also, the COM21:0 bits control the OC2 pin output source. Figure 17-4 shows a simplified sche-

matic of the logic affected by the COM21:0 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 COM21:0 bits are shown. When referring to the OC2 state, the

reference is for the internal OC2 Register, not the OC2 pin. If a System Reset occur, the OC2

Figure 17-4. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Output Compare (OC2) from the Waveform

Generator if either of the COM21:0 bits are set. However, the OC2 pin direction (input or output)

is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction Regis-

PORT

DDR

OCn

Waveform

Generator

COMn1

COMn0

FOCn

clk

I/O

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