ATmega644PA datasheet(145/468 Pages) ATMEL

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8152G–AVR–11/09

ATmega164PA/324PA/644PA/1284P

16.5.3

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 Com-

pare (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.

16.6

Compare Match Output Unit

The Compare Output mode (COM2x1:0) bits have two functions. The Waveform Generator 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 16-4 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 Register, not the OC2x pin.

Figure 16-4. 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

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

FOCnx

clk

I/O

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