ATmega325P datasheet(139/364 Pages) ATMEL

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139

8023F–AVR–07/09

ATmega325P/3250P

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 unit,

independently of whether the Timer/Counter is running or not. If the value written to TCNT2

equals the OCR2A 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

counting down.

The setup of the OC2A should be performed before setting the Data Direction Register for the

port pin to output. The easiest way of setting the OC2A value is to use the Force Output Com-

pare (FOC2A) strobe bit in Normal mode. The OC2A Register keeps its value even when

changing between Waveform Generation modes.

Be aware that the COM2A1:0 bits are not double buffered together with the compare value.

Changing the COM2A1:0 bits will take effect immediately.

16.6

Compare Match Output Unit

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

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 16-4 shows a sim-

plified 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 Regis-

ters (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 Register, not the OC2A pin.

Figure 16-4. Compare Match Output Unit, Schematic

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

FOCnx

clk

I/O

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