ATmega169P datasheet(142/393 Pages) ATMEL

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8018N–AVR–08/09

ATmega169P

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.

17.5

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 17-4 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 17-4. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Output Compare (OC2A) from the Waveform

Generator if either of the COM2A1:0 bits are set. However, the OC2A pin direction (input or out-

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

ible on the pin. The port override function is independent of the Waveform Generation mode.

The design of the Output Compare pin logic allows initialization of the OC2A state before the

output is enabled. Note that some COM2A1:0 bit settings are reserved for certain modes of

operation. See ”8-bit Timer/Counter Register Description” on page 153.

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

FOCnx

clk

I/O

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