ATmega324P datasheet(121/377 Pages) ATMEL

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7674F–AVR–09/09

ATmega164P/324P/644P

14.8

Compare Match Output Unit

The Compare Output mode (COMnx1:0) bits have two functions. The Waveform Generator uses

the COMnx1:0 bits for defining the Output Compare (OCnx) state at the next compare match.

Secondly the COMnx1:0 bits control the OCnx pin output source. Figure 14-5 shows a simplified

schematic of the logic affected by the COMnx1:0 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 COMnx1:0 bits are shown. When referring to the

OCnx state, the reference is for the internal OCnx Register, not the OCnx pin. If a system reset

occur, the OCnx Register is reset to “0”.

Figure 14-5. Compare Match Output Unit, Schematic

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

Generator if either of the COMnx1:0 bits are set. However, the OCnx pin direction (input or out-

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

ble on the pin. The port override function is generally independent of the Waveform Generation

mode, but there are some exceptions. Refer to Table 14-2, Table 14-3 and Table 14-4 for

details.

The design of the Output Compare pin logic allows initialization of the OCnx state before the out-

put is enabled. Note that some COMnx1:0 bit settings are reserved for certain modes of

operation. See “Register Description” on page 131.

The COMnx1:0 bits have no effect on the Input Capture unit.

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

FOCnx

clk

I/O

ATmega324P Datasheet(PDF) 121 Page - ATMEL Corporation