Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P

 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 126

16.9 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 16-5 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 16-5. 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 output) is still controlled by the

Data Direction Register (DDR) for the port pin. The Data Direction Register bit for the OCnx pin (DDR_OCnx)

must be set as output before the OCnx value is visible on the pin. The port override function is generally

independent of the Waveform Generation mode, but there are some exceptions. Refer to Table 16-2, Table 16-

3 and Table 16-4 for details.

The design of the Output Compare pin logic allows initialization of the OCnx state before the output is enabled.

Note that some COMnx1:0 bit settings are reserved for certain modes of operation. See “Register description”

on page 136.

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

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

DATA BU S

FOCnx

clk

I/O