ATmega324P datasheet(132/377 Pages) ATMEL

132 / 377 page

132

7674F–AVR–09/09

ATmega164P/324P/644P

The COMnA1:0 and COMnB1:0 control the Output Compare pins (OCnA and OCnB respec-

tively) behavior. If one or both of the COMnA1:0 bits are written to one, the OCnA output

overrides the normal port functionality of the I/O pin it is connected to. If one or both of the

COMnB1:0 bit are written to one, the OCnB output overrides the normal port functionality of the

I/O pin it is connected to. However, note that the Data Direction Register (DDR) bit correspond-

ing to the OCnA or OCnB pin must be set in order to enable the output driver.

When the OCnA or OCnB is connected to the pin, the function of the COMnx1:0 bits is depen-

dent of the WGMn3:0 bits setting. Table 14-2 on page 132 shows the COMnx1:0 bit functionality

when the WGMn3:0 bits are set to a Normal or a CTC mode (non-PWM).

Table 14-3 on page 132 shows the COMnx1:0 bit functionality when the WGMn3:0 bits are set to

the fast PWM mode.

Note:

1. A special case occurs when OCRnA/OCRnB equals TOP and COMnA1/COMnB1 is set. In

this case the compare match is ignored, but the set or clear is done at BOTTOM. See “Fast

PWM Mode” on page 123. for more details.

Table 14-2.

Compare Output Mode, non-PWM

COMnA1/COMnB1

COMnA0/COMnB0

Description

Normal port operation, OCnA/OCnB disconnected.

Toggle OCnA/OCnB on Compare Match.

Clear OCnA/OCnB on Compare Match (Set output to

low level).

Set OCnA/OCnB on Compare Match (Set output to

high level).

Table 14-3.

Compare Output Mode, Fast PWM

(1)

COMnA1/COMnB1

COMnA0/COMnB0

Description

Normal port operation, OCnA/OCnB disconnected.

WGMn3:0 = 14 or 15: Toggle OC1A on Compare

Match, OC1B disconnected (normal port operation).

For all other WGM1 settings, normal port operation,

OC1A/OC1B disconnected.

Clear OCnA/OCnB on Compare Match, set

OCnA/OCnB at BOTTOM (non-inverting mode)

Set OCnA/OCnB on Compare Match, clear

OCnA/OCnB at BOTTOM (inverting mode)

ATmega324P Datasheet(PDF) 132 Page - ATMEL Corporation

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