ATmega325P datasheet(150/364 Pages) ATMEL

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8023F–AVR–07/09

ATmega325P/3250P

• Bit 5:4 – COM2A1:0: Compare Match Output Mode A

These bits control the Output Compare pin (OC2A) behavior. If one or both of the COM2A1:0

bits are set, the OC2A output overrides the normal port functionality of the I/O pin it is connected

to. However, note that the Data Direction Register (DDR) bit corresponding to OC2A pin must be

set in order to enable the output driver.

When OC2A is connected to the pin, the function of the COM2A1:0 bits depends on the

WGM21:0 bit setting. Table 16-3 shows the COM2A1:0 bit functionality when the WGM21:0 bits

are set to a normal or CTC mode (non-PWM).

Table 16-4 shows the COM2A1:0 bit functionality when the WGM21:0 bits are set to fast PWM

mode.

Note:

1. A special case occurs when OCR2A equals TOP and COM2A1 is set. In this case, the com-

pare match is ignored, but the set or clear is done at TOP. See ”Fast PWM Mode” on page 142

for more details.

Table 16-5 shows the COM21:0 bit functionality when the WGM21:0 bits are set to phase cor-

rect PWM mode.

Note:

1. A special case occurs when OCR2A equals TOP and COM2A1 is set. In this case, the com-

pare match is ignored, but the set or clear is done at TOP. See ”Phase Correct PWM Mode” on

page 143 for more details.

Table 16-3.

Compare Output Mode, non-PWM Mode

COM2A1

COM2A0

Description

Normal port operation, OC2A disconnected.

Toggle OC2A on compare match.

Clear OC2A on compare match.

Set OC2A on compare match.

Table 16-4.

Compare Output Mode, Fast PWM Mode

(1)

COM2A1

COM2A0

Description

Normal port operation, OC2A disconnected.

Reserved

Clear OC2A on compare match, set OC2A at BOTTOM,

(non-inverting mode)

Set OC2A on compare match, clear OC2A at BOTTOM,

(inverting mode)

Table 16-5.

Compare Output Mode, Phase Correct PWM Mode

(1)

COM2A1

COM2A0

Description

Normal port operation, OC2A disconnected.

Reserved

Clear OC2A on compare match when up-counting. Set OC2A on

compare match when counting down.

Set OC2A on compare match when up-counting. Clear OC2A on

compare match when counting down.

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