ATmega16 datasheet(122/315 Pages) ATMEL

122 / 315 page

122

ATmega16(L)

2466E–AVR–10/02

Note:

1. The CTC2 and PWM2 bit definition names are now obsolete. Use the WGM21:0 def-

initions. However, the functionality and location of these bits are compatible with

previous versions of the timer.

• Bit 5:4 – COM21:0: Compare Match Output Mode

These bits control the Output Compare pin (OC2) behavior. If one or both of the

COM21:0 bits are set, the OC2 output overrides the normal port functionality of the I/O

pin it is connected to. However, note that the Data Direction Register (DDR) bit corre-

sponding to OC2 pin must be set in order to enable the output driver.

When OC2 is connected to the pin, the function of the COM21:0 bits depends on the

WGM21:0 bit setting. Table 51 shows the COM21:0 bit functionality when the WGM21:0

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

Table 52 shows the COM21:0 bit functionality when the WGM21:0 bits are set to fast

PWM mode.

Note:

1. A special case occurs when OCR2 equals TOP and COM21 is set. In this case, the

compare match is ignored, but the set or clear is done at TOP. See “Fast PWM Mode”

on page 117 for more details.

Table 53 shows the COM21:0 bit functionality when the WGM21:0 bits are set to phase

correct PWM mode

Table 50. Waveform Generation Mode Bit Description(1)

Mode

WGM21

(CTC2)

WGM20

(PWM2)

Timer/Counter Mode of

Operation

TOP

Update of

OCR2

TOV2 Flag

Set on

Normal

0xFF

Immediate

MAX

PWM, Phase Correct

0xFF

TOP

BOTTOM

CTC

OCR2

Immediate

MAX

Fast PWM

0xFF

TOP

MAX

Table 51. Compare Output Mode, non-PWM Mode

COM21

COM20

Description

Normal port operation, OC2 disconnected.

Toggle OC2 on compare match

Clear OC2 on compare match

Set OC2 on compare match

Table 52. Compare Output Mode, Fast PWM Mode(1)

COM21

COM20

Description

Normal port operation, OC2 disconnected.

Reserved

Clear OC2 on compare match, set OC2 at TOP

Set OC2 on compare match, clear OC2 at TOP

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