Datasheet

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Figure 22-3. Output Compare Unit, Block Diagram

OCFn (Int. Req.)

= (8-bit Comparator )

OCRn

OCxy

DATA BUS

TCNTn

WGMn1:0

Waveform Generator

TOP

FOCn

COMn1:0

BOTTOM

The OCR2 Register is double buffered when using any of the Pulse Width Modulation (PWM) modes. For

the normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The

double buffering synchronizes the update of the OCR2 Compare Register to either top or bottom of the

counting sequence. The synchronization prevents the occurrence of odd-length, non-symmetrical PWM

pulses, thereby making the output glitch-free.

The OCR2 Register access may seem complex, but this is not case. When the double buffering is

enabled, the CPU has access to the OCR2 Buffer Register, and if double buffering is disabled the CPU

will access the OCR2 directly.

22.5.1 Force Output Compare

In non-PWM Waveform Generation modes, the match output of the comparator can be forced by writing a

one to the Force Output Compare (FOC2) bit. Forcing Compare Match will not set the OCF2 Flag or

reload/clear the timer, but the OC2 pin will be updated as if a real Compare Match had occurred (the

COM21:0 bits settings define whether the OC2 pin is set, cleared or toggled).

22.5.2 Compare Match Blocking by TCNT2 Write

All CPU write operations to the TCNT2 Register will block any Compare Match that occurs in the next

timer clock cycle, even when the timer is stopped. This feature allows OCR2 to be initialized to the same

value as TCNT2 without triggering an interrupt when the Timer/Counter clock is enabled.

22.5.3 Using the Output Compare Unit

Since writing TCNT2 in any mode of operation will block all compare matches for one timer clock cycle,

there are risks involved when changing TCNT2 when using the Output Compare channel, independently

of whether the Timer/Counter is running or not. If the value written to TCNT2 equals the OCR2 value, the

AVR 8-Bit Microcontroller

8-bit Timer/Counter2 with PWM and Asynchronous Ope...

Datasheet Complete

40001974A-page 153