
142
ATmega64A [DATASHEET]
8160D–AVR–02/2013
Figure 18-3. Output Compare Unit, Block Diagram
The OCR2 Register is double buffered when using any of the Pulse Width Modulation (PWM) modes. For the nor-
mal 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.
18.5.1
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).
18.5.2
Compare Match Blocking by TCNT2 Write
All CPU write operations to the TCNT2 Register will block any Compare Match that occur 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 with-
out triggering an interrupt when the Timer/Counter clock is enabled.
18.5.3
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 Compare Match will be
missed, resulting in incorrect waveform generation. Similarly, do not write the TCNT2 value equal to BOTTOM
when the counter is downcounting.
OCFn (Int.Req.)
= (8-bit Comparator )
OCRn
OCn
DATA BUS
TCNTn
WGMn1:0
Waveform Generator
top
FOCn
COMn1:0
bottom