Datasheet

Figure 21-3. Output Compare Unit, Block Diagram
OCFn (Int. Re q.)
= (8-bit Compa rator )
OCRn
OCn
DATA BUS
TCNTn
WGMn1:0
Waveform Generator
TOP
FOCn
COMn1:0
BOTTOM
The OCR0 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 OCR0 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 OCR0 Register access may seem complex, but this is not case. When the double buffering is
enabled, the CPU has access to the OCR0 Buffer Register, and if double buffering is disabled the CPU
will access the OCR0 directly.
21.5.1. 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 (FOC0) bit. Forcing Compare Match will not set the OCF0 Flag or
reload/clear the timer, but the OC0 pin will be updated as if a real Compare Match had occurred (the
COM01:0 bits settings define whether the OC0 pin is set, cleared or toggled).
21.5.2. Compare Match Blocking by TCNT0 Write
All CPU write operations to the TCNT0 Register will block any Compare Match that occurs in the next
timer clock cycle, even when the timer is stopped. This feature allows OCR0 to be initialized to the same
value as TCNT0 without triggering an interrupt when the Timer/Counter clock is enabled.
21.5.3. Using the Output Compare Unit
Since writing TCNT0 in any mode of operation will block all compare matches for one timer clock cycle,
there are risks involved when changing TCNT0 when using the Output Compare channel, independently
of whether the Timer/Counter is running or not. If the value written to TCNT0 equals the OCR0 value, the
Compare Match will be missed, resulting in incorrect waveform generation. Similarly, do not write the
TCNT0 value equal to BOTTOM when the counter is downcounting.
The setup of the OC0 should be performed before setting the Data Direction Register for the port pin to
output. The easiest way of setting the OC0 value is to use the Force Output Compare (FOC0) strobe bit
in Normal mode. The OC0 Register keeps its value even when changing between waveform generation
modes.
Be aware that the COM01:0 bits are not double buffered together with the compare value. Changing the
COM01:0 bits will take effect immediately.
Atmel ATmega32A [DATASHEET]
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