Rainbow Electronics ATtiny861_V - Manual (Page 97)

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2588B–AVR–11/06
ATtiny261/461/861
Figure 16-5. Effects of Unsynchronized OCR Latching
16.4.1 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 (FOC1x) bit. Forcing Compare Match will not set the
OCF1x Flag or reload/clear the timer, but the Waveform Output (OCW1x) will be updated as if a
real Compare Match had occurred (the COM1x1:0 bits settings define whether the Waveform
Output (OCW1x) is set, cleared or toggled).
16.4.2 Compare Match Blocking by TCNT1 Write
All CPU write operations to the TCNT1 Register will block any Compare Match that occur in the
next timer clock cycle, even when the timer is stopped. This feature allows OCR1x to be initial-
ized to the same value as TCNT1 without triggering an interrupt when the Timer/Counter clock is
enabled.
16.4.3 Using the Output Compare Unit
Since writing TCNT1 in any mode of operation will block all Compare Matches for one timer
clock cycle, there are risks involved when changing TCNT1 when using the Output Compare
Unit, independently of whether the Timer/Counter is running or not. If the value written to TCNT1
equals the OCR1x value, the Compare Match will be missed, resulting in incorrect waveform
generation. Similarly, do not write the TCNT1 value equal to BOTTOM when the counter is
down-counting.
The setup of the Waveform Output (OCW1x) should be performed before setting the Data Direc-
tion Register for the port pin to output. The easiest way of setting the OCW1x value is to use the
Force Output Compare (FOC1x) strobe bits in Normal mode. The OC1x keeps its value even
when changing between Waveform Generation modes.
Be aware that the COM1x1:0 bits are not double buffered together with the compare value.
Changing the COM1x1:0 bits will take effect immediately.
Output Compare
Waveform OCWnx
Output Compare
Wafeform OCWnx
Unsynchronized WFnx Latch
Synchronized WFnx Latch
Counter Value
Compare Value
Counter Value
Compare Value
Compare Value changes
Glitch
Compare Value changes