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 (FOC2A) bit. Forcing compare
match will not set the OCF2A flag or reload/clear the timer, but the OC2A pin will be
updated as if a real compare match had occurred (the COM2A1:0 bits settings define
whether the OC2A pin is set, cleared or toggled).
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
OCR2A to be initialized to the same value as TCNT2 without triggering an interrupt
when the Timer/Counter clock is enabled.
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 OCR2A 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.
The setup of the OC2A should be performed before setting the Data Direction Register
for the port pin to output. The easiest way of setting the OC2A value is to use the Force
Output Compare (FOC2A) strobe bit in Normal mode. The OC2A Register keeps its
value even when changing between Waveform Generation modes.
Be aware that the COM2A1:0 bits are not double buffered together with the compare
value. Changing the COM2A1:0 bits will take effect immediately.
144
AT90CAN128
4250E–CAN–12/04
ATMEL [ ATMEL ]
