Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 125
16.8.1 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 (FOCnx) bit. Forcing compare match will not set the OCFnx Flag or reload/clear
the timer, but the OCnx pin will be updated as if a real compare match had occurred (the COMn1:0 bits settings
define whether the OCnx pin is set, cleared or toggled).
16.8.2 Compare Match Blocking by TCNTn Write
All CPU writes to the TCNTn Register will block any compare match that occurs in the next timer clock cycle,
even when the timer is stopped. This feature allows OCRnx to be initialized to the same value as TCNTn without
triggering an interrupt when the Timer/Counter clock is enabled.
16.8.3 Using the Output Compare Unit
Since writing TCNTn in any mode of operation will block all compare matches for one timer clock cycle, there
are risks involved when changing TCNTn when using any of the Output Compare channels, independent of
whether the Timer/Counter is running or not. If the value written to TCNTn equals the OCRnx value, the
compare match will be missed, resulting in incorrect waveform generation. Do not write the TCNTn equal to
TOP in PWM modes with variable TOP values. The compare match for the TOP will be ignored and the counter
will continue to 0xFFFF. Similarly, do not write the TCNTn value equal to BOTTOM when the counter is
downcounting.
The setup of the OCnx should be performed before setting the Data Direction Register for the port pin to output.
The easiest way of setting the OCnx value is to use the Force Output Compare (FOCnx) strobe bits in Normal
mode. The OCnx Register keeps its value even when changing between Waveform Generation modes.
Be aware that the COMnx1:0 bits are not double buffered together with the compare value. Changing the
COMnx1:0 bits will take effect immediately.