Datasheet
146
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
Figure 15-5. Effects of Unsynchronized OCR Latching
15.4.1 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 (FOC4x) bit. Forcing Compare Match will not set the OCF4x Flag or reload/clear the
timer, but the Waveform Output (OCW4x) will be updated as if a real Compare Match had occurred (the
COM4x1:0 bits settings define whether the Waveform Output (OCW4x) is set, cleared or toggled).
15.4.2 Compare Match Blocking by TCNT4 Write
All CPU write operations to the TCNT4 Register will block any Compare Match that occur in the next timer clock
cycle, even when the timer is stopped. This feature allows OCR4x to be initialized to the same value as TCNT4
without triggering an interrupt when the Timer/Counter clock is enabled.
15.4.3 Using the Output Compare Unit
Since writing TCNT4 in any mode of operation will block all Compare Matches for one timer clock cycle, there
are risks involved when changing TCNT4 when using the Output Compare Unit, independently of whether the
Timer/Counter is running or not. If the value written to TCNT4 equals the OCR4x value, the Compare Match will
be missed, resulting in incorrect waveform generation. Similarly, do not write the TCNT4 value equal to
BOTTOM when the counter is down-counting.
The setup of the Waveform Output (OCW4x) should be performed before setting the Data Direction Register for
the port pin to output. The easiest way of setting the OCW4x value is to use the Force Output Compare (FOC4x)
strobe bits in Normal mode. The OC4x keeps its value even when changing between Waveform Generation
modes.
Be aware that the COM4x1:0 bits are not double buffered together with the compare value. Changing the
COM4x1:0 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