atmega32u4-16mu ATMEL Corporation, atmega32u4-16mu Datasheet - Page 163

• Manufacturer Part Number: atmega32u4-16mu
• Description: Atmega32u4 8-bit Avr Microcontroller With 32k Bytes Of Isp Flash And Usb Controller
• Manufacturer: ATMEL Corporation

16.4.1
16.4.2
16.4.3
7766A–AVR–03/08
Force Output Compare
Compare Match Blocking by TCNT4 Write
Using the Output Compare Unit
Figure 16-5. Effects of Unsynchronized OCR Latching
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).
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 initial-
ized to the same value as TCNT4 without triggering an interrupt when the Timer/Counter clock is
enabled.
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 Direc-
tion 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.
Unsynchronized WFnx Latch
Synchronized WFnx Latch
Compare Value changes
Compare Value changes
Glitch
Counter Value
Compare Value
Output Compare
Waveform OCWnx
Counter Value
Compare Value
Output Compare
Wafeform OCWnx
ATmega32U4
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