ATmega16U4 datasheet(145/408 Pages) ATMEL

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145

7766D–AVR–01/09

ATmega16U4/ATmega32U4

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 initial-

ized 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 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.

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

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