ATtiny261-ESSZ datasheet(98/227 Pages) ATMEL

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7753C–AVR–07/09

ATtiny261/ATtiny461/ATtiny861

Figure 16-5. Effects of Unsynchronized OCR Latching

16.4.1

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 (FOC1x) bit. Forcing Compare Match will not set the

OCF1x Flag or reload/clear the timer, but the Waveform Output (OCW1x) will be updated as if a

real Compare Match had occurred (the COM1x1:0 bits settings define whether the Waveform

Output (OCW1x) is set, cleared or toggled).

16.4.2

Compare Match Blocking by TCNT1 Write

All CPU write operations to the TCNT1 Register will block any Compare Match that occur in the

next timer clock cycle, even when the timer is stopped. This feature allows OCR1x to be initial-

ized to the same value as TCNT1 without triggering an interrupt when the Timer/Counter clock is

enabled.

16.4.3

Using the Output Compare Unit

Since writing TCNT1 in any mode of operation will block all Compare Matches for one timer

clock cycle, there are risks involved when changing TCNT1 when using the Output Compare

Unit, independently of whether the Timer/Counter is running or not. If the value written to TCNT1

equals the OCR1x value, the Compare Match will be missed, resulting in incorrect waveform

generation. Similarly, do not write the TCNT1 value equal to BOTTOM when the counter is

down-counting.

The setup of the Waveform Output (OCW1x) should be performed before setting the Data Direc-

tion Register for the port pin to output. The easiest way of setting the OCW1x value is to use the

Force Output Compare (FOC1x) strobe bits in Normal mode. The OC1x keeps its value even

when changing between Waveform Generation modes.

Be aware that the COM1x1:0 bits are not double buffered together with the compare value.

Changing the COM1x1:0 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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