ATMEGA640 datasheet(118/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

Figure 16-3. Output Compare Unit, Block Diagram

The OCR0x Registers are double buffered when using any of the Pulse Width Modulation (PWM) modes. For the

normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The double buff-

ering synchronizes the update of the OCR0x Compare Registers to either top or bottom of the counting sequence.

The synchronization prevents the occurrence of odd-length, non-symmetrical PWM pulses, thereby making the

output glitch-free.

The OCR0x Register access may seem complex, but this is not case. When the double buffering is enabled, the

CPU has access to the OCR0x Buffer Register, and if double buffering is disabled the CPU will access the OCR0x

directly.

16.5.1

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 (FOC0x) bit. Forcing Compare Match will not set the OCF0x Flag or reload/clear the

timer, but the OC0x pin will be updated as if a real Compare Match had occurred (the COM0x1:0 bits settings

define whether the OC0x pin is set, cleared or toggled).

16.5.2

Compare Match Blocking by TCNT0 Write

All CPU write operations to the TCNT0 Register will block any Compare Match that occur in the next timer clock

cycle, even when the timer is stopped. This feature allows OCR0x to be initialized to the same value as TCNT0

without triggering an interrupt when the Timer/Counter clock is enabled.

16.5.3

Using the Output Compare Unit

Since writing TCNT0 in any mode of operation will block all Compare Matches for one timer clock cycle, there are

risks involved when changing TCNT0 when using the Output Compare Unit, independently of whether the

Timer/Counter is running or not. If the value written to TCNT0 equals the OCR0x value, the Compare Match will be

missed, resulting in incorrect waveform generation. Similarly, do not write the TCNT0 value equal to BOTTOM

when the counter is down-counting.

OCFnx (Int.Req.)

= (8-bit Comparator )

OCRnx

OCnx

DATA BUS

TCNTn

WGMn1:0

Waveform Generator

top

FOCn

COMnX1:0

bottom

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