ATmega328P datasheet(147/448 Pages) ATMEL

147 / 448 page

147

8161D–AVR–10/09

ATmega48PA/88PA/168PA/328P

Figure 17-3. Output Compare Unit, Block Diagram

The OCR2x Register is 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 buffering synchronizes the update of the OCR2x Compare

occurrence of odd-length, non-symmetrical PWM pulses, thereby making the output glitch-free.

The OCR2x Register access may seem complex, but this is not case. When the double buffering

is enabled, the CPU has access to the OCR2x Buffer Register, and if double buffering is dis-

abled the CPU will access the OCR2x directly.

17.5.1

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 (FOC2x) bit. Forcing compare match will not set the

OCF2x Flag or reload/clear the timer, but the OC2x pin will be updated as if a real compare

match had occurred (the COM2x1:0 bits settings define whether the OC2x pin is set, cleared or

toggled).

17.5.2

Compare Match Blocking by TCNT2 Write

All CPU write operations to the TCNT2 Register will block any compare match that occurs in the

next timer clock cycle, even when the timer is stopped. This feature allows OCR2x to be initial-

ized to the same value as TCNT2 without triggering an interrupt when the Timer/Counter clock is

enabled.

17.5.3

Using the Output Compare Unit

Since writing TCNT2 in any mode of operation will block all compare matches for one timer clock

cycle, there are risks involved when changing TCNT2 when using the Output Compare channel,

independently of whether the Timer/Counter is running or not. If the value written to TCNT2

equals the OCR2x value, the compare match will be missed, resulting in incorrect waveform

generation. Similarly, do not write the TCNT2 value equal to BOTTOM when the counter is

downcounting.

OCFnx (Int.Req.)

= (8-bit Comparator )

OCRnx

OCnx

DATA BUS

TCNTn

WGMn1:0

Waveform Generator

top

FOCn

COMnX1:0

bottom

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