ATtiny43U datasheet(82/182 Pages) ATMEL

82 / 182 page

8048B–AVR–03/09

ATtiny43U

Figure 12-3. Output Compare Unit, Block Diagram

The OCRnx 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 dou-

ble buffering is disabled. The double buffering synchronizes the update of the OCRnx 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 OCRnx Register access may seem complex, but this is not case. When the double buffering

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

abled the CPU will access the OCRnx directly.

12.5.1

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

OCFnx Flag or reload/clear the timer, but the OCnx pin will be updated as if a real Compare

Match had occurred (the COMnx1:0 bits settings define whether the OCnx pin is set, cleared or

toggled).

12.5.2

Compare Match Blocking by TCNTn Write

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

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

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

enabled.

12.5.3

Using the Output Compare Unit

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

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

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

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

OCFnx (Int.Req.)

= (8-bit Comparator )

OCRnx

OCnx

DATA BUS

TCNTn

WGMn1:0

Waveform Generator

top

FOCn

COMnx1:0

bottom

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