Datasheet

21.7.1 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 OC1x pin will be updated as if a real Compare Match had occurred (the

COM1x1:0 bits settings define whether the OC1x pin is set, cleared or toggled).

21.7.2 Compare Match Blocking by TCNT1 Write

All CPU writes to the TCNT1 Register will block any Compare Match that occurs in the next timer clock

cycle, even when the timer is stopped. This feature allows OCR1x to be initialized to the same value as

TCNT1 without triggering an interrupt when the Timer/Counter clock is enabled.

21.7.3 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 any of the Output Compare channels,

independent 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. Do not

write the TCNT1 equal to TOP in PWM modes with variable TOP values. The Compare Match for the

TOP will be ignored and the counter will continue to 0xFFFF. Similarly, do not write the TCNT1 value

equal to BOTTOM when the counter is downcounting.

The setup of the OC1x should be performed before setting the Data Direction Register for the port pin to

output. The easiest way of setting the OC1x value is to use the Force Output Compare (FOC1x) strobe

bits in Normal mode. The OC1x Register 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.

21.8 Compare Match Output Unit

The Compare Output mode (COM1x1:0) bits have two functions. The waveform generator uses the

COM1x1:0 bits for defining the Output Compare (OC1x) state at the next Compare Match. Secondly the

COM1x1:0 bits control the OC1x pin output source. The figure below shows a simplified schematic of the

logic affected by the COM1x1:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the figure are

shown in bold. Only the parts of the general I/O Port Control Registers (DDR and PORT) that are affected

by the COM1x1:0 bits are shown. When referring to the OC1x state, the reference is for the internal OC1x

AVR 8-Bit Microcontroller

16-bit Timer/Counter1

Datasheet Complete

40001974A-page 124