written first. When the High byte I/O location is written by the CPU, the TEMP Register will be
updated by the value written. Then when the Low byte (OCR1xL) is written to the lower eight
bits, the High byte will be copied into the upper 8-bits of either the OCR1x buffer or OCR1x Com-
pare Register in the same system clock cycle.
For more information of how to access the 16-bit registers refer to “Accessing 16-bit Registers”
on page 79.
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).
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.
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 wave-
form 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 Com-
pare (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.
86
ATmega8(L)
2486T–AVR–05/08
ATMEL [ ATMEL ]
