83
ATmega16M1/32M1/64M1 [DATASHEET]
8209E–AVR–11/2012
Figure 14-3. 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.
14.5.1
Force output compare
In non-PWM waveform generating 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).
14.5.2
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.
14.5.3
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 generating. Similarly, do not write the TCNT0 value equal to BOTTOM
when the counter is down counting.
The setup of the OC0x should be performed before setting the Data Direction Register for the port pin to output.
The easiest way of setting the OC0x value is to use the Force Output Compare (FOC0x) strobe bits in Normal
mode. The OC0x Registers keep their values even when changing between Waveform Generating modes.
OCFnx (Int.req.)
= (8-bit comparator )
OCRnx
OCnx
DATA BUS
TCNTn
WGMn1:0
Waveform generator
top
FOCn
COMnx1:0
bottom