ATmega88PA Automotive Atmel Corporation, ATmega88PA Automotive Datasheet - Page 147

• Manufacturer Part Number: ATmega88PA Automotive
• Manufacturer: Atmel Corporation

18.5.1
18.5.2
18.5.3
18.6
9223B–AVR–09/11
Compare Match Output Unit
Force Output Compare
Compare Match Blocking by TCNT2 Write
Using the Output Compare Unit
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 com-
pare match had occurred (the COM2x1:0 bits settings define whether the OC2x pin is set,
cleared or toggled).
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
initialized to the same value as TCNT2 without triggering an interrupt when the Timer/Counter
clock is enabled.
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.
The setup of the OC2x should be performed before setting the Data Direction Register for the
port pin to output. The easiest way of setting the OC2x value is to use the Force Output Com-
pare (FOC2x) strobe bit in Normal mode. The OC2x Register keeps its value even when
changing between Waveform Generation modes.
Be aware that the COM2x1:0 bits are not double buffered together with the compare value.
Changing the COM2x1:0 bits will take effect immediately.
The Compare Output mode (COM2x1:0) bits have two functions. The Waveform Generator
uses the COM2x1:0 bits for defining the Output Compare (OC2x) state at the next compare
match. Also, the COM2x1:0 bits control the OC2x pin output source.
plified schematic of the logic affected by the COM2x1:0 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 Reg-
isters (DDR and PORT) that are affected by the COM2x1:0 bits are shown. When referring to
the OC2x state, the reference is for the internal OC2x Register, not the OC2x pin.
Atmel ATmega48PA/88PA/168PA [Preliminary]
Figure 18-4
shows a sim-
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