Atmel ATmega328P Xplained Mini MEGA328P-XMINI MEGA328P-XMINI Data Sheet
Product codes
MEGA328P-XMINI
145
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
Figure 18-3.
Output Compare Unit, Block Diagram
The OCR2x Register is 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
buffering synchronizes the update of the OCR2x Compare Register 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.
Normal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The double
buffering synchronizes the update of the OCR2x Compare Register 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 OCR2x Register access may seem complex, but this is not case. When the double buffering is enabled, the
CPU has access to the OCR2x Buffer Register, and if double buffering is disabled the CPU will access the
OCR2x directly.
CPU has access to the OCR2x Buffer Register, and if double buffering is disabled the CPU will access the
OCR2x directly.
18.5.1 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 (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 compare match had occurred (the COM2x1:0 bits settings
define whether the OC2x pin is set, cleared or toggled).
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 compare match had occurred (the COM2x1:0 bits settings
define whether the OC2x pin is set, cleared or toggled).
18.5.2 Compare Match Blocking by TCNT2 Write
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.
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.
18.5.3 Using the Output Compare Unit
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.
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 Compare (FOC2x) strobe bit in Normal
mode. The OC2x Register keeps its value even when changing between Waveform Generation modes.
The easiest way of setting the OC2x value is to use the Force Output Compare (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.
COM2x1:0 bits will take effect immediately.
OCFnx (Int.Req.)
=
(8-bit Comparator )
OCRnx
OCnx
DATA BUS
TCNTn
WGMn1:0
Waveform Generator
top
FOCn
COMnX1:0
bottom