ATmega48PA Automotive Atmel Corporation, ATmega48PA Automotive Datasheet - Page 96
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ATmega48PA Automotive
Manufacturer Part Number
ATmega48PA Automotive
Description
Manufacturer
Atmel Corporation
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15.5.1
15.5.2
15.5.3
96
Atmel ATmega48PA/88PA/168PA [Preliminary]
Force Output Compare
Compare Match Blocking by TCNT0 Write
Using the Output Compare Unit
Figure 15-3
Figure 15-3. 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 buffering synchronizes the update of the OCR0x
Compare Registers to either top or bottom of the counting sequence. The synchronization pre-
vents 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 buffer-
ing is enabled, the CPU has access to the OCR0x Buffer Register, and if double buffering is
disabled the CPU will access the OCR0x directly.
In non-PWM waveform generation 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 com-
pare match had occurred (the COM0x1:0 bits settings define whether the OC0x pin is set,
cleared or toggled).
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 initial-
ized to the same value as TCNT0 without triggering an interrupt when the Timer/Counter clock
is enabled.
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 generation. Similarly, do not write the TCNT0 value equal to BOTTOM when the
counter is downcounting.
shows a block diagram of the Output Compare unit.
bottom
FOCn
top
OCRnx
Waveform Generator
WGMn1:0
=
(8-bit Comparator )
DATA BUS
COMnx1:0
TCNTn
OCFnx (Int.Req.)
OCnx
9223B–AVR–09/11
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