ATMEGA128L-8MC View Datasheet(PDF) - Atmel Corporation
Part Name
Description
MFG CO.
'ATMEGA128L-8MC' PDF : 395 Pages View PDF
Figure 36. Output Compare Unit, Block Diagram
DATA BUS
ATmega128
OCRn
TCNTn
top
bottom
FOCn
= (8-bit Comparator )
OCFn (Int.Req.)
Waveform Generator
OCxy
WGMn1:0 COMn1:0
Force Output Compare
Compare Match Blocking by
TCNT0 Write
Using the Output Compare
Unit
The OCR0 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
OCR0 Compare Register to either top or bottom of the counting sequence. The synchro-
nization prevents the occurrence of odd-length, non-symmetrical PWM pulses, thereby
making the output glitch-free.
The OCR0 Register access may seem complex, but this is not case. When the double
buffering is enabled, the CPU has access to the OCR0 buffer Register, and if double
buffering is disabled the CPU will access the OCR0 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 (FOC0) bit. Forcing compare
match will not set the OCF0 flag or reload/clear the timer, but the OC0 pin will be
updated as if a real compare match had occurred (the COM01:0 bits settings define
whether the OC0 pin is set, cleared or toggled).
All CPU write operations to the TCNT0 Register will block any compare match that
occurs in the next timer clock cycle, even when the timer is stopped. This feature allows
OCR0 to be initialized 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 channel, independently of whether the Timer/Counter is running or not. If the
value written to TCNT0 equals the OCR0 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.
95
2467O–AVR–10/06
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