Rainbow Electronics ATmega128L - Manual (Page 90)

90
ATmega128(L)
2467B–09/01
Figure 35. Output Compare Unit, Block Diagram
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.
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 (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).
Compare Match Blocking by
TCNT0 Write 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.
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 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.
OCFn (Int.Req.)
=
(8-bit Comparator )
OCRn
OCxy
DATA BUS
TCNTn
WGMn1:0
Waveform Generator
top
FOCn
COMn1:0
bottom