Rainbow Electronics ATmega32L - Manual (Page 115)

115
ATmega32(L)
2503C–AVR–10/02
The OCR2 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
OCR2 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 OCR2 Register access may seem complex, but this is not case. When the double
buffering is enabled, the CPU has access to the OCR2 Buffer Register, and if double
buffering is disabled the CPU will access the OCR2 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 (FOC2) bit. Forcing compare
match will not set the OCF2 flag or reload/clear the timer, but the OC2 pin will be
updated as if a real compare match had occurred (the COM21:0 bits settings define
whether the OC2 pin is set, cleared or toggled).
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
OCR2 to be initialized to the same value as TCNT2 without triggering an interrupt when
the Timer/Counter clock is enabled.
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 OCR2 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 OC2 should be performed before setting the Data Direction Register for
the port pin to output. The easiest way of setting the OC2 value is to use the Force Out-
put Compare (FOC2) strobe bit in Normal mode. The OC2 Register keeps its value even
when changing between Waveform Generation modes.
Be aware that the COM21:0 bits are not double buffered together with the compare
value. Changing the COM21:0 bits will take effect immediately.
Compare Match Output
Unit The Compare Output mode (COM21:0) bits have two functions. The Waveform Genera-
tor uses the COM21:0 bits for defining the Output Compare (OC2) state at the next
compare match. Also, the COM21:0 bits control the OC2 pin output source. Figure 56
shows a simplified schematic of the logic affected by the COM21:0 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 Registers (DDR and PORT) that are affected by the COM21:0
bits are shown. When referring to the OC2 state, the reference is for the internal OC2
Register, not the OC2 pin.