ATMEGA8_14 Datasheet, PDF (105/331 Pages) ATMEL Corporation – High-performance, Low-power Atmel

ATmega8(L)
Output Compare
Unit
The 8-bit comparator continuously compares TCNT2 with the Output Compare Register
(OCR2). Whenever TCNT2 equals OCR2, the comparator signals a match. A match will set the
Output Compare Flag (OCF2) at the next timer clock cycle. If enabled (OCIE2 = 1), the Output
Compare Flag generates an Output Compare interrupt. The OCF2 Flag is automatically cleared
when the interrupt is executed. Alternatively, the OCF2 Flag can be cleared by software by writ-
ing a logical one to its I/O bit location. The waveform generator uses the match signal to
generate an output according to operating mode set by the WGM21:0 bits and Compare Output
mode (COM21:0) bits. The max and bottom signals are used by the waveform generator for han-
dling the special cases of the extreme values in some modes of operation (see “Modes of
Operation” on page 108).
Figure 47 shows a block diagram of the Output Compare unit.
Figure 47. Output Compare Unit, Block Diagram
DATA BUS
OCRn
TCNTn
TOP
BOTTOM
FOCn
= (8-bit Comparator )
Waveform Generator
OCFn (Int. Req.)
OCxy
WGMn1:0 COMn1:0
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 buff-
ering is disabled. The double buffering synchronizes the update of the OCR2 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 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.
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