Datasheet
119
7728G–AVR–06/10
ATtiny87/ATtiny167
12.7 Output Compare Units
The 16-bit comparator continuously compares TCNT1 with the Output Compare Register
(OCR1A/B). If TCNT equals OCR1A/B the comparator signals a match. A match will set the
Output Compare Flag (OCF1A/B) at the next timer clock cycle. If enabled (OCIE1A/B = 1), the
Output Compare Flag generates an Output Compare interrupt. The OCF1A/B flag is automati-
cally cleared when the interrupt is executed. Alternatively the OCF1A/B flag can be cleared by
software by writing a logical one to its I/O bit locations. The Waveform Generator uses the
match signal to generate an output according to operating mode set by the Waveform Gener-
ation mode (WGM13:0) bits and Compare Output mode (COM1A/B1:0) bits. The TOP and
BOTTOM signals are used by the Waveform Generator for handling the special cases of the
extreme values in some modes of operation (See “Modes of Operation” on page 122.)
A special feature of Output Compare unit A allows it to define the Timer/Counter TOP value
(i.e., counter resolution). In addition to the counter resolution, the TOP value defines the
period time for waveforms generated by the Waveform Generator.
Figure 12-4 shows a block diagram of the Output Compare unit. The elements of the block dia-
gram that are not directly a part of the Output Compare unit are gray shaded.
Figure 12-4. Output Compare Unit, Block Diagram
The OCR1A/B Register is double buffered when using any of the twelve Pulse Width Modula-
tion (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
OCR1A/B Compare Register to either TOP or BOTTOM of the counting sequence. The syn-
chronization prevents the occurrence of odd-length, non-symmetrical PWM pulses, thereby
making the output glitch-free.
OCFnx (Int.Req.)
=
(16-bit Comparator )
OCRnx Buffer (16-bit Register)
OCRnxH Buf.(8-bit)
OCnxV
OCnxU
OCnxW
TEMP (8-bit)
DATA BUS
(8-bit)
OCRnxL Buf.(8-bit)
TCNTn (16-bit Counter)
TCNTnH (8-bit) TCNTnL (8-bit)
COMnx1:0WGMn3:0
OCRnx (16-bit Register)
OCRnxH (8-bit) OCRnxL (8-bit)
Waveform Generator
TOP
BOTTOM
OCnxX