Datasheet

90
7728G–AVR–06/10
ATtiny87/ATtiny167
Depending on the mode of operation used, the counter is cleared, incremented, or decre-
mented at each timer clock (clk
T
0). clk
T
0 can be generated from an external or internal clock
source, selected by the Clock Select bits (CS02:0). When no clock source is selected (CS02:0
= 0) the timer is stopped. However, the TCNT0 value can be accessed by the CPU, regardless
of whether clk
T
0 is present or not. A CPU write overrides (has priority over) all counter clear or
count operations.
The counting sequence is determined by the setting of the WGM01 and WGM00 bits located
in the Timer/Counter Control Register (TCCR0A). There are close connections between how
the counter behaves (counts) and how waveforms are generated on the Output Compare out-
put OC0A. For more details about advanced counting sequences and waveform generation,
see “Modes of Operation” on page 92.
The Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation selected
by the WGM01:0 bits. TOV0 can be used for generating a CPU interrupt.
10.5 Output Compare Unit
The 8-bit comparator continuously compares TCNT0 with the Output Compare Register
(OCR0A). Whenever TCNT0 equals OCR0A, the comparator signals a match. A match will set
the Output Compare Flag (OCF0A) at the next timer clock cycle. If enabled (OCIE0A = 1), the
Output Compare Flag generates an Output Compare interrupt. The OCF0A flag is automati-
cally cleared when the interrupt is executed. Alternatively, the OCF0A flag can be cleared by
software by writing 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 WGM01:0 bits and
Compare Output mode (COM0A1:0) bits. The max and bottom signals are used by the Wave-
form Generator for handling the special cases of the extreme values in some modes of
operation (“Modes of Operation” on page 92).
Figure 10-3 shows a block diagram of the Output Compare unit.
Figure 10-3. Output Compare Unit, Block Diagram
OCFnx (Int.Req.)
= (8-bit Comparator )
OCRnx
OCnx
DATA BUS
TCNTn
WGMn1:0
Waveform Generator
top
FOCn
COMnX1:0
bottom