91
ATmega64A [DATASHEET]
8160D–AVR–02/2013
direction
Selects between increment and decrement.
clear
Clear TCNT0 (set all bits to zero).
clk
T0
Timer/Counter clock.
top
Signalizes that TCNT0 has reached maximum value.
bottom
Signalizes that TCNT0 has reached minimum value (zero).
Depending on the mode of operation used, the counter is cleared, incremented, or decremented at each timer
clock (clk
T0). clkT0 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
T0 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 (TCCR0). There are close connections between how the counter behaves (counts) and how
waveforms are generated on the Output Compare output OC0. For more details about advanced counting
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.
15.5
Output Compare Unit
The 8-bit comparator continuously compares TCNT0 with the Output Compare Register (OCR0). Whenever
TCNT0 equals OCR0, the comparator signals a match. A match will set the Output Compare Flag (OCF0) at the
next timer clock cycle. If enabled (OCIE0 = 1), the Output Compare Flag generates an Output Compare interrupt.
The OCF0 flag is automatically cleared when the interrupt is executed. Alternatively, the OCF0 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 gener-
ate an output according to operating mode set by the WGM01:0 bits and Compare Output mode (COM01:0) bits.
The max and bottom signals are used by the Waveform Generator for handling the special cases of the extreme
Output Compare unit.
Figure 15-3. Output Compare Unit, Block Diagram
The OCR0 Register is double buffered when using any of the Pulse Width Modulation (PWM) modes. For the nor-
mal and Clear Timer on Compare (CTC) modes of operation, the double buffering is disabled. The double buffering
OCFn (Int.Req.)
= (8-bit Comparator )
OCRn
OCxy
DATA BUS
TCNTn
WGMn1:0
Waveform Generator
top
FOCn
COMn1:0
bottom