79
Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
Signal description (internal signals):
●
count
Increment or decrement TCNT0 by 1.
●
direction
Select between increment and decrement.
●
clear
Clear TCNT0 (set all bits to zero).
●
clkT
n
Timer/Counter clock, referred to as clkT0 in the following.
●
top
Signalize that TCNT0 has reached maximum value.
●
bottom
Signalize that TCNT0 has reached minimum value (zero).
Depending of the mode of operation used, the counter is cleared, incremented, or decremented at each timer clock (clkT0). 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
clkT0 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) and the WGM02 bit located in the Timer/Counter control register B (TCCR0B). There are close connections
between how the counter behaves (counts) and how waveforms are generated on the output compare outputs OC0A and
The Timer/Counter overflow flag (TOV0) is set according to the mode of operation selected by the WGM02:0 bits. TOV0 can be
used for generating a CPU interrupt.
12.4
Output Compare Unit
The 8-bit comparator continuously compares TCNT0 with the output compare registers (OCR0A and OCR0B). Whenever
TCNT0 equals OCR0A or OCR0B, the comparator signals a match. A match will set the output compare flag (OCF0A or
OCF0B) at the next timer clock cycle. If the corresponding interrupt is enabled, the output compare flag generates an output
compare interrupt. The output compare flag is automatically cleared when the interrupt is executed. Alternatively, the 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 WGM02:0 bits and compare output mode (COM0x1:0) bits. The max and
bottom signals are used by the waveform generator for handling the special cases of the extreme values in some modes of
Figure 12-3 shows a block diagram of the output compare unit.
Figure 12-3. Output Compare Unit, Block Diagram
OCFnx (Int. Req.)
= (8-bit Comparator)
OCRnx
Waveform Generator
TCNTn
OCnx
Top
Bottom
FOCn
WGMn1:0
COMnx1:0
DATA BUS