ATmega128 datasheet(95/386 Pages) ATMEL

95 / 386 page

2467S–AVR–07/09

ATmega128

Signal description (internal signals):

count

Increment or decrement TCNT0 by 1.

direction

Selects between increment and decrement.

clear

Clear TCNT0 (set all bits to zero).

clk

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 sequences and waveform generation, see

“Modes of Operation” on page 98.

The Timer/Counter overflow (TOV0) flag is set according to the mode of operation selected by

the WGM01:0 bits. TOV0 can be used for generating a CPU interrupt.

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 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 WGM01:0 bits and compare output

mode (COM01:0) 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 (“Modes of Operation”

on page 98). Figure 36 shows a block diagram of the output compare unit.

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