ATMEGA640 datasheet(121/449 Pages) ATMEL

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121

ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

Depending of the mode of operation used, the counter is cleared, incremented, or dec-

remented 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 (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 OC0B. For more details about advanced counting

sequences and waveform generation, see “Modes of Operation” on page 124.

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.

Output Compare Unit

The 8-bit comparator continuously compares TCNT0 with the Output Compare Regis-

ters (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 Out-

put 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 Gener-

ator 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 sig-

nals are used by the Waveform Generator for handling the special cases of the extreme

values in some modes of operation (“Modes of Operation” on page 124).

Figure 39 shows a block diagram of the Output Compare unit.

Figure 39. 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

ATMEGA640 Datasheet(PDF) 121 Page - ATMEL Corporation

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