ATMEGA640 datasheet(116/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

16.2.1

Registers

The Timer/Counter (TCNT0) and Output Compare Registers (OCR0A and OCR0B) are 8-bit registers. Interrupt

request (abbreviated to Int.Req. in the figure) signals are all visible in the Timer Interrupt Flag Register (TIFR0). All

interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK0). TIFR0 and TIMSK0 are not

shown in the figure.

The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the T0 pin. The

Clock Select logic block controls which clock source and edge the Timer/Counter uses to increment (or decrement)

its value. The Timer/Counter is inactive when no clock source is selected. The output from the Clock Select logic is

referred to as the timer clock (clkT0).

The double buffered Output Compare Registers (OCR0A and OCR0B) are compared with the Timer/Counter value

at all times. The result of the compare can be used by the Waveform Generator to generate a PWM or variable fre-

quency output on the Output Compare pins (OC0A and OC0B). See “Output Compare Unit” on page 117. for

details. The Compare Match event will also set the Compare Flag (OCF0A or OCF0B) which can be used to gener-

ate an Output Compare interrupt request.

16.2.2

Definitions

Many register and bit references in this section are written in general form. A lower case “n” replaces the

Timer/Counter number, in this case 0. A lower case “x” replaces the Output Compare Unit, in this case Compare

Unit A or Compare Unit B. However, when using the register or bit defines in a program, the precise form must be

used, that is, TCNT0 for accessing Timer/Counter0 counter value and so on.

The definitions in Table 16-1 are also used extensively throughout the document.

16.3 Timer/Counter Clock Sources

The Timer/Counter can be clocked by an internal or an external clock source. The clock source is selected by the

Clock Select logic which is controlled by the Clock Select (CS02:0) bits located in the Timer/Counter Control Reg-

ister (TCCR0B). For details on clock sources and prescaler, see “Timer/Counter 0, 1, 3, 4, and 5 Prescaler” on

page 164.

16.4 Counter Unit

The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure 16-2 on page 117

shows a block diagram of the counter and its surroundings.

Table 16-1.

Definitions

BOTTOM

The counter reaches the BOTTOM when it becomes 0x00.

MAX

The counter reaches its MAXimum when it becomes 0xFF (decimal 255).

TOP

The counter reaches the TOP when it becomes equal to the highest value in the count

sequence. The TOP value can be assigned to be the fixed value 0xFF (MAX) or the value stored

in the OCR0A Register. The assignment is dependent on the mode of operation.

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