AT90USB646 datasheet(102/434 Pages) ATMEL

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102

7593A–AVR–02/06

AT90USB64/128

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 Gen-

erator to generate a PWM or variable frequency output on the Output Compare pins (OC0A and

OC0B). See “Output Compare Unit” on page 103. for details. The Compare Match event will also

set the Compare Flag (OCF0A or OCF0B) which can be used to generate an Output Compare

interrupt request.

13.1.2

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 Com-

pare 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, i.e., TCNT0 for accessing

Timer/Counter0 counter value and so on.

The definitions in Table 13-1 are also used extensively throughout the document.

13.2 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 Register (TCCR0B). For details on clock sources and pres-

caler, see “Timer/Counter0, Timer/Counter1, and Timer/Counter3 Prescalers” on page 99.

13.3 Counter Unit

The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure

13-2 shows a block diagram of the counter and its surroundings.

Figure 13-2. Counter Unit Block Diagram

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).

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.

DATA BUS

TCNTn

Control Logic

count

TOVn

(Int.Req.)

Clock Select

top

Edge

Detector

( From Prescaler )

clk

bottom

direction

clear

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