ATMEGA8_08 Datasheet, PDF(70/308 Page) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

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ATMEGA8_08 Datasheet, PDF (70/308 Pages) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

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Timer/Counter

Clock Sources

Counter Unit

Operation

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 (TCCR0). For details on clock sources and prescaler, see

âTimer/Counter0 and Timer/Counter1 Prescalersâ on page 74.

The main part of the 8-bit Timer/Counter is the programmable counter unit. Figure 27 shows a

block diagram of the counter and its surroundings.

Figure 27. Counter Unit Block Diagram

DATA BUS

TOVn

(Int. Req.)

Clock Select

TCNTn

count

Control Logic

clkTn

Edge

Detector

Tn

( From Prescaler )

max

Signal description (internal signals):

count

Increment TCNT0 by 1.

clkTn

max

Timer/Counter clock, referred to as clkT0 in the following.

Signalize that TCNT0 has reached maximum value.

The counter is incremented 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 direction is always up (incrementing), and no counter clear is performed. The

counter simply overruns when it passes its maximum 8-bit value (MAX = 0xFF) and then restarts

from the bottom (0x00). In normal operation the Timer/Counter Overflow Flag (TOV0) will be set

in the same timer clock cycle as the TCNT0 becomes zero. The TOV0 Flag in this case behaves

like a ninth bit, except that it is only set, not cleared. However, combined with the timer overflow

interrupt that automatically clears the TOV0 Flag, the timer resolution can be increased by soft-

ware. A new counter value can be written anytime.

70 ATmega8(L)

2486TâAVRâ05/08

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