ATMEGA8 datasheet(68/303 Pages) ATMEL

68 / 303 page

ATmega8(L)

2486M–AVR–12/03

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 (TCCR0). For details on

clock sources and prescaler, see “Timer/Counter0 and Timer/Counter1 Prescalers” on

page 72.

Counter Unit

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

Signal description (internal signals):

count

Increment TCNT0 by 1.

clk

Timer/Counter clock, referred to as clk

T0 in the following.

max

Signalize that TCNT0 has reached maximum value.

The counter is incremented 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.

Operation

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 software. A new counter value can be

written anytime.

DATA BUS

TCNTn

Control Logic

count

TOVn

(Int. Req.)

Clock Select

max

Edge

Detector

( From Prescaler )

clk

ATMEGA8 Datasheet(PDF) 68 Page - ATMEL Corporation

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