Rainbow-electronics ATmega8515L Bedienungsanleitung PDF herunterladen (Seite 78)

ATmega8515(L)

2512A–AVR–04/02

inactive when no clock sourceisselected.Theoutput from the clock select logic is

referred to as thetimerclock (clk

The double bufferedOutput CompareRegister(OCR0) iscomparedwith the

Timer/Counter value at all times.The resultof the compare can beusedbythe Wave-

form Generator to generateaPWM or variable frequency output on the Output Compare

Pin (OC0). See “Output Compare Unit” on page 79. fordetails.The comparematch

event will also set the Compare Flag (OCF0)which can beused to generate an output

compareinterrupt request.

Definitions Manyregister andbit references in thisdocument are written in generalform. A lower

case“n”replaces theTimer/Counter number, in thiscase0.However, when using the

accessing Timer/Counter0 counter value andso on.

The definitions in Table43 arealsoused extensively throughout the document.

Timer/Counter Clock

Sources

TheTimer/Countercan be clockedbyan internal or an externalclock source. The clock

sourceisselectedbythe clock select logicwhich iscontrolledbythe Clock Select

(CS02:0)bitslocated in theTimer/CounterControl Register(TCCR0). Fordetails on

clock sources andprescaler, see “Timer/Counter0and Timer/Counter1Prescalers”on

page 92.

Counter Unit Themainpartof the8-bit Timer/Counter is the programmable bi-directionalcounter unit.

Figure 34 shows a block diagram of the counter and itssurroundings.

Figure 34. CounterUnit Block Diagram

Signaldescription (internalsignals):

count Increment ordecrement TCNT0 by 1.

direction Select between increment anddecrement.

clear Clear TCNT0 (set all bits to zero).

Table 43. Definitions

BOTTOM The counterreaches the BOTTOM when it becomes 0x00.

MAX The counterreaches itsMAXimum when it becomes 0xFF (decimal 255).

TOPThe counterreaches theTOP when it becomes equal to the highest

value in the count sequence. TheTOPvalue can beassigned to bethe

fixed value 0xFF (MAX) or thevalue stored in the OCR0 Register.The

assignment isdependent on themodeof operation.

DATA B U S

TCNTn Control Logic

count

TOVn

(Int.Req.)

Clock Select

top

Edge

Detector

( From Prescaler )

clk

bottom

direction

clear

1 2 ... 73 74 75 76 77 78 79 80 81 82 83 ... 222 223

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