timer 11

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CS RD WR A1 A0 OPERATION

0 1 0 0 0 WRITE COUNTER 0



0 1 0 1 1 WRITE CONTROL WORD

0 0 1 0 0 READ COUNTER 0



0 0 1 1 1 NO OPERATION ( TRISTATED )

0 1 1 X X NO OPERATION ( TRISTATED )

1 X X X X 8254 NOT SELECTED

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Mode 0: INTERRUPT ON TERMINAL COUNT

The output becomes a logic 0 when the controlword is written

Remains low even after count value loaded incounter

Counter starts decrementing after falling edge ofclock

The OUT goes high upon reaching the terminalcount & remains high till reloading

OUT can be used as interrupt

Writing a count register , when previous countingis in process first byte when loaded stops theprevious count, second byte when loaded startsnew count

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Mode 1: One-shot mode.

monostable multivibrator

gate input is used as trigger input

output remains high till the count is

loaded

After application of trigger, output goeslow and remains low till count becomes zero

Another count loaded, when outputalready low it does not disturb countinguntil a new trigger is applied at the gate

New counting starts after new trigger

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Mode 2: RATE GENERATOR / DIVIDE BY NCOUNTER

When N is loaded as count after N pulses OUT goes low for only one clock cycle then, countN is reloaded OUT becomes high for N clockpulses

The number of clock pulses between the two lowpulses is equal to the count loaded

gate logic 0 no countingGate logic 1 normal counting

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Mode 3: SQUARE WAVE RATE

GENERATORWhen count N loaded is even outputremains HIGH for half the count and LOW for

the rest half of the countWhen count N loaded is odd outputremains HIGH for (N+1)/2 and low for (N-1)/2.

Repeated operation gives square wave

Generates a continuous square-wave with Gset to 1.

If count is even, 50% duty cycle otherwise

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Mode 4: Software triggered Strobe

After mode is set output goes high

When count is loaded counting downstartsOn reaching terminal count outputgoes low for only one clock cycle, andthen again output goes HIGHThe above said low pulse can be usedas a strobe for interfacing MP with

peripheralsWhen GATE is LOW counting isinhibited and count is latchedIf a new count is loaded while

counting, previous counting stops and

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Mode 5: Hardware triggered Strobe

This mode generates a strobe in response to therising edge at the triggerMode is used to generate a delayed strobe inresponse to an externally generated signalOnce mode is programmed and counter loaded,

OUT goes HIGHCounter starts counting after the rising edge of thetrigger (GATE)The OUTPUT goes LOW for one clock period, when

the terminal count is reachedOutput will not go LOW until the counter contentbecomes zero after the rising edge of any triggerGATE is used as trigger input

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Read Operations

There are three possible methods forreading the counters:

a simple read operation

the Counter Latch Command

the Read-Back Command

Simple read operation :

The Counter which is selected with the A1,A0 inputs, the CLK input of the selectedCounter must be inhibited by using either theGATE input or external logic.

Otherwise, the count may be in the processof changing when it is read, giving an

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ead-back control command:

The read-back control,word is used, when it isnecessary for thecontents of more thanone counter to be readat a same time.

Count : logic 0, selectone of theCounter to be latched

Status : logic 0, Statusmust be latched to beread status of a counterand is accessed by a readfrom that counter

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Status register:

shows the state of the outputpincheck the counter is in NULLstate (0) or not

how the counter is programmed

timer 11

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