Programmable Interval Timer8253

By,Daggubati.Sirisha

• The Intel 8253 and 8254 are Programmable Interval Timers (PITs), which perform timing and counting functions. They were primarily designed for the Intel 8080/8085-processors, but later used in x86-systems.

• There are several applications where we need to generate clock signals of different frequency, Delays of different duration and strobe signals after certain interval

• These Signals can be generated by using different techniques

• Software,Hardware and combined • Programmable interval Timer is used in

Software based approachClock

using 2 delay routine we generateclock

Generating delay

Strobe signal

Limitation

Some application e need to generate clock and delay at a time , which is not possible using this method, although it is Flexible to generate everything by program ,due to above Drawback

Hardware based approach

Hardware based approach

555 Timer

Drawback:If we need to change the frequency of clock need to change the capacitor andResistor values ie hardware need to be changed, so we go for combined approach

trigger

Combined approach-Programmable Interval Timer-8253

Max clk freq 2mhz

8253

• Control word register used to control mode of operation

• three16 bit registers called Counters used to generate delay, clock signal and strobe signal

Addressing and CONTROL REGISTER

Interrupt on TC-Mode-0

Count down can be inhibited

Gate is inhibiting the clock to decrement the count

Mode:1(Monostable multivibrator)

When trigger is applied it will start count, here gate line acts like trigger input

Rate generator

If u write n ,it keeps gnerating strobe after tc reaches 0-called rate generatorGate input used to load reset,the count value reset and start from n value

Square wave generator

Output will remain high or low for this durationDifference of period is only one clock cycle between odd and even number

Softare triggered strobe

It generate the strobe at the output, here write operation acts as the triggerOperation can be delayed by gating

Hardare triggered strobe

This gate line is used as the trigger input, count down is initiated by gate