8254 microprocessor

Programmable Interval Timer

T.Shakeel, Lovely Professional University.

IC 8254 is an programmable interval Timer, whose function is similar to software program(counter and Time delay).

Applications are• Used to generate time delay• Clock pulses• Square wave generators and etc


8254 consists of 3- 16 bit counters, which works individually.

This counters operates in 6 different modes 8254 is a 24 pin dual in line package IC It requires +5v power supply. To operate as an counter, 16 bit count value is

to be stored in register and required command should be given to start the decrement operation

When the count is decrement to zero, and signal is given to MPU( consider as a interrupt).

We can also read the contents of counter while its value is decrementing.


8254 consists of 3 individual counter register(0,1,2).

Data bus (D7-D0) R/W control logic Control register Counter is having 2 input pins and one

output pin.Input pins: CLOCK(CLK) and GATEOutput Pins: OUT• Data pins are connected to the data bus pins of

MPUT.Shakeel, Lovely Professional University.

In 8254 , 5 control logic pins are present • RD(AL): • WR(AL):

RD and WR control lines are connected with IOM, IOR, MEMR and MEMW control lines of MPU.

• CS(AL): This pins is used to Disable and enable the 8254 chip and this is done by decoding the address lines

• A1,A0: This lines are used to select the 3 different counter.


Each counter is assigned with individual port address.

Control register is Common to all counters and has its own port address.


8254 can be programmed to provide various types of output through write operation and we can also check the count while counting through Read operation.

Write Operation:To initialize a Counter, following steps are required

• Write a control word into control register• Load the low-order byte of count in the counter register• Load the high-order byte of count in the counter register

Read Operation:This operation can be done in 2 ways:1. Reading the count after stopping the counter 2. Second method involves reading a count, while count is in

progress( READING ON FLY).


Problem Statement:1. Identify the port addresses of the control register and

counter 2 below.2. Write a subroutine to initialize counter 2 in mode 0 with a

count of 50,000. The sub routine should also include reading a counts on the fly, when the count reaches zero, it should return to the main program.


COUNTER: MVI A, B0H; control word initialization to c2OUT 83H;Write in the control registerMVI A, LOWBYTE; move lower byte value into AOUT 82H; load the lower value of countMVI A, HIBYTE ; move upper byte value into AOUT 82H; load the upper byte value of count

READ: MVI A, 80H; control word for reading a count OUT 83H; write in the control registerIN 82H; read lower order byte from 8254MOV D,A; store lower order byte in DIN 82H; read higher order byte from 8254ORA D; or operation to set zero flagJNZ READ; if counter not equal to 0 go back to READRET


Mode 0 Interrupt on terminal count

Mode 1 Programmable one-shot

Mode 2 Rate Generator

Mode 3 Square wave rate generator

Mode 4 Software triggered strobe

Mode 5 Hardware trigger strobe


The output in this mode is initially low, and will remain low for the duration of the count if GATE = 1.

Width of low pulse = NT

Where N is the clock count loaded into counter, and T is the clock period of the CLK input.


When the terminal (zero)count is reached, the output will go high and remain high until a new control word or new count number is loaded.

• In this mode, if GATE input becomes low at the middle of the count, the count will stop and the output will be low.

• The count resumes when the gate becomes high again.


• This mode is also called hardware triggerable one-shot.

• The triggering must be done through the GATE input by sending a 0-to-1 pulse to it.

• The following two steps must be performed:

1. Load the count registers.

2. A 0-to-1 pulse must be sent to the GATE input to trigger the counter.

• Contrast this with mode 0, in which the counter produces the output immediately after the counter is loaded as long as GATE = 1.

• In mode 1 after sending the 0-to-1 pulse to GATE, OUT becomes low and stays low for a duration of NT, then becomes high and stays high until the gate is triggered again.T.Shakeel, Lovely Professional University.

Mode 2 is also called divide-by-N counter.

• In this mode, if GATE = 1, OUT will be high for the NT clock period, goes low for only one clock pulse, then the count is reloaded automatically, and the process continues indefinitely.


In this mode if GATE = 1, OUT is a square wave where the high pulse is equal to the low pulse if N is an even number.

• In this case the high part and low part of the pulse have the same duration and are equal to (N/2)T (50% duty cycle)

• If N is an odd number, the high pulse is one clock pulse longer.

• This mode is widely used as a frequency divider and audio-tone generator.


•In this mode if GATE = 1, the output will go high upon loading the count.

• It will stay high for the duration of NT.

• After the count reaches zero (terminal count), it becomes low for one clock pulse, then goes high again and stays high until a new command word or new count is loaded.

• To repeat the strobe, the count must be reloaded again.

• Mode 4 is similar to mode 2, except that the counter is not reloaded automatically.

• In this mode, the count starts the moment the count is written into the counter. T.Shakeel, Lovely Professional University.

• This mode is similar to mode 4 except that the trigger must be done with the GATE input.

• In this mode after the count is loaded, we must send a low-to-high pulse to the gate to start the counter.


8254 microprocessor

Documents