AIM: Write a Program to generate a square wave using 8254.

Write an assembly language program in 8085 microprocessor which generates 1 KHz square waveform by using counter 2 as a binary counter if clock frequency of 8254 is 2 MHz.

Objective:

To understand the mode of operation of 8254

To understnd the calculation of count value

To develop the program for generating a square wave pulse using 8254

Assumption –

Assume the port addresses are 8C01 H, 8C03 H, 8C05 H, 8C07 H for Counter-0, Counter-1, Counter-2, Control Register.

Modes of Operation:

8254 can operate in six mode of operations. These are:

Mode-0: Interrupt on Terminal Count –Mode 0 is typically used for event counting

Mode-1: Hardware Retriggreable One Shot

Mode-2: Rate Generator) –Initially value of OUT is low. When counting is enabled, it becomes high and this process repeats periodically.

Mode-3: Square Wave Generator

Mode-4: Software Triggered Strobe

Mode-5: Hardware Triggered Strobe –OUT will initially be high. Counting is triggered by a rising edge of GATE. When the initial count has expired, OUT will go low for one clock pulse and then go high again

Mode-3 is used for square wave generation. So it is explained in detail.

Mode 3 (Square Wave Generator) – Counting is enabled when GATE = 1 and disabled when GATE = 0. This mode is used to generate square waveform and time period (equal to count) is generated.

 

Control Word Format:

 

If N is count and is even then ontime of wave = N/2 and offtime = N/2
If N is odd the on time = (N + 1) / 2 and offtime = (N – 1) / 2

For the above problem, 8254 must work in Mode 3 which is the square wave generator. Count for register is given as clock frequency / square wave frequency

count = 2 MHz / 1 KHz
= 2000

decimal to hex:

16 | 2000

16 | 125 -→0

16 | 7 → 13 → D

16 | 0 → 7

2000  =  (07C0) H

Now the data is 16 bit so value of RL1 = 1 and RL0 = 1 in Control Register. As we want to select C2 (Counter 2) the value of SC1 = 0 and SC0 = 1 in Control Register. Value of M2 = 0, M1 = 1 and M2 = 1 for Mode 3 in Control Register. For binary counter value of LSB in CR is 0.

Hence the Control Register (CR) is given by,

D7	D6	D5	D4	D3	D2	D1	D0
1	0	1	1	0	1	1	0
Control Word FORMAT; Counter-1 in Mode-3, and Binary COUNT  For Counter-0 CW =36h For Counter-1 CW = 76h For Counter-2 CW = B6h

Programming the 8254 for Square wave generation

Algorithm Steps:

Step-1: Set CWR address in DX

Step-2: Transfer the CW value (B6 for counter-2 ) at CWR address

Step-3: Set the counter address for counter-2

Step-4: Set the count value in counter 2, with LS Byte first then the MS Byte

Program:

Address	Opcode and Operands	Mnemonic	Comments
1000: 0200	BA 07 8C	MOV DX, 8C07h	; CWR Address
1000:0203	B0 B6	MOV AL, B6H
1000:0205	EE	OUT DX AL	LOAD CWR
1000:0206	BA 05 8C	MOV DX, 8C05H	COUNTER 2 ADDRESS
1000:0209	B0 D0	MOV AL, 0D0H
1000:020B	EE	OUT DX, AL	Load counter-2 LS Byte
1000:020C	B0 07	MOV AL, 07
1000:020E	EE	OUT DX, AL	Load counter-2 MS Byte
1000:020F	EB F8	JMP 0209	LOOP

Before executing the program, you have to provide the clock input to the CLK2 of the 8253(timer-2). This can be done by shorting the jumper (out of the black of two) located below 8253 IC (U-22)

1. * *

   ViVa Questions:

   Q1. name the different modes of operations of 8254

2. Ans:8254 can operate in six mode of operations. These are:

   Mode-0: Interrupt on Terminal Count –Mode 0 is typically used for event counting

   Mode-1: Hardware Retriggreable One Shot

   Mode-2: Rate Generator) –Initially value of OUT is low. When counting is enabled, it becomes high and this process repeats periodically.

   Mode-3: Square Wave Generator

   Mode-4: Software Triggered Strobe

   Mode-5: Hardware Triggered Strobe –OUT will initially be high. Counting is triggered by a rising edge of GATE. When the initial count has expired, OUT will go low for one clock pulse and then go high again

3. Q2. Write the control word for generating a square wave pulse using counter-0 binary count.Ans:
   

   D7	D6	D5	D4	D3	D2	D1	D0
   0	0	1	1	0	1	1	0
   Counter-0		LSB first then MSB		Mode-3			Binary

   Q3. What will be the value of the count if the clock frequency top 8254 is 1MHz and it is required to get a pulse of 1KHz square wave.

   Ans:

count = 2 MHz / 1 KHz
= 2000

decimal to hex:

16 | 1000

16 | 65 -→4

16 | 4 →    1

16 | 0 →    4

1000  =  (0414) H

Thus the count to be loaded in the counter is 0414h