UNIT-5 I/O INTERFACES€¦ · 8255 programmable Peripheral Interface o Various modes of operation o Interfacing to 8086 8051 interfacing with o Seven segment LED displays o Stepper

UNIT-5 I/O INTERFACES ECE DEPARTMENT

MICROPROCESSORS AND MICROCONTROLLERS Page 1

UNIT-V I/O INTERFACES

Contents at a glance:

8255 programmable Peripheral Interface

o Various modes of operation

o Interfacing to 8086

8051 interfacing with

o Seven segment LED displays

o Stepper motor interfacing

o D/A converter interfacing

o Interfacing DC motor

o Interfacing 4*4 Matrix Keypad

o Interfacing to Alphanumeric Displays (LCD)

o A/D converter interfacing.

Introduction to I/O Ports Interfacing:

I/O ports or Input/output ports are the devices through which the microprocessor communicates with other

devices or external data source/destinations.

Input activity, as one may expect, is the activity that enables the microprocessor to read data from external

devices, for example keyboards. These devices are known as input devices as they feed data into microprocessor

system.

Output activity transfers data from the microprocessor to the external devices, for example CRT display. These

devices which accept the data from a microprocessor system are called output devices.

Thus for a microprocessor the input activity is similar to read operation, while the output activity is similar to

write operation.

Steps in Interfacing an I/O Device:

Connect the data bus of the microprocessor system with the data bus of the I/O port.

Derive a device address pulse by decoding the required address of the device and use it as the chip select of

the device.

Use a suitable control signal i.e. IORD or IOWR to carry out device operations

Input Port:

The input device is connected to the microprocessor through buffer. The simplest form of a input port is a buffer as shown in the figure.

This buffer is a tri-state buffer and its output is available only when enable signal is active. When microprocessor wants to read data from the input device (keyboard), the control signals from the microprocessor activates the buffer by asserting enable input of the buffer. Once the buffer is enabled, data from the device is available on the data bus. Microprocessor reads this data by initiating read command.

Output Port:

It is used to send the data to the output device such as display from the microprocessor. The simplest

form of the output port is a latch.



The output device is connected to the microprocessor through latch as shown in the figure.

When microprocessor wants to send data to the output device it puts the data on the data bus and activates

the clock signal of the latch, latching the data from the data bus at the output of latch. It is then available at

the output of latch for the output device.

I/O Interfacing Techniques:

Input/output devices can be interfaced with microprocessor systems in two ways :

1. I/O mapped I/O

2. Memory mapped I/O

1. I/O mapped I/O:

8086 has special instructions IN and OUT to transfer data through the input/output ports in I/O mapped I/O

system.

The IN instruction copies data from a port to the Accumulator. If an 8-bit port is read data will go to AL and if

16-bit port is read the data will go to AX.

The OUT instruction copies a byte from AL or a word from AX to the specified port.

The M/IO signal is always low when 8086 is executing these instructions. In this address of I/O device is 8-bit

or 16-bit. It is 8-bit for direct addressing and 16-bit for indirect addressing.

Memory mapped I/O:

In this type of I/O interfacing, the 8086 uses 20 address lines to identify an I/O device. The I/O device is connected as if it is a memory device. The 8086 uses same control signals and instructions to

access I/O as those of memory, here RD and WR signals are activated indicating memory bus cycle.

Comparison of Memory mapping I/O and I/O mapping I/O:

Memory Mapping I/O I/O mapping I/O

1. 20-bit addresses are provided for I/O devices. 1. 8-bit or 16-bit address are provided for I/O devices

2. The I/O ports or peripherals can be treated like

memory locations and so all instructions related to

memory can be used for data transfer.

2. Only IN and OUT instructions can be used for data

transfer between I/O device and the processor.

3. In memory mapped ports, the data can be moved

from any register to port and vice versa

3. In I/O mapped ports, the data transfer can take only

between the accumulator and the ports

4. When memory mapping is used for I/O devices, the

full memory address space cannot be used for

addressing memory.

4. When I/O mapping is used for I/O devices, then the full

address space can be used for addressing memory.



5. For accessing memory mapped devices, the

processors executes the memory read or write cycle.

During this cycle, is asserted high.

5. For accessing I/O mapped devices, the processors

executes the I/O read or write cycle. During this cycle,

is asserted low.

Problem:

Interface an input port 74LS245 to read the status of the switches SW1 to SW8. the switches when shorted,

input a ‘1’ else input a ‘0’ to the microprocessor system. Store the status in register BL. The address of the port

is 0740H

Solution:

The hardware interface circuit is shown in figure. The address, control and data lines are assumed to be readily available at the microprocessor system The ALP is given as follows:

MOV BL, 00H ; clear BL for status MOV DX, 0740H ; 16-bit Port address in DX IN AL,DX ; Read Port 0740H for switch positions. MOV BL, AL; Store status of switches from AL into BL HLT; Stop

Programmable Peripheral Interface (PPI) – 8255:

The parallel input-output port chip 8255 is also known as programmable peripheral input-output port.

It has 24 input/output lines which may be individually programmed in two groups of twelve lines each, or

three groups of eight lines.

The two groups of I/O pins are named as Group A and Group B.

Each of these two group contains a subgroup of eight I/O lines called as 8-bit port and another subgroup of

four I/O lines or a 4-bit port.

Thus Group A contains an 8-bit port A along with a 4-bit port, C upper. The port A lines are identified by

symbols PA0 –PA7 while the port C lines are identified as PC4-PC7.

Similarly Group B contains an 8-bit port and a 4-bit port C with lower bits.

The port C upper and port C lower can be used in combination as an 8-bit port C.

All of these ports function independently either as input or as output ports.

This can be achieved by programming the bits of internal register of 8255 called as Control Word Register.

The 8-bit data bus buffer is controlled by read/write control logic.

The read/write control logic manages all of the internal and external transfer of both data and control words.



Programmable Peripheral Interface (PPI) Architecture:

Data Bus Buffer:

This three-state bi-directional 8-bit buffer is used to interface the 82C55A to the system data bus. Data is

transmitted or received by the buffer upon execution of input or output instructions by the CPU. Control

words and status information are also transferred through the data bus buffer.

Read/Write and Control Logic

The function of this block is to manage all of the internal and external transfers of both Data and Control or

Status words. It accepts inputs from the CPU Address and Control busses and in turn, issues commands to both

of the Control Groups.

(A0 and A1) Port Select0 and Port Select1:

These input signals, in conjunction with the RD and WR inputs, control the selection of one of the three ports

or the control word register. They are normally connected to the least significant bits of the address bus (A0

and A1).

(CS) Chip Select:

A “low” on this input pin enables the communication between the 82C55A and the CPU.

(RD) Read:

A “low” on this input pin enables82C55A to send the data or status information to the CPU on the data bus. In

essence, it allows the CPU to “read from” the 82C55A.

(WR) Write:

A “low” on this input pin enables the CPU to write data or control words into the 82C55A.

(RESET) Reset:

A “high” on this input initializes the control register to 9Bh and all ports (A, B, C)are set to the input mode.

“Bus hold” devices internal to the 82C55A will hold the I/O port inputs to a logic “1” state with a maximum

hold current of 400mA.



Modes of Operation of 8255:

There are two basic modes of operation of 8255 - I/O mode and BSR (Bit-Set-Reset) mode.

In the I/O mode, the 8255 ports work as programmable I/O ports, while in BSR mode only port C (PC0 – PC7)

can be used to set or reset its individual port bits.

Under the I/O mode of operation, further there are three modes of operation of 8255,so as to support

different types of applications, viz. mode 0, mode 1 and mode 2.

All these modes can be selected by programming a register internal to 8255, known as Control Word Register

(CWR).

BSR Mode:

In this mode, any of the 8-bits of port C can be set or reset depending on B0 of the control word.

The bit to be set or reset is selected by bit select flags B3, B2 and B1 of the CWR as given in the table

below.

The CWR format is as shown below:



I/O Modes:

The CWR format is as shown below:

I/O Modes – MODE 0 (Basic I/O Mode) :

This mode is also known as basic input/output mode.

This mode provides input and output capability using each of the three ports.

Data can be simply read from or written to the input and output ports respectively, after appropriate

initialization.

The salient features of this mode are

1. Two 8 bit ports (port A and port B) and two 4-bits ports(port C upper and lower) are available. The two 4-

bit ports can be combinedly used as a third 8-bit port.

2. Any port can be used as an input and output port

3. Output ports are latched. Input ports are not latched.

4. A maximum of four ports are available so that overall 16 I/O configurations are possible.

I/O Modes – Mode 1 (Strobed I/O Mode):

This mode is also known as strobed input/output mode. In this mode the handshaking signals control the input

or output action of the specified port. Port C lines PC0-PC2, provide strobe or handshake lines for port B. Port C

lines PC3-PC5, and provide strobe or handshake lines for port A.

Features of MODE 1

1. Two groups- group A and group B are available for strobed data transfer.

2. Each group contains one 8-bit data I/O port and one 4-bit control/data port.

3. The 8-bit port can be either used as input or an output port.

4. Out of 8-bit port C, PC0-PC2 are used to generate control signals for port B and PC3-PC5 are used to

generate control signals for port A. the lines PC6,PC7 may be used as independent data lines.



I/O Modes – Mode 2 (Strobed Bidirectional I/O Mode) :

This mode is also known as strobed Bidirectional input/output mode. This mode of operation provides 8255

with an additional feature for communicating with a peripheral device on an 8-bit data bus. Handshaking

signals are provided to maintain proper data flow and synchronization between data transmitter and receiver.

Features of MODE 2

1. The single 8-bit port in group A is available.

2. The 8-bit port is bidirectional and additionally a 5-bit control port is available.

3. Three I/O lines are available at port C

4. Input and output ports are both latched

5. The 5-bit control port C is used for generating/accepting handshake signals for 8-bit data transfer on

port A.



Interfacing 8255 to 8086:

Problem: Interface an 8255 with 8086 to work as an I/O port. Initialize port A as output port, port B as input port and

port C as output port. Port A address should be 0740H. Write a program to sense switch positions SW0-SW7

connected to port B. the sensed pattern is to display on port A, to which 8 LEDs are connected, while the port C lower

displays number of on switches out of the total eight switches.

Solution:

Thus 82H is the control word for the requirements in the problem. The port address can be done as given

below.



The 8255 is to be interfaced with lower order data bus, i.e. D0-D7

The A0 and A1 pins of 8255 are connected to A01 and A02 pins of microprocessor respectively.

The A00 pin of the microprocessor is used for selecting lower byte of data bus.

Hence any change in the status of A00 does not affect the port to be selected.

Let us use absolute decoding scheme that uses all the 16 address lines for deriving the device address pulse.

Out of A0-A15 lines, two address lines A02 and A01 are directly required by 8255 for three port and CWR address

decoding.

Hence only A3 to A15 are used for decoding address.

Flow Chart



Program:

MOV DX, 0746H ; Initialize CWR with

MOV AL, 82H ; control word 82H

OUT DX, AL ;

MOV DX, 0742H ; Get address of port B in DX

IN AL, DX ; Read port B for switch

MOV DX, 0740H ; positions in to AL and get port A address in DX

OUT DX, AL ; Display switch positions on port A

MOV BL, 00H ; Initialize BL for switch count

MOV CL, 08H

Back: ROL AL , 1 ; Rotate AL through carry to check,

JNC Next ; whether the switches are on or

INC BL ; off, i.e. either 1 or 0

Next: LOOP Back ; Check for next switch. If all switch are checked, the

MOV AL, BL ; number of on switches are

MOV DX, 0744H ; in BL. Display it on port C

OUT DX, AL ; lower

HLT

8051 interfacing with LED:

LEDs can be connected with the port line in two ways as presented in below figure.

In fir 18.8 (a) a logic level ‘1’ needs to be applied for glowing the LED. The resistance R is a current limiting

resistance usually in the range of 220Ω to 470Ω.

In this configuration, the current required for glowing the LED is sourced by the pin. Hence it is also called as

source mode.

In fir 18.8 (b) logic level ‘0’ needs to be applied for glowing the LED. In this configuration, the current required

for glowing the LED is sourced by the ‘+5V’ supply while it is sinked by the microcontroller pin. Hence it is also

called as sink mode.

Problem: Design the hardware circuit to interface an LED to 8051 port pin P1.5 and write a ALP to blink LED with some

delay.



Program:

ORG 0000H

CLR P1.5

AGAIN: CPL P1.5

ACALL DELAY

SMP AGAIN

DELAY: MOV R3,#0AH

UP: MOV R4, #0FFH

HERE: DJNZ R4, HERE

DJNZ R3, UP

RET

END

Seven segment interfacing with 8051:

Seven segment displays are used to indicate numerical information. Seven segments display can display digits

from 0 to 9 and even we can display few characters like A, b, C, H, E, e, F, etc. These are very popular and have

many more applications.

Seven segment displays internally consist of 8 LEDs. In these LEDs, 7 LEDs are used to indicate the digits 0 to 9

and single LED is used for indicating decimal point. Generally seven segments are two types, one is common

cathode and the other is common anode.

In common cathode, all the cathodes of LEDs are tied together and labelled as com. and the anode are left

alone. In common anode, seven segment display all the anodes are tied together and cathodes are left freely.

Below figure shows the internal connections of seven segment Display.

To display the digits on common cathode 7 segment, we need to glow different logic combinations of

segments.

For example if you want to display the digit 3 on seven segment then you need to glow the segments a, b, c, d

and g.

The below table show you the Hex decimal values what we need to send from PORT to Display the digits from

0 to 9.



Problem: Interfacing common cathode 7 segment display with 8051. Write an ALP to display the decimal

counter (0- 9) with a delay of 10msec.

F=11.08592MHz, T=1/11.08592MHz=1.085s

Count=10msec/1.085s=9216

65536-9216=56320D

=DC00H

ORG 0000H

START: MOV R2, #0AH

MOV TMOD, #01H

MOV DPTR, #0400H

MOV A, #00H

MOV P1, #00H

MOV R1, #00H

BACK: MOVC A,@A+DPTR

MOV P1, A

MOV TH0, #0DCH

MOV TL0, #00H

SETB TR0

HERE: JNB TF0, HERE

CLR TF0

CLR TR0

INC R1

MOV A, R1

DJNZ R2, BACK

SJMP START

ORG 0400H

DB 3FH,06H,5BH,4F,66H,6DH,7DH,07H,7FH,6FH

END



Problem: Write an ALP to display 0-99 on seven segment display.

ORG 000H

MOV P2,#00H

CLR P3.0

CLR P3.1

MOV DPTR,#0400H

AGAIN:MOV R1,#00H

MSB: MOV R2,00H

LSB: MOV A,@A+DPTR

SETB P3.0

CLR P3.1

MOV P2,A

MOV A,R2

MOVC A,@A+DPTR

CLR P3.0

SETB P3.1

MOV P2,A

ACALL DELAY

MOV A,,R1

INC R2

CJNE R2,#0AH,LSB

INC R1

CJNE R1,#0AH,MSB

SJMP AGAIN

END

DELAY:MOV R3,#0AH

UP: MOV R4,#OFFH

HERE: DJNZ R4,HERE

DJNZ R3,UP

RET

ORG 0400H

DB 3FH,06H,5BH,4F,66H,6DH,7DH,07H,7FH,6FH

END



Stepper Motor Interfacing:

Stepper motor is a widely used device that translates electrical pulses into mechanical movement.

Stepper motor is used in applications such as; disk drives, dot matrix printer, robotics etc,.

The construction of the motor is as shown in figure 1 below.

It has a permanent magnet rotor called the shaft which is surrounded by a stator.

Commonly used stepper motors have four stator windings that are paired with a center – tapped

common.

Such motors are called as four-phase or unipolar stepper motor.

The stator is a magnet over which the electric coil is wound. One end of the coil are connected

commonly either to ground or +5V.

The other end is provided with a fixed sequence such that the motor rotates in a particular direction.

Stepper motor shaft moves in a fixed repeatable increment, which allows one to move it to a precise

position.

Direction of the rotation is dictated by the stator poles. Stator poles are determined by the current sent

through the wire coils.

Step angle:

Step angle is defined as the minimum degree of rotation with a single step.

No of steps per revolution = 360° / step angle

Steps per second = (rpm x steps per revolution) / 60

Example: step angle = 2°

No of steps per revolution = 180

Switching Sequence of Motor:

As discussed earlier the coils need to be energized for the rotation. This can be done by sending a bits

sequence to one end of the coil while the other end is commonly connected.

The bit sequence sent can make either one phase ON or two phases ON for a full step sequence or it can

be a combination of one and two phase ON for half step sequence. Both are tabulated below.



Problem: Interfacing a 4 phase 200 teeth, 9v , stepper motor with 8051 using ULN and write ALP for

a) Rotating shift of the stepper motor at a speed of 2 rotations per min in clock wise direction.

ORG 0000H

MOV P1, #00H

MOV A, #11H

AGAIN: MOV PL, A

ACALL DELAY (150 msec)

RR A

SJMP AGAIN

DELAY: MOV TMOD, #01H ; time delay as 15x10msec

MOV R1, #15

UP: MOV TH0, #0DCH

MOV TL0, #00H

SETB TR0

HERE: JNB TF0, HERE

CLR TF0

CLR TR0



DJNZ R1, UP

END

b) Rotating the shaft 180 anti-clock wise in one min

ORG 0000H

MOV P1, #00H

AGAIN: MOV R0, #00H

MOV A, #88H

CONTINUE: MOV P1, A

ACALL DELAY (600MS)

INC R0

RL A

CJNE R0, #100, CONITUE

SJMP AGAIN

DELAY: MOV TMOD, #01H

MOV R1, #60

UP: MOV TH0, #0DCH

MOV TL0, #00H

SETB TR0

HERE: JNB TF0, HERE

CLR TF0

CLR TR0

DJNZ R1, UP

END

c) Rotating the shaft 90 back and forward in 30 seconds each continuously

ORG 0000H

MOV P1, 00H

AGAIN: MOV R0, #00H

MOV A, #88H

CONTINUE: MOV P1, A

ACALL DELAY (600mS)

INC R0

RR A

CJNE R0, #50, CONTINUE

CONT1: MOV P1, A

ACALL DELAY (600mS)

DEC R0

RL A

CJNE R0, #0FFh, CONT1

SJMP AGAIN



DC Motor Interfacing with 8051:

The DC motor is another widely used device that translates electrical pulses into mechanical movement.

Motor has 2 leads +ve and – ve , connecting them to a DC voltage supply moves the motor in one direction.

On reversing the polarity rotates the motor in the reverse direction.

Basic difference between Stepper and DC motor is stepper motor moves in steps while DC motor moves

continuously.

Another difference is with stepper motor the number of steps can be counted while it is not possible in DC

motor.

Unidirectional Control:

Figure 3 shows the rotation of the DC motor in clockwise and anticlockwise direction.

Bidirectional Control:



Problem: Interface the DC motor with 8051 and write an ALP to rotate the motor in clock wise direction

continuously.

CLR P1.0

CLR P1.1

SETB P1.0

CLR P1.1

HERE: SJMP HERE

END



Example: A switch is connected to pin P2.7. Write an ALP to monitor the status of the SW. If SW = 0, DC

motor moves clockwise and if SW = 1, DC motor moves anticlockwise.

Program:

CLR P1.0

CLR P1.1

SETB P2.7

CHECK: JNB P2.7, CLOCK

CLR P1.0

SETB P1.1

SJMP CHECK

CLOCK: SETB P1.0

CLR P1.1

SJMP CHECK

END

Analog-to-digital converter (ADC) interfacing:

ADCs (analog-to-digital converters) are among the most widely used devices for data acquisition. A physical

quantity, like temperature, pressure, humidity, and velocity, etc., is converted to electrical (voltage, current)

signals using a device called a transducer, or sensor We need an analog-to-digital converter to translate the

analog signals to digital numbers, so microcontroller can read them.

ADC0808/0809 chip:

ADC0808/0809 has 8 analog inputs. It allows us to monitor up to 8 different transducers using only single chip. The chip has 8-bit data output. The 8 analog input channels are multiplexed and selected according to the values given to the three address

pins, A, B, and C. that is; if CBA=000, CH0 is selected; CBA=011, CH3 is selected and so on. The pin details of ADC0808 are as shown in the figure below.





Steps to Program ADC0808/0809

1. Select an analog channel by providing bits to A, B, and C addresses.

2. Activate the ALE pin. It needs an H-to-L pulse to latch in the address.

3. Activate SC (start conversion) by an H-to-L pulse to initiate conversion.

4. Monitor EOC (end of conversion) to see whether conversion is finished.

5. Activate OE (output enable) to read data out of the ADC chip. An H-to-L pulse to the OE pin will bring digital

data out of the chip.

Problem: Interface an 8-bit ADC 0808/0809 with 8051 ports. Further write ALP for

a) Reading the digital equivalent of the analog inputs applied at channel 0 and channel 5 and stores them in R0

and R1.

b) Reading the digital equivalent of all the channel (0-7) and store those starting form address 50H in internal

RAM.

Programs:

a) Reading the digital equivalent of the analog inputs applied at channel 0 and channel 5 and stores them in R0 and R1.

ORG 0000H

MOV P0, #0FF H

MOV P2, #00H

CLR P1.0

CLR P1.1

SETB P1.3

CLR P1.2

MOV P2, #00H

SETB P1.0

NOP

CLR P1.0

SETB P1.1

NOP

CLR P1.1

WAIT: JB P1.3, WAIT

HERE: JNB P1.3, HERE

SETB P1.2

MOV A, P0

CLR P1.2

MOV R0, A

MOV P2, #O5H

SETB P1.0

NOP

CLR P1.0

SETB P1.1

NOP

CLR P1.1

WAIT1: JB P1.3, WAIT1

HERE1: JNB P1.3, HERE1

SETB P1.2

MOV A, P0

CLR P1.2

MOV R1, A

END

b) Reading the digital equivalent of all the channel (0-7) and store those starting form address 50H in internal RAM.



ORG 0000H

MOV P0, #0FFH

MOV P2, #00H

CLR P1.0

CLR P1.1

SETB P1.3

CLR P1.2

MOV R1, #50H

MOV R2, #00H

CONTINUE: MOV A, R2

MOV P2, A

SETB P1.0

NOP

CLR P1.0

SETB P1.1

NOP

CLR P1.1

WAIT: JB P1.3, WAIT

HERE: JNB P1.3, HERE

SETB P1.2

MOV A, P0

CLR P1.2

MOV @R1, A

INC R2

INC R1

CJNE R2, #08, CONTINUE

END

Digital-to-Analog (DAC) converter:

The DAC is a device widely used to convert digital pulses to analog signals. In this section we will discuss the

basics of interfacing a DAC to 8051.

The two method of creating a DAC is binary weighted and R/2R ladder.

DAC0808:

The digital inputs are converter to current Iout, and by connecting a resistor to the Iout pin, we can convert the result to voltage.

The 8051 connection to DAC0808 is as shown in the figure below.



Problem: interface DAC 0808 with 8051 port P0 and write assembly language programs,

a) To generate a triangular wave of 0 to 3V with frequency 100Hz

b) To generate a square waveform of 0-5V frequency 500Hz

c) To generate the following pattern

Solution:

The interfacing circuit for (a) and (b) of the problem is shown in above figure (18.8(a)). In the above circuit Vref + and

Vref – are directly tied with +5V and 0V respectively. The maximum output swing will be 0 to 5V.

Program:

a) To generate a triangular wave of 0 to 3V with frequency 100Hz

Resolution of DAC = 5 / 28 – 1 = 5/255 = 0.0196V = 19.6mV

Binary equivalent of 3V = 3/19.6mV = 153 = 99H

Frequency f = 100Hz; T = 1/f = 1/100 = 10000 µsec

Delay for one binary value = 5000 µsec/153 = 32.6 µsec

AGAIN: MOV P0, 00H

MOV R1, #00H

INCR: MOV A, R1

MOV P0, A

ACALL DELAY (32.6µS)

INC R1

CJNE R1, #9A, INCR

DEC R1

DECR: MOV A, R1

MOV P0, A

ACALL DELAY (32.6µS)

DEC R1

CJNE R1, #0FFH, DECR

SJMP AGAIN

; Timer 0 in mode 2 (8 – bit auto reload timer) used to generate a delay of 32.6µsec



No of pulses to be count for delay of 32.6µsec is = 32.6µsec/1.085 µsec =~ 30

Initial value to be load into timer registers is = 256 – 30 = 226 = E2H


MOV TH0, #0E2H

SETB TR0

HERE: JNB TF0, HERE

CLR TF0

RET

END

b) To generate a square waveform of 0-5V frequency 500Hz

f = 500Hz; T = 1/f = 1/500 = 2 ms; TLOW = 1ms, THIGH = 1 ms

Program:

ORG 0000H

MOV A, #00H

AGAIN: MOV P0, A

ACALL DELAY (1ms)

CPL A

SJMP AGAIN

; Timer 0 in mode 1 (16 – bit timer) used to generate a delay of 1msec

No of pulses to be count for delay of 1msec is = 1msec/1.085 µsec =~ 922

Initial value to be load into timer registers is = 65556 – 922 = 64614 = FC66 H


MOV TH0, #0FCH

MOV TL0, #66H

SETB TR0

HERE: JNB TF0, HERE

CLR TF0

CLR TR0

RET

END

c) Program to generate the given pattern

For generate the given waveform. The above interfacing circuit will be useful with Vref + tied to 10V and Vref – tied to

0V.

Program:

ORG 0000H

MOV A, #00H

MOV P0, A

REPEAT: MOV A, #80H

MOV P0, A

ACALL DELAY (1ms)

MOV A, #0FFH

MOV P0, A

ACALL DELAY (1ms)

ACALL DELAY (1ms)

MOV A, #80H

MOV P0, A

ACALL DELAY (1ms)

MOV A, #00H

MOV P0, A

ACALL DELAY (1ms)



ACALL DELAY (1ms)

SJMP REPEAT


MOV TH0, #0FCH

MOV TL0, #66H

SETB TR0

HERE: JNB TF0, HERE

CLR TF0

CLR TR0

RET

END

4x4 Matrix Keyboard Interfacing with 8051:

Keyboards are organized in a matrix of rows and columns. The CPU accesses both rows and columns through

ports.

Therefore, with two 8-bit ports, an 8 x 8 matrix of keys can be connected to a microcontroller. When a key is

pressed, a row and a column make a contact.

Otherwise, there is no connection between rows and columns. A 4x4 matrix connected to a port.

The rows and the columns are connected to an input port like as shown in below figure.

Problem: Interface a 4x4 matrix keyboard with 8051 ports and get the key number (from 0-15) after a key is pressed in

R0.

Assembly Language Program:

ORG 0000H

START: MOV R2, #00H ; for row count MOV R3, #03H ; for column count WAIT: MOV P2, #0FFH MOV A, P2 CJNE A, #0FFH, KEY SJMP WAIT KEY: CALL DEBOUNCE MOV P2, #0FFH MOV A, P2 MOV R1, A ANL A, #0FH ROW: RRC A JNC COL INC R2

SJMP ROW COL: MOV A, R1 ANL A, #0F0H COL1: RLC A JNC KEYCODE DEC R3 SJMP COL1

KEYCODE: MOV A, R2 RL A RL A ADD A, R3 MOV R0, A SJMP START END



16X2 LCD Interfacing with 8051:

LCD is finding widespread use replacing LEDs for the following reasons:

o The declining prices of LCD

o The ability to display numbers, characters, and graphics

o Incorporation of a refreshing controller into the LCD, thereby relieving the CPU of

o the task of refreshing the LCD

o Ease of programming for characters and graphics The below figure show the 16x2 LCD pin diagram

Pin Descriptions for LCD:



Sending Data/ Commands to LCDs with Time Delay:

To send any of the commands to the LCD, make pin RS=0. For data, make RS=1.

Then send a high-to-low pulse to the E pin to enable the internal latch of the LCD.

Problem: Interface 16x2 LCD with 8051 and write an ALP to initialize the LCD and display message “ECE”.

Program:

ORG 0000H MOV A, #38H ACALL COMNWRT ACALL DELAY MOV A, #0EH ACALL COMNWRT ACALL DELAY MOV A, #01 ACALL COMNWRT ACALL DELAY

MOV A, #06H ACALL COMNWRT ACALL DELAY MOV A, #87H ACALL COMNWRT ACALL DELAY MOV A,#’E’ ACALL DATAWRT ACALL DELAY MOV A,#’C’



ACALL DATAWRT ACALL DELAY MOV A,#’E’ ACALL DATAWRT

ACALL DELAY AGAIN: SJMP AGAIN

COMNWRT: MOV P1, A CLR P2.0; RS=0 for command CLR P2.1; R/W=0 for write SETB P2.2;E=1 for high pulse ACALL DELAY CLR P2.2; E=0 for H-to-L pulse RET

DATAWRT: MOV P1, A SETB P2.0; RS=1 for data CLR P2.1; R/W=0 for write SETB P2.2; E=1 for high pulse ACALL DELAY CLR P2.2; E=0 for H-to-L pulse RET

DELAY: MOV R3,#50 HERE2: MOV R4,#255 HERE: DJNZ HERE

DJNZ R3, HERE2 RET END

Problem: Write an assembly language program to display a string “VardhamanCollege of Engineering” on 16x2 LCD

display.

ORG 0000H MOV A, #38H ACALL COMNWRT ACALL DELAY MOV A, #0EH ACALL COMNWRT ACALL DELAY MOV A, #01 ACALL COMNWRT ACALL DELAY MOV A, #06H ACALL COMNWRT ACALL DELAY MOV A, #80H ACALL COMNWRT ACALL DELAY MOV DPTR, #0500H MOV R1, #00H

NEXT: MOV A, R1 MOVC A,@A+DPTR ACALL DATAWRT ACALL DELAY INC R1 CJNE R1,#10H, NEXT MOV A, #0C0H ACALL COMNWRT ACALL DELAY

NEXT1: MOV A, R1 MOVC A,@A+DPTR ACALL DATAWRT ACALL DELAY INC R1 CJNE R1,#20H, NEXT1 AGAIN: SJMP AGAIN

COMNWRT: MOV P1, A CLR P2.0; RS=0 for command



CLR P2.1; R/W=0 for write SETB P2.2; E=1 for high pulse ACALL DELAY CLR P2.2; E=0 for H-to-L pulse RET



DJNZ R3, HERE2 RET

ORG 0500H DB “VardhamanCollege of Engineering” END

Problem: Write an assembly language program to display a string “Microprocessors & Microcontrollers” on 16x2 LCD

display.

ORG 0000H MOV A, #38H ACALL COMNWRT ACALL DELAY MOV A, #0EH ACALL COMNWRT ACALL DELAY MOV A, #01 ACALL COMNWRT ACALL DELAY MOV A, #06H ACALL COMNWRT ACALL DELAY MOV A, #80H ACALL COMNWRT ACALL DELAY MOV DPTR, #0500H MOV R1, #00H

NEXT: MOV A, R1 MOVC A,@A+DPTR ACALL DATAWRT ACALL DELAY INC R1 CJNE R1,#10H, NEXT MOV A, #0C0H ACALL COMNWRT ACALL DELAY

NEXT1: MOV A, R1 MOVC A,@A+DPTR ACALL DATAWRT ACALL DELAY INC R1



CJNE R1,#20H, NEXT1 AGAIN: SJMP AGAIN

COMNWRT: MOV P1, A CLR P2.0; RS=0 for command CLR P2.1; R/W=0 for write SETB P2.2;E=1 for high pulse ACALL DELAY CLR P2.2; E=0 for H-to-L pulse RET



DJNZ R3, HERE2 RET

ORG 0500H DB “Microprocessors & Microcontrollers” END

Documents

UNIT-5 I/O INTERFACES€¦ · 8255 programmable Peripheral Interface o Various modes of operation o Interfacing to 8086 8051 interfacing with o Seven segment LED displays o Stepper