VTech Alphabert s Magic Readport User Manual download pdf (Page 66)

Lab Manual of Microcontroller & Interfacing Page 66

pulses decreases when we press the switch SW2. This will control speed of the motor. Transistor

acts as a switch which becomes ON when port pin P1.1 is set and becomes OFF when port pin

P1.1 is reset.

Program:

ORG 00h

SJMP START

ORG 03h

INC B ; Increase R7 to increase width of pulse

RETI

ORG 13h

DEC B ;Decrease R7 to decrease width of pulse

RETI

START: MOV IE,#85h ; Enable external interrupt 0 and 1

SETB IT0 ; Negative edge triggered interrupt

SETB IT1 ;Negative edge triggered interrupt

CLR P1.1 ; Make P1.1 output pin

MOV B,#80h ; Initial speed

next: SETB P1.0

ACALL ON_delay ; Call delay for ON time

CLR P1.0 ; Complement P1.0 to generate square wave

ACALL OFF_delay ; Call delay for OFF time

SJMP next

OFF_delay:

MOV R4,#00h

L1: DJNZ R4, L1

RET

ON_delay:

MOV R7,B ; Load count value from register B

L3: MOV R6,#0FFh

L2: DJNZ R6, L2 ; Decrement R6 until it becomes zero

DJNZ R7,L3

NOP ; No operation

RET ; Return to main routine

END

Optical isolation:

It is better to use opto-isolator between motor circuit and microcontroller because it will

protect microcontroller from EMI created by the motor brushes. If motor voltage rating is higher

than the voltage used for microcontroller then opto-isolator prevents damage to the

microcontroller by providing optical isolation. If there is any fault in motor circuit or power

supply, microcontroller is safe. Modified circuit using optoisolator is shown below:

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VTech Alphabert s Magic Readport User Manual Page 66