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PWM or Pulse Width Modulation is an important and essential
concept in embedded systems. We can control speed of a motor, brightness of a
bulb or a LED etc by supplying a train of pulses. If the pulse has larger ON
time i.e. duty cycle, the average voltage supplied will be more, thus if we have
connected a motor to the output, its speed increases, if we reduce the pulse
width, average voltage supplied is less, thus the speed of the motor decreases.
Thus we can digitally control the speed of a motor without the need for an ADC.
So now we will construct a pulse width modulator using
8051(89s52) to illustrate the concept of PWM I have constructed a simple
circuit which is shown below, where a LED or small lamp is connected to the output of
microcontroller (you can also connect a small motor or fan with an additional
transistor). The microcontroller is programmed such that the brightness of
the LED increases for some time, when it reaches maximum brightness, the LED's
brightness starts to fade till its brightness is minimum and this continues.
The program appears to be bigger but it is just replication
of first main block with different values loaded in registers. The second
program published in the second table uses a concept of lookup tables.
;*************************************************
;
;Program: PWM
;Author: Srikanth
;Website: http://shree-electronics.com/
;Description: Varies brightness of an LED
;connected to any pin of P2 continuously
;
;*************************************************
outp equ P2;
org 0h;
main:mov r4,#0ffh;
up0: mov r0,#100d;
mov r1,#01d;
acall out;
djnz r4,up0;
mov r4,#0ffh;
up1: mov r0,#90d;
mov r1,#10d;
acall out;
djnz r4,up1;
mov r4,#0ffh;
up2: mov r0,#80d;
mov r1,#20d;
acall out;
djnz r4,up2;
mov r4,#0ffh;
up3: mov r0,#70d;
mov r1,#30d;
acall out;
djnz r4,up3;
mov r4,#0ffh;
up4: mov r0,#60d;
mov r1,#40d;
acall out;
djnz r4,up4;
mov r4,#0ffh;
up5: mov r0,#50d;
mov r1,#50d;
acall out;
djnz r4,up5;
mov r4,#0ffh;
up6: mov r0,#40d;
mov r1,#60d;
acall out;
djnz r4,up6;
mov r4,#0ffh;
up7: mov r0,#30d;
mov r1,#70d;
acall out;
djnz r4,up7;
mov r4,#0ffh;
up8: mov r0,#20d;
mov r1,#80d;
acall out;
djnz r4,up8;
mov r4,#0ffh;
up9: mov r0,#10d;
mov r1,#90d;
acall out;
djnz r4,up9;
mov r4,#0ffh;
up11: mov r0,#10d;
mov r1,#90d;
acall out;
djnz r4,up11;
mov r4,#0ffh;
up12: mov r0,#20d;
mov r1,#80d;
acall out;
djnz r4,up12;
mov r4,#0ffh;
up13: mov r0,#30d;
mov r1,#70d;
acall out;
djnz r4,up13;
mov r4,#0ffh;
up14: mov r0,#40d;
mov r1,#60d;
acall out;
djnz r4,up14;
mov r4,#0ffh;
up15: mov r0,#50d;
mov r1,#50d;
acall out;
djnz r4,up15;
mov r4,#0ffh;
up16: mov r0,#60d;
mov r1,#40d;
acall out;
djnz r4,up16;
mov r4,#0ffh;
up17: mov r0,#70d;
mov r1,#30d;
acall out;
djnz r4,up17;
mov r4,#0ffh;
up18: mov r0,#80d;
mov r1,#20d;
acall out;
djnz r4,up18;
mov r4,#0ffh;
up19: mov r0,#90d;
mov r1,#10d;
acall out;
djnz r4,up19;
mov r4,#0ffh;
up20: mov r0,#100d;
mov r1,#01d;
acall out;
djnz r4,up20;
ajmp main;
**************************************************
out: mov outp,#0ffh;
djnz r0,out;
out1:mov outp,#00h;
djnz r1,out1;
ret;
**************************************************
end;
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Program using lookup tables.
;*************************************************
;
;Program: PWM
;Author: Srikanth
;Website: http://shree-electronics.com/
;Description: Varies brightness of an LED
;connected to any pin of P2 continuously
;
;*************************************************
outp equ P0;
org 0h;
main:mov dptr,#pwm_data;
clr a;
movc a,@a+dptr;
mov r2,a;
inc dptr;
up: clr a;
movc a,@a+dptr;
mov 40h,a;
clr a;
inc dptr;
movc a,@a+dptr;
mov 41h,a;
inc dptr;
acall out;
djnz r2,up;
sjmp main;
;*************************************************
out:
mov r4,#02h
lp0: mov r3,#0ffh
lp: mov r0,40h;
mov r1,41h;
lp1: mov outp,#0ffh;
djnz r0,lp1;
lp2: mov outp,#00h;
djnz r1,lp2;
djnz r3,lp;
djnz r4,lp0;
ret;
;*************************************************
;Lookup table
pwm_data: db 15h ;no of elements in the
db 100d,001d;lookup table
db 090d,010d;
db 080d,020d;each element consisting
db 070d,030d;of 2 bytes
db 060d,040d;
db 050d,050d;first byte is value
db 040d,060d;for ON period
db 030d,070d;second byte is value
db 020d,080d;for OFF period
db 010d,090d;
db 001d,100d;
db 010d,090d;
db 020d,080d;
db 030d,071d;
db 040d,060d;
db 050d,050d;
db 060d,040d;
db 070d,030d;
db 080d,020d;
db 090d,010d;
db 100d,001d;
;*************************************************
end;
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