►Idea Behind Implementation

The basic idea behind PWM implementation on 8051 is using timers and switching port pin high/low at defined intervals. As we have discussed in the introduction of PWM that by changing the Ton time, we can vary the width of square wave keeping same time period of the square wave.

We will be using 8051 Timer0 in Mode 0. Values for high and low level will be loaded in such a way that total delay remains same. If for high level we load a value X in TH0 then for low level TH0 will be loaded with 255-X so that total remains as 255.

►Assembly Code Example

►Timer setup for PWM
	PWMPIN EQU P1.0		; PWM output pin
	MOV TMOD,#00H		; Timer0 in Mode 0
	MOV R7, #160		; Set pulse width control
	; The value loaded in R7 is value X as
	; discussed above.
	SETB EA		; Enable Interrupts
	SETB ET0		; Enable Timer 0 Interrupt
	SETB TR0		; Start Timer

►Interrupt Service Routine
	JB F0, HIGH_DONE	; If F0 flag is set then we just finished
				; the high section of the
LOW_DONE:			; cycle so Jump to HIGH_DONE
	SETB F0			; Make F0=1 to indicate start of high section
	SETB PWMPIN		; Make PWM output pin High
	MOV TH0, R7		; Load high byte of timer with R7
				; (pulse width control value)
	CLR TF0			; Clear the Timer 0 interrupt flag
	RETI			; Return from Interrupt to where
					; the program came from
	CLR F0			; Make F0=0 to indicate start of low section
	CLR PWMPIN		; Make PWM output pin low
	MOV A, #0FFH	; Move FFH (255) to A
	CLR C			; Clear C (the carry bit) so it does
					; not affect the subtraction
	SUBB A, R7		; Subtract R7 from A. A = 255 - R7.
	MOV TH0, A		; so the value loaded into TH0 + R7 = 255
	CLR TF0			; Clear the Timer 0 interrupt flag
	RETI			; Return from Interrupt to where
				; the program came from

In your main program you need to call this PWM_SETUP routine and your controller will have a PWM output. Timer Interrupt service routine will take care of PWM in the background. The width of PWM can be changed by changing the value of R7 register. In above example I am using 160, you can choose any value from 0 to 255. R7 = 0 will give you o/p 0V approx and R7 = 255 will give you 5V approx.

You can also make use of Timer1 if you want. And the output pin can be changed to whatever pin you want.
►C Code Example

►Timer setup for PWM in C
//Global variables and definition
#define PWMPIN P1_0

unsigned char pwm_width;
bit pwm_flag = 0;

void pwm_setup(){
	TMOD = 0;
	pwm_width = 160;
	EA = 1;
	ET0 = 1;
	TR0 = 1;

►Interrupt Service Routine
void timer0() interrupt 1 {
	if(!pwm_flag) {	//Start of High level
		pwm_flag = 1;	//Set flag
		PWMPIN = 1;	//Set PWM o/p pin
		TH0 = pwm_width;	//Load timer
		TF0 = 0;		//Clear interrupt flag
		return;		//Return
	else {	//Start of Low level
		pwm_flag = 0;	//Clear flag
		PWMPIN = 0;	//Clear PWM o/p pin
		TH0 = 255 - pwm_width;	//Load timer
		TF0 = 0;	//Clear Interrupt flag
		return;		//return

Now your normal 8051 is capable of PWM output.

◄ Introduction to PWM  |  8051 PWM Example Code  |  AVR PWM Example Code ►

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Thu Nov 20 2014, 06:00 PM
@Mehboob.. I don't think you can do that.
Sat Nov 15 2014, 06:41 AM
Pls can you send me code in c of PWM generation using UART/serial port in 8051.
Fri Nov 14 2014, 02:51 PM
mohammad aqil
Thu Nov 13 2014, 02:15 PM
please code hex project avr blood pressure monitor
Thu Nov 13 2014, 05:56 AM
hi mate, whats the ajay-bhargav? tell us more pls
Wed Nov 12 2014, 12:56 AM
@tmtm000.. have you been able to join ?
Wed Nov 12 2014, 12:54 AM
@Alice..become a member,then send a PM to ajay_bhargav the site owner.
Tue Nov 11 2014, 03:58 AM
Abhishek Bansal
Thanks a lot for your informative tutorials!
Mon Nov 10 2014, 09:03 AM
Hello, This is Alice from a PCB rototype manufacturer. Our company is interested in your website contents, and wonder if we can advertise our company with payment on your website. There are two ways to advertise: first, if we an get a instant quote section on your main site columns; second, whether we can get a advertising section if you have any avaliable one. If any of above ways avaliable, please contact me with your detailed scheme and advertising charge. Thanks. Alice
Sat Nov 08 2014, 11:58 AM
Signals and Systems, 2nd Edition by Alan V. Oppenheim Alan S.willsky with S.Hamid Nawab?



Tue Nov 18 2014, 09:13 AM
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Posted By ExperimenterUK
Thu Nov 20 2014, 05:56 PM
You set the on/off times here as 47%float PWMcom...
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Right now i am studying Bootloader.I want to writ...
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How can i get PWM on-off time..?pls check the atta...
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I am using PSoC 5LP.pls check the attached design ...
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Thanks for the prompt response, I will try it out ...
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Set duty cycle to 50% you will get equal on/off ti...
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Posted By ajay_bhargav
Wed Nov 19 2014, 10:41 AM
Which controller are you using?