PWM signal problem - Electronic Circuits Projects Diagrams Free

iznalgnaij · 21st March 2008, 07:51 PM

I need to use PIC16F877A to generate a PWM signal to control my motor driver--MD22(http://www.robot-electronics.co.uk/htm/md22tech.htm), the RC servo mode of MD22 need a PWM signal which has PWM period of 20ms and duty cycle range from 1ms to 2ms.
I look for the frequency problem online regarding PIC16F877A, but it seems that the lowest PWM frequency I can get is about 488Hz.
How can I generate the PWM signal in this case?

I need some help...
thank you

**Nigel Goodwin** · 21st March 2008, 09:45 PM

It's usual to do it completely in software, even a BASIC STAMP can do it.

dknguyen · 24th March 2008, 05:24 AM

You can make a timer interrupt that goes as long as you want and base some code that manually turns a pin high and low off of that to make slow PWM.

iznalgnaij · 25th March 2008, 08:50 AM

Quote:

Originally Posted by dknguyen

You can make a timer interrupt that goes as long as you want and base some code that manually turns a pin high and low off of that to make slow PWM.

I am a bird to this PIC programming, is there any example code which show how to write this kind of interrupt?

second286 · 25th March 2008, 12:40 PM

iznalgnaij:

Just use a slower xtal oscillator (32.768 kHz would work), or maybe an RC osc; read up on the different osc types (this is all explained in the PIC16F877A data sheet). The duty cycle pulse is no problem no matter what xtal you use. The Period=P is:

P=[(PR2+1)4(Tosc)(PSV)] where PSV=prescale value

Ex. If use a 32.768 kHz xtal, with PSV=1 then PR2+1=163.84 ->164
Period(actual) = 20.0195 mS, 0.002 % difference from 20 mS

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21st March 2008, 07:51 PM	(permalink)
iznalgnaij	PWM signal problem I need to use PIC16F877A to generate a PWM signal to control my motor driver--MD22(http://www.robot-electronics.co.uk/htm/md22tech.htm), the RC servo mode of MD22 need a PWM signal which has PWM period of 20ms and duty cycle range from 1ms to 2ms. I look for the frequency problem online regarding PIC16F877A, but it seems that the lowest PWM frequency I can get is about 488Hz. How can I generate the PWM signal in this case? I need some help... thank you

21st March 2008, 09:45 PM	(permalink)
Nigel Goodwin	It's usual to do it completely in software, even a BASIC STAMP can do it. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

24th March 2008, 05:24 AM	(permalink)
dknguyen	You can make a timer interrupt that goes as long as you want and base some code that manually turns a pin high and low off of that to make slow PWM.

25th March 2008, 12:40 PM	(permalink)
second286	iznalgnaij: Just use a slower xtal oscillator (32.768 kHz would work), or maybe an RC osc; read up on the different osc types (this is all explained in the PIC16F877A data sheet). The duty cycle pulse is no problem no matter what xtal you use. The Period=P is: P=[(PR2+1)4(Tosc)(PSV)] where PSV=prescale value Ex. If use a 32.768 kHz xtal, with PSV=1 then PR2+1=163.84 ->164 Period(actual) = 20.0195 mS, 0.002 % difference from 20 mS Last edited by second286; 25th March 2008 at 09:51 PM.