Pulse Width Modulation

Pulse-width modulation (PIC16F877A)

In this tutorial we are using PIC 16F877A for demonstrating PWM generation using CCP module. PIC 16F877A contains two CCP modules.

Pulse Width Modulation, or PWM, is a technique for getting analog results with digital means. Digital control is used to create a square wave, a signal switched between on and off.

In creating pulse with various widths for PWM two factors are important.

1.    Period  of the pulse.

2.    Its Duty Cycle.

DC = Duty Cycle is pulse that stays high relative to entire period.

It is stated in the form of percentages(%).

For example = Pulse with a 4ms,2ms period that stays HIGH for 1ms has DC of 25%.

                       

                     (1ms/4ms=25%)

                     (1ms/2ms=50%)….

PERIOD OF PWM

Frequency  of PWM is a fraction of the Fosc, the crystal frequency.

PIC16F877A microcontroller is used here  PWM control.

Use PR2 register to set the PWM period .

Tpwm=[(PR2) +1]4xNxTosc.

Tosc=1/Fosc,the crystal frequency.

Tpwm=Desired PWM period.

N=Prescaler.

SETUP FOR PWM OPERATION

The following steps should be taken when configuring

the CCP module for PWM operation:

1. Set the PWM period by writing to the PR2 register.

2. Set the PWM duty cycle by writing to the

CCPR1L register and CCP1CON<5:4> bits.

3. Make the CCP1 pin an output by clearing the

TRISC<2> bit.

4. Set the TMR2 prescale value and enable Timer2

by writing to T2CON.

5. Configure the CCP1 module for PWM operation.

 Note 2 : All PWM modules in a PIC Microcontroller uses Timer 2 for its operation, so you cannot set different frequencies for different PWM modules.

In Pulse Width Modulation mode, the CCPx pin produces up to a 10-bit resolution PWM output. The PWM duty cycle is specified by writing to the CCPR1L register and to the CCP1CON<5:4> bits. Up to 10-bit resolution is available. The CCPR1L contains the eight MSbs and the CCP1CON<5:4> contains the two LSbs. This 10-bit value is represented by CCPR1L:CCP1CON<5:4>.

CODE

#include<pic.h>

#include<htc.h>

#define _PIC16F877A_H _CONFIG(WDTE_OFF & FOSC_HS & PWRTE_OFF & LVP_OFF & BOREN_OFF);

void main()

{

 PR2=0xFF;                            // setting minmum frequency i.e 255.

CCPR1L=80;                         // 8 upper bit resolution or PWM duty cycle

TRISC2=0;                             //setting CCP1 as output

CCP1CON=0x3C;                 //2 lower bit resolution

T2CON=0x01;                       // setting prescaler.

TMR2ON=1;                         //timer2 on

{

while(!TMR2IF) ;                  //checking timer2 flag overflow 

        TMR2IF=0;                   //reset the timer 2 flag to 0

    }

}

NOTE: By changing the the value of CCPR1L you can change the PWM duty cycle.

PROTEUS DIAGRAM

 WAVEFORM

 For more information contact me at chandershh@gmail.com.