Help !PIC 16F887 Generating 50Hz PWM Frequency for DC Motor Speed Control

[twinsratio]

Hi there,

Is that possible to generate 50Hz PWM frequency with using 20MHz external crystal ?
From what I heard, with 20Mhz crystal frequency it couldn't create such low PWM frequency, it that correct ?

Is the any other method to create PWM 50Hz by using 20MHz external crystal?

Below is my current coding basically I'm using timer0 to interrupt 1 second for the counter to get the pulse/sec from my motor. As now, my task is to create a PWM frequency at 50Hz to control the speed of the motor for 25%,50% n 75% duty cycle.
How can I edit my code to achieve 50Hz PWM frequency ? I'm using PIC 16F887 MCU

#include <htc.h>
#include <stdlib.h>
#include <string.h>

__CONFIG( HS & WDTDIS & PWRTDIS & MCLRDIS & UNPROTECT & DUNPROTECT & BORDIS & IESODIS & FCMDIS & LVPDIS & DEBUGDIS & BORV21);

#define RS RE0
#define RW RE1
#define EN RE2
unsigned int reload=76;
unsigned int rpm;
//High speed Osc (> 4mhz)

unsigned long s, k, w, a, b, c, y;


//LCD instruction function
void instruction(char C)
{

PORTE = 0x04; //RS =1 , W/R = 0 , E = 0
PORTD = C;
PORTE = 0x00;
for (s=0;s<30;s++){}
}


// LCD display function
void lcd (char C)
{

PORTE = 0x05; //RS =1 , W/R = 0 , E = 1
PORTD = C;
PORTE = 0x01; //RS =0 , W/R = 0 , E = 1

PORTE = 0x01; //RS =0 , W/R = 0 , E = 1
PORTD = C;
PORTE = 0x04; //RS =1 , W/R = 0 , E = 0

for (s=0;s<5;s++){}
}


/* write a string of chars to the LCD */
// LCD lcdout function for character input
void lcdout (const char *C)
{
PORTE = 0x04;
while (*C)
{
lcd(*C++);
}
}

void out ()
{

unsigned long counter = b;
char output[12];
ltoa(output, counter, 10);
instruction(0x80);
lcdout("Pulse/sec:");
lcdout(output); //output
instruction (0xC0);
lcdout("RPM:");
lcdout(output);
}
void out2 ()
{
instruction(0x80);
lcdout("Pulse/sec:0 ");
instruction (0xC0);
lcdout("RPM:0 ");
}





// Main Function



void main(void)
{
OSCCON= 0b01101110;


TRISA = 0xFF; //set PORTA as Input
TRISD = 0x00; //Set PORTD as Output (Data)
TRISE = 0x00; //Set PORTE as Output (EN, R/W, RS)
TRISC = 0x00; //Set PORT C as Output
ANSEL = 0x00;
ANSELH = 0x00;
ADCON0 =0x00; //’disable ADC
CM1CON0 =0x00; //’disable COMPARATOR 1
CM2CON0 =0x00; //’disable COMPARATOR 2
T1CON =0x00; //’disable TIMER1
OPTION = 0x07;
T0IE = 1; // Enable Timer
GIE = 1; // Enable Global Interrupt
y=0; //Initial counter value=0



//rpm=((y*60)/(3*20)); //formula for rpm




//LCD Function
instruction(0x0C); //Display on/off & cursor HEX 08-0F (1st)
instruction(0x38); //Function Set HEX 20-3F
instruction(0x01); //Clear Display HEX 01 (last)


while(1)
{
if(w==0)
{ out2();
}




switch (T0IE==1)
{
case 1:
if((RA0==0)&&(RA1==0)){
for(s=0;s<35;s++){} ////////pulse..
y++;
out();
w=1;
break;
}

}





}



}


static void interrupt
isr(void)
{

if(reload==0){
b=y;
rpm=y;

T0IE = 0; // disable Timer
y=0;
w=0;
instruction(0x01);
T0IE = 1; // disable Timer
reload = 76;
}
reload--;
T0IF = 0; // Clear Timer Interrupt Flag
}

Thank you inadvance

Thank you,

 

[be80be]

You could use timer one preset to 2 and load offset setting with

TMR1 = 15626 which gives 50.09 hz

 

[Mr RB]

50 Hz is a poor choice of PWM freq for a DC motor. Why not just use a few hundred Hz like most well designed controllers?