18f2420 & MCC18 - How to Derive linear frequency output from ADC Input?

[Angry Badger]

Hi,

Can anyone give me some guidance on how to 'do' pfm i.e. pulse frequency modualtion in MCC18
I've built a little stepper driver board around a L297. Using an 18f2420 I want to provide a variable clock signal so as to vary the speed of the stepper. Can do this at the moment but the the outputted clock signal ain't linear.

Thanks in advance for any help.

// Read adc value and output value as a frequency to clk L297

#include <p18f2420.h>
#include <adc.h>                         //the code for the adc routine is in here
#include <stdlib.h>
#include <delays.h>                    //the code for the delay routines is in here

#pragma config OSC  = INTIO7 //Internal oscillator with Fosc/4 on RA6, I/O on RA7
#pragma config WDT = OFF

int result;                                     //put this here to clr the 'out of scope' msg in watch window

#define clk     PORTAbits.RA5    //pin 7

void clksig (void)                          //clk signal to L297 stepper controller inversely proportional to ADRESH
{
    clk = 1;
    Delay1KTCYx(result);    //delay = 1000 x instruction cylce time x ADRESH
    clk = 0;
    Delay1KTCYx(result);
}        

void main (void)
{
    OSCCON = 0b01100010; //set Fosc = 4Mhz
       TRISA = 0x1;
       TRISAbits.TRISA0=1;    // ADC input on pin 2

       OpenADC(ADC_FOSC_RC & 
            ADC_LEFT_JUST & 
            ADC_0_TAD, ADC_CH0 & 
            ADC_INT_OFF & 
            ADC_VREFPLUS_VDD & 
            ADC_VREFMINUS_VSS, 0 );

   while (1)
   {
        ConvertADC(); // Start conversion
          while(BusyADC()); // Wait for completion
          result = 256 - ADRESH;    //result = ADRES high byte (i.e. left justified) 
        clksig();
   }
        
}

**broken link removed**

**broken link removed**









**broken link removed**

 

[Mike - K8LH]

What is the desired frequency range of the output pulse?

 

[Angry Badger]

Hi Mike,

Thanks for your reply. I'm thinking 0 to 100Hz or thereabouts.