James,

James Williams wrote:
 >  The duty cycle is not the issue.  The issue is trying to change
 >  the period or frequency of the PWM once it has been set up.

I don't understand what goes wrong with you, but I have no problem
with changing PWM frequency on a 18F242. I don't write in Asm, but
in C (for CC8E compiler). I've added below the source of a
stripped down program with ADC input and PWM output. The code is
very simple and the essential part is practically one-to-one
translatable to Asm.  The PWM frequency is dropped by a factor 4
by changing the prescaler setting when the analog input becomes
higher than approx 90% of +Vref (maybe not of any practical value,
but it shows how it works).
Hope this helps you!  (If you want I'll send you the hex-file to
check if your chip is OK).

Regards, Rob.


---------------------------------------------------------------------------------------

// PWM + ADC experiment on PIC18F242 (20MHz)  -  Rob Hamerling 2003

#include <18c242.h>

#pragma  config[00] = 0b0000.0000       //
#pragma  config[01] = 0b0010.0010       // HS oscillator, PLL disabled
#pragma  config[02] = 0b0000.0001       // BOR disabled, pwrt enabled
#pragma  config[03] = 0b0000.0000       // WDT disabled
#pragma  config[04] = 0b0000.0000       //
#pragma  config[05] = 0b0000.0001       //
#pragma  config[06] = 0b1000.0001       // no bg debug, no LVP, STVREN
#pragma  config[07] = 0b0000.0000       //
#pragma  config[08] = 0b0000.1111       //
#pragma  config[09] = 0b1100.0000       //
#pragma  config[10] = 0b0000.1111       //
#pragma  config[11] = 0b1110.0000       //
#pragma  config[12] = 0b0000.1111       //
#pragma  config[13] = 0b0100.0000       //

#define ADCGO  0b0000.0100               //  start of ADC

#define  delay()   W = 12;        \
                   X: W = W + 255;   \
                      skipIfZero(W); \
                      goto  X;


void main(void) {

     PORTA   = 0x00000000;               // PortA all off
     TRISA   = 0b00001011;               // PortA (3 pins An. input)
     PORTB   = 0;                                // PortB all off
     TRISB   = 0b00000000;               // PortB all output
     PORTC   = 0b00000000;              // PortC all off
     TRISC   = 0b00000000;               // PortC all output

     T2CKPS0 = 1;                       // ) prepare for 1:4
prescaling
     T2CKPS1 = 1;                       // ) start with 1:16
prescaling
     TMR2ON  = 1;                      // activate Timer2
     PR2     = 255;                         // long period ('low'
PWM freq)
     CCP1CON = 0b0000.1100;     // select PWM mode

     ADCON0  = 0b10.001.001;     // ADC: FRC, AN1, enable A/D
     ADCON1  = 0b0.0.00.0100;    // ADC: Left justified, Fosc/32,, 3/0


     for (;;) {                                  // forever
       delay();                                 // approx 12(?)
mus delay
       ADCON0 |= ADCGO;                // start ADC
       while (ADCON0 & ADCGO)     // poll for cleared state
         ;
       if (ADRESH > 225)                    // higher than approx 90%
         T2CKPS1 = 0;                            // set 1:4 prescaling
       else                                                // lower
         T2CKPS1 = 1;                           // set 1:16 prescaling
       CCPR1L = ~ADRESH;               // update duty cycle
       }

}

----------------------------------------------------------

--
Rob Hamerling, Vianen, NL phone +31-347-322822
homepage: http://www.robh.nl/

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