Low freq PWM problem

[Mike, K8LH]

You might also consider using the PWM module along with a small interrupt driven "helper" to link together several smaller PWM period "frames" into the much longer 120-Hz PWM period. This method produces a relatively low overhead high resolution jitter free PWM output.

I don't have PBP but perhaps you and other Forum members can muddle through this pseudo C code example.

This example uses a 12-MHz clock (Tcy = 0.3334-usecs) with PWM module settings that produce a PWM period of 333.3334-usecs (250 x 1.3334-usec 'steps'). The interrupt "helper" links together 25 of these short 333.3334-usec PWM period "frames" to produce a precise 120-Hz (8333.3334-usec) period with an output pulse width of 0 to 6250 x 1.3334-usec "steps".

Regards, Mike

;******************************************************************
;
;  // setup PWM for a 333.3 usec period (12-MHz xtal, Timer 2
;  // prescale 4:1, PR2 = 249) and use 25 of these 333.3 usec
;  // PWM "frames" for a precise 120-Hz (8.3334 msec) period.
;
;  char frame = 1;              // initialize "frame" var'
;  int width = 100;             // 0..6250 (1.333 usec steps)
;  int pulse;                   // isr work variable, 0..6250
;
;  void interrupt()             // 120-Hz example
;  { pir1.TMR2IF = 0;           // clear timer 2 interrupt flag
;    frame--;                   // decrement 'frame' number
;    if(frame == 0)             // if end-of-period
;    { frame = 25;              // reset for new period
;      pulse = width;           // refresh work variable
;    }
;    if(pulse > 250)            //
;    { ccpr1l = 250;            // do a 100% duty cycle frame
;      pulse -= 250;            // update 'pulse'
;    }
;    else                       //
;    { ccpr1l = pulse;          // do variable duty cycle
;      pulse = 0;               // force remaining frames to 0%
;    }
;
;
;
;  void Main()                  //
;  { cmcon0 = 7;                // comparators off
;    ansel = 0;                 // a2d convertor off
;    pir1 = 0;                  // clear all interrupt flags
;    pie1.TMR2IE = 1;           // enable TMR2 interrupts
;    ccpr1l = 0;                // set 0% duty cycle (output off)
;    t2con = 0b00000101;        // '0-------' unimplemented bit
;                               // '-0000---' TOUTPS<3:0>, postscale 1
;                               // '-----1--' TMR2ON, turn Timer 2 on
;                               // '------01' T2CKPS<1:0>, prescale 4
;    pr2 = 250-1;               // 250 x 1333-nsec 'ticks' = 333 usecs
;    intcon = 0b11000000;       // '1-------' GIE, enable global ints
;                               // '-1------' PEIE, enable peripheral ints
;                               // '--0-----' T0IE, TMR0 ints disabled
;                               // '----0---' GPIE, IOC disabled
;                               // '-----000' T0IF/INTF/GPIF flags
;    while(1)                   //
;    {                          //
;    }                          //
;  }
;