Servo Jumpy with DT Interrupt

[Mike, K8LH]

Have you considered using the PWM module? Many people don't realize you can run the PWM module at very low frequencies by using a software "helper" ISR (interrupt service routine) to link together several much smaller/faster PWM period "frames" to form a much longer/slower overall period.

This method provides many of the advantages of the PWM module (zero jitter, interrupt tolerant, very high resolution pulse width, etc.) at the cost of using a small low overhead ISR software "helper". You can also use the 1-msec interrupt interval as a basis for timer operations.

Perhaps one of the PBP gurus can convert, test, and qualify this code for you?

Happy Holidays!

Mike, K8LH

  //  setup a 1000 usec PWM period with PR2 = 249 and prescaler
  //  16:1 (16 MHz clock) or 4:1 (4 MHz clock) and then link 20
  //  of these 1000 usec PWM "frames" together to form the much
  //  longer 20-ms servo period (1-usec pulse width resolution)

  int servo = 1500;            // 0..20000 in 1 usec steps
  int frame = 1;               // frame number 1..20

  void interrupt()             //
  { int width;                 // work variable
    pir1.TMR2IF = 0;           // clear Timer 2 interrupt flag
    frame--;                   //
    if (frame == 0)            // if end of the 20 msec period
    { frame = 20;              // reset for new 20 msec period
      width = servo;           // reset 'width' work variable
    }
    if ((width > 1000)         // if width > 1000 usecs
    { ccpr1l = 250;            // do a 100% duty cycle frame
      ccp1con.CCP1Y = 0;       //
      ccp1con.CCP1X = 0;       //
      width -= 1000;           // subtract 1000 usecs
    }
    else                       // else do a 0..1000 usec frame
    { ccpr1l = (width >> 2);   // 0..250 (0..1000 usecs)
      ccp1con.CCP1Y = (width & 1);
      ccp1con.CCP1X = (width & 2);
      width = 0;               // force remaining frames to 0%
    }
  }