PIC Micro Controller C Input / Output Routine

Quadrature Encoder ISR

by David Kott

struct RotaryEncoderStruct {
      int unused:4;  // RB[0:3]
      int A:1;   // RB4
      int B:1;   // RB5
      int unused2:2;  // RB6-7
};

struct RotaryEncoderStruct RotaryEncoder;

#pragma BYTE PORTB = 0x06

#pragma BYTE RotaryEncoder = 0x06 // Place structure right over PORTB at
location 0x06


USHORT g_usEncoderCount = 0;

#pragma INT_RB
void
PortBInterrupt (void)
{

      static BYTE OldPortB;
      BYTE byTemp;
      static struct RotaryEncoderStruct OldEncoder;


      if (OldPortB != (byTemp=(PORTB & 0x30))) {


      OldPortB = byTemp;

      if (RotaryEncoder.A == OldEncoder.A) {
     if (RotaryEncoder.B == RotaryEncoder.A)
    --g_usEncoderCount;
     else
    ++g_usEncoderCount;
      } else {
     OldEncoder.A = RotaryEncoder.A;
     if (RotaryEncoder.B == RotaryEncoder.A)
    ++g_usEncoderCount;
     else
    --g_usEncoderCount;
      }



     }

   // Handle more PORTB stuff here, like a button, or something.


}

Questions:

• Ken T Macfarlane of p.s. autogrinding asks:

  How many machine cycles does this take to run (worst case)? This sets the maximum rotational speed of encoder that can be tracked (tempered by the processor clock speed and the encoder resolution in lines/rev, obviously)

  +

Comments:

• karl.ratcliff at spamtiscali.co.uk " Doesn't check for overrun or error conditions. The output of the encoder will be gray code changing by one bit, so a two bit change = error condition" +

  .