File content as of revision 0:75e3cf27c60b:

#include "mbed.h"
 
DigitalIn phaseA( p5 ); // phase a of the quadrature encoder
DigitalIn phaseB( p6 ); // phase b of the quadrature encoder
 
int encoderClickCount    = 0; // hold the signed value corresponding to the number of clicks left or right since last sample
int previousEncoderState = 0; // keep a record of the last actioned sample state of the Qb+Qa outputs for comparison on each interrupt
 
void quadratureDecoder( void )
{
    int currentEncoderState = (phaseB.read() << 1) + phaseA.read(); // create a two bit value out of phaseB and phaseA
    
    if( currentEncoderState == previousEncoderState )
    {
        return;
    }
    
    switch( previousEncoderState )
    {
        case 0:
            if( currentEncoderState == 1 )
            {
                encoderClickCount--;
            }
            else if( currentEncoderState == 2 )
            {
                encoderClickCount++;
            }
            break;
            
        case 1:
            if( currentEncoderState == 3 )
            {        
                encoderClickCount--;
            }
            else if( currentEncoderState == 0 )
            {
                encoderClickCount++;
            }
            break;
            
        case 2:
            if( currentEncoderState == 0 )
            {
                encoderClickCount--;
            }
            else if( currentEncoderState == 3 )
            {
                encoderClickCount++;
            }
            break;
            
        case 3:
            if( currentEncoderState == 2 )
            {
                encoderClickCount--;
            }
            else if( currentEncoderState == 1 )
            {
                encoderClickCount++;
            }
            break;

        default:
            break;
    }
    previousEncoderState = currentEncoderState;
}
 
 // read the rotation accumulator, and once read, reset it to zero
int getClicks( void )
{
   int res = encoderClickCount; // this allows the knob to be rotated "while im not looking at it' and still return the 
    
    encoderClickCount = 0;      // actual number of clicks that have been rotated since last time I was checking it.

    return res;
}
 
int main()
{
    Serial pc( USBTX, USBRX );
    Ticker sampleTicker;                                // create a timer to sample the encoder
    int    currentClicks;

    phaseA.mode( PullUp );                              // phaseA is set with a built in pull-up resistor
    phaseB.mode( PullUp );                              // phaseB is set with a built in pull-up resistor    
    sampleTicker.attach_us( &quadratureDecoder, 1000 ); // make the quadrature decoder function check the knob once every 1000us = 1ms

    pc.printf( "Encoder test started\r\n" );
    while( 1 )
    {
        currentClicks = getClicks();
        if( currentClicks != 0 )
        {
            pc.printf( "click count = %d,%d\r\n", currentClicks, previousEncoderState );
        }
        wait_ms( 200 );
    }
}