encoder.c

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/***************************************************************************
***  Fraunhofer IPA 
***  Robotersysteme 
***  Projekt: 3D cartography demonstrator
****************************************************************************
****************************************************************************
***  Autor: Julio Sagardoy (goa-js)
***************************************************************************/


#include <stdlib.h>

#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h>

#include "encoder.h"
#include "utils.h"

#define PHASE_A BITREAD(PIND,PD7)
#define PHASE_B BITREAD(PINE,PE6)

volatile int8_t enc_delta;      // Contains the difference between the last sample and the current sample in binary, if any
volatile int16_t enc_accum;     // Contains the absolute count of steps
static int16_t last;            // Contains last sampling binary values

void encoder_reset()
{
        enc_accum = 0;
        enc_delta = 0;
        last = 0;
}

int16_t encoder_read()
{
        return ~enc_accum;
}

ISR( TIMER3_COMPA_vect )
{
        int8_t new, diff;
        
        // convert from gray to binary, code by Peter Dannegger
        new = 0;
        if( PHASE_A )
                new = 3;
        if( PHASE_B )
                new ^= 1;

        diff = last - new;
        
        // See if there is difference with last
        if(diff & 1)    //      Bit 0 = value
        {
                last = new;                     //Store new as next last
                enc_delta += (diff & 2) - 1;    //BIt 1 = direction. enc_delta: 1/-1
                enc_accum += enc_delta;
                enc_delta = 0;
        }
}

void encoder_init()
{
        int8_t new;
        
        new=0;
        //Encoder output is gray-coded. At each sampling of the encoder input, we convert gray to binary, so value can be easily added or substracted. Gray code is:
        //____^^^^____^^^^____ PHASE_A
        //  ____^^^^____^^^^____        PHASE_B
        //  0 1 1 0 0 1 1 0 0 
        //  0 0 1 1 0 0 1 1 0

        if( PHASE_A )
                new = 3;
        if( PHASE_B )
                new ^= 1;
        last = new;
        
        enc_delta = 0;
        
        TCCR3A = 0x03;  //PWM, TOP=OCR3A, BOTTOM=TOV
        TCCR3B = 0x11;  //no preescaling
        OCR3A = 3500;// Sampling rate for loseless sampling calculation: stepper steps at 3ms/step max. 1 step of stepper are 5 encoder steps. This is 0.6ms/e_step. Nyquist with factor 2.5 = 0.24ms/e_sambple --> around OCR3A = 3500
        BITSET(TIMSK3,OCIE3A);  //enable interrupt at OC3A

        // Index signal is not sampled, instead, it triggers an external interrupt.
        BITSET(PCICR,PCIE0);
        BITSET(PCMSK0,PCINT0);
}