quadrature_encoder.c

/**
 * Copyright (c) 2021 pmarques-dev @ github
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

#include <stdio.h>
#include "pico/stdlib.h"
#include "hardware/pio.h"
#include "hardware/timer.h"

#include "quadrature_encoder.pio.h"

//
// ---- quadrature encoder interface example
//
// the PIO program reads phase A/B of a quadrature encoder and increments or
// decrements an internal counter to keep the current absolute step count
// updated. At any point, the main code can query the current count by using
// the quadrature_encoder_*_count functions. The counter is kept in a full
// 32 bit register that just wraps around. Two's complement arithmetic means
// that it can be interpreted as a 32-bit signed or unsigned value, and it will
// work anyway.
//
// As an example, a two wheel robot being controlled at 100Hz, can use two
// state machines to read the two encoders and in the main control loop it can
// simply ask for the current encoder counts to get the absolute step count. It
// can also subtract the values from the last sample to check how many steps
// each wheel as done since the last sample period.
//
// One advantage of this approach is that it requires zero CPU time to keep the
// encoder count updated and because of that it supports very high step rates.
//

int main() {
    int new_value, delta, old_value = 0;

    // Base pin to connect the A phase of the encoder.
    // The B phase must be connected to the next pin
    const uint PIN_AB = 10;

    stdio_init_all();

    PIO pio = pio0;
    const uint sm = 0;

    uint offset = pio_add_program(pio, &quadrature_encoder_program);
    quadrature_encoder_program_init(pio, sm, offset, PIN_AB, 0);

    while (1) {
        // note: thanks to two's complement arithmetic delta will always
        // be correct even when new_value wraps around MAXINT / MININT
        new_value = quadrature_encoder_get_count(pio, sm);
        delta = new_value - old_value;
        old_value = new_value;

        printf("position %8d, delta %6d\n", new_value, delta);
        sleep_ms(100);
    }
}