MCLK for I2S arduino-pico using PIO -- see https://github.com/earlephilhower/arduino-pico/issues/647

MCLK.cpp

#include <Arduino.h>
#include "MCLK.h"
#include "mclk.pio.h"


MCLK::MCLK() {
  _running = false;
  _pin = 29;
  _freq = 48000;
}

MCLK::~MCLK() {
}

bool MCLK::setPin(pin_size_t pin) {
  if (_running) {
    return false;
  }
  _pin = pin;
  return true;
}

bool MCLK::setFrequency(unsigned int newFreq, unsigned int newMultiplier) {
  _freq = newFreq;
  _multiplier = newMultiplier;
  if (_running) {
    float bitClk = _freq * _multiplier * 2.0 /* edges per clock */;
    pio_sm_set_clkdiv(_pio, _sm, (float)clock_get_hz(clk_sys) / bitClk);
  }
  return true;
}


bool MCLK::begin() {
  _running = true;
  int off = 0;
  _mclk = new PIOProgram(&mclk_program);
  _mclk->prepare(&_pio, &_sm, &off);
  mclk_program_init(_pio, _sm, off, _pin);
  setFrequency(_freq, _multiplier);
  pio_sm_set_enabled(_pio, _sm, true);
  return true;
}

void MCLK::end() {
  _running = false;
  delete _mclk;
  _mclk = nullptr;
}

MCLK.h

#pragma once

class MCLK {
  public:
    MCLK();
    virtual ~MCLK();

    bool setPin(pin_size_t pin);
    // set MCLK clock, based on audio sampling rate (e.g. 48000hz) and multiplier (typically 256, check specs)
    bool setFrequency(unsigned int newFreq, unsigned int newMultiplier);

    bool begin(unsigned int sampleRate, unsigned int multiplier ) {
      setFrequency(sampleRate, multiplier);
      return begin();
    }

    bool begin();
    void end();

  private:
    pin_size_t _pin;
    unsigned int _freq = 48000;
    unsigned int _multiplier = 256;

    bool _running;

    PIOProgram *_mclk;
    PIO _pio;
    int _sm;
};

mclk.pio

 .program mclk

; Generate mclk clock

.wrap_target
    set pins, 1 ; Turn on
    set pins, 0 ; Turn off, most complicated ever
.wrap

% c-sdk {

// Helper function (for use in C program) to initialize this PIO program
void mclk_program_init(PIO pio, uint sm, uint offset, uint pin) {

    // Sets up state machine and wrap target. This function is automatically
    // generated in mclk.pio.h.
    pio_sm_config c = mclk_program_get_default_config(offset);

    // Allow PIO to control GPIO pin (as output)
    pio_gpio_init(pio, pin);

    // Connect pin to SET pin (control with 'set' instruction)
    sm_config_set_set_pins(&c, pin, 1);

    // Set the pin direction to output (in PIO)
    pio_sm_set_consecutive_pindirs(pio, sm, pin, 1, true);
    
    // Load configuration and jump to start of the program
    pio_sm_init(pio, sm, offset, &c);
}

%}

mclk.pio.h

// -------------------------------------------------- //
// This file is autogenerated by pioasm; do not edit! //
// -------------------------------------------------- //

#pragma once

#if !PICO_NO_HARDWARE
#include "hardware/pio.h"
#endif

// ---- //
// mclk //
// ---- //

#define mclk_wrap_target 0
#define mclk_wrap 1

static const uint16_t mclk_program_instructions[] = {
            //     .wrap_target
    0xe001, //  0: set    pins, 1                    
    0xe000, //  1: set    pins, 0                    
            //     .wrap
};

#if !PICO_NO_HARDWARE
static const struct pio_program mclk_program = {
    .instructions = mclk_program_instructions,
    .length = 2,
    .origin = -1,
};

static inline pio_sm_config mclk_program_get_default_config(uint offset) {
    pio_sm_config c = pio_get_default_sm_config();
    sm_config_set_wrap(&c, offset + mclk_wrap_target, offset + mclk_wrap);
    return c;
}

// Helper function (for use in C program) to initialize this PIO program
void mclk_program_init(PIO pio, uint sm, uint offset, uint pin) {
    // Sets up state machine and wrap target. This function is automatically
    // generated in mclk.pio.h.
    pio_sm_config c = mclk_program_get_default_config(offset);
    // Allow PIO to control GPIO pin (as output)
    pio_gpio_init(pio, pin);
    // Connect pin to SET pin (control with 'set' instruction)
    sm_config_set_set_pins(&c, pin, 1);
    // Set the pin direction to output (in PIO)
    pio_sm_set_consecutive_pindirs(pio, sm, pin, 1, true);
    // Load configuration and jump to start of the program
    pio_sm_init(pio, sm, offset, &c);
}

#endif

test_mclk.ino

// minimal example to setup MCLK clock with PIO, used that in combination with I2S library
#include "MCLK.h"
// changing clock
#include <pico/stdlib.h>

MCLK mclk;
#define pMCLK 18 // pin 24

// chose arbitrary samplerate so we can have a cpu clock which is a divider of the MCLK
const int sampleRate =  40000;
// typical for many DAC
const int mclkMultiplier = 256;

void setup() {
  // set CPU speed for a good ratio with 256*fs -- flag to bock if requested speed cannot be achieved
  set_sys_clock_khz(102400, true);

  Serial.begin(115200);
  mclk.setPin(pMCLK);
  if (!mclk.begin(sampleRate, mclkMultiplier)) {
    while (1) {
      Serial.println("Failed to initialize MCLK!");
      delay(1000);
    }
  }
}

void loop() {
  // do your things
  delay(1000);
}