esp8266/libraries/I2S/examples/SimpleTone/SimpleTone.ino

/*
  This example generates a square wave based tone at a specified frequency
  and sample rate. Then outputs the data using the I2S interface to a
  MAX08357 I2S Amp Breakout board.

  created 17 November 2016
  by Sandeep Mistry
  modified for ESP8266 by Earle F. Philhower, III <earlephilhower@yahoo.com>
*/

#include <I2S.h>

const int frequency = 440;    // frequency of square wave in Hz
const int amplitude = 500;    // amplitude of square wave
const int sampleRate = 8000;  // sample rate in Hz

const int halfWavelength = (sampleRate / frequency);  // half wavelength of square wave

short sample = amplitude;  // current sample value
int count = 0;

void setup() {
  Serial.begin(115200);
  Serial.println("I2S simple tone");

  // start I2S at the sample rate with 16-bits per sample
  if (!I2S.begin(I2S_PHILIPS_MODE, sampleRate, 16)) {
    Serial.println("Failed to initialize I2S!");
    while (1)
      ;  // do nothing
  }
}

void loop() {
  if (count % halfWavelength == 0) {
    // invert the sample every half wavelength count multiple to generate square wave
    sample = -1 * sample;
  }

  // write the same sample twice, once for left and once for the right channel
  I2S.write(sample);
  I2S.write(sample);

  // increment the counter for the next sample
  count++;
}