#include <Wire.h>
#include <ESP8266WiFi.h>
#include <PolledTimeout.h>
#define SDA_PIN 4
#define SCL_PIN 5
const int16_t I2C_MASTER = 0x42;
const int16_t I2C_SLAVE = 0x08;
String i2c_clock = "";   // stores the frequency request

void setup() {
  Serial.begin(74880);           // start serial for output
  Serial.println();
  Serial.println("Starting...");
  Wire.begin(SDA_PIN, SCL_PIN, I2C_MASTER); // join i2c bus (address optional for master)
  Wire.setClock(100000L);
  WiFi.persistent(false);
  WiFi.mode(WIFI_OFF);
  delay(100);
  WiFi.forceSleepBegin();
  yield();
}

byte x = 0;

void loop() {
  using periodic = esp8266::polledTimeout::periodicMs;
  static periodic nextPing(100);

  if (nextPing) {
    Wire.beginTransmission(I2C_SLAVE); // transmit to device #8
    Wire.write("x is ");        // sends five bytes
    Wire.write(x);              // sends one byte
    Wire.endTransmission();    // stop transmitting

//    Serial.println (x);

    x++;
  }
    if (Serial.available()) {
    i2c_clock = Serial.readStringUntil('\n');  // enter the I2C bus speed in the serial monitor
    Serial.print("bus clock = ");
    Serial.println(i2c_clock);
    unsigned int freq = (i2c_clock.toInt());
    unsigned int period_nS = (500000000/freq);  // half-cycle period in ns
    Serial.print("period = ");
    Serial.print(period_nS);
    Serial.println("ns");
#if F_CPU == FCPU80
    unsigned int twi_dcount = (1000*(period_nS - 1120)) / 62500;  // (half cycle period - overhead) / busywait loop time 
#else
    unsigned int twi_dcount = (1000*(period_nS - 560)) / 31250;  // (half cycle period - overhead) / busywait loop time 
#endif
    Serial.print("busywait = ");
    Serial.print(twi_dcount);
    Serial.println(" loops");
    Serial.println();
    Wire.setClock(freq);
    }
}