added i2c recovery code from esp8266/Arduino#1025

ESP8266CO2PM25/ESP8266CO2PM25.ino

@@ -54,6 +54,8 @@
 #define LED2     14     // led 2
 #define LEDON    LOW
 #define LEDOFF   HIGH
+#define SDAPIN   4
+#define CLKPIN   5
 
 // objects
 SoftwareSerial mSerial1(13, 15); // RX, TX (13,15)
@@ -62,6 +64,49 @@ piTimer          ptimer;
 hdc1080          hdc;
 bme280           bme;
 
+void i2crecovery() {
+  DEBUG_PRINTLN(F("I2C bus recovery..."));
+  delay(500);
+  //try i2c bus recovery at 100kHz = 5uS high, 5uS low
+  pinMode(SDAPIN, OUTPUT_OPEN_DRAIN);//keeping SDA high during recovery
+  pinMode(CLKPIN, OUTPUT_OPEN_DRAIN);
+  digitalWrite(SDAPIN, HIGH);
+  digitalWrite(CLKPIN, HIGH);
+
+  for (int i = 0; i < 10; i++) { //9nth cycle acts as NACK
+    digitalWrite(CLKPIN, HIGH);
+    delayMicroseconds(5);
+    digitalWrite(CLKPIN, LOW);
+    delayMicroseconds(5);
+  }
+
+  //a STOP signal (SDA from low to high while CLK is high)
+  digitalWrite(SDAPIN, LOW);
+  delayMicroseconds(5);
+  digitalWrite(CLKPIN, HIGH);
+  delayMicroseconds(2);
+  digitalWrite(SDAPIN, HIGH);
+  delayMicroseconds(2);
+  //bus status is now : FREE
+
+  //return to power up mode
+  pinMode(SDAPIN, INPUT_PULLUP);
+  pinMode(CLKPIN, INPUT_PULLUP);
+  delay(500);
+
+  bool isok = digitalRead(SDAPIN) && digitalRead(CLKPIN);
+  if (!digitalRead(SDAPIN))
+    DEBUG_PRINTLN(F("I2C bus error: SDA still LOW."));  
+  if (!digitalRead(CLKPIN))
+    DEBUG_PRINTLN(F("I2C bus error: CLK still LOW."));  
+
+  Wire.pins(SDAPIN, CLKPIN); 
+  Wire.begin(SDAPIN, CLKPIN);
+
+  if (isok)
+    DEBUG_PRINTLN(F("I2C bus recovery complete."));  
+}
+
 ///////////////////////////////////////////////////////////////////////////
 //   Setup() and loop()
 ///////////////////////////////////////////////////////////////////////////
@@ -100,8 +145,8 @@ void setup() {
   DEBUG_PRINTLN(str);
 
   // hdc1080
-  Wire.pins(4, 5); // (SDA, SCL) changes default values
-  Wire.begin(4, 5); // (SDA, SCL)
+  Wire.pins(SDAPIN, CLKPIN); // (SDA, SCL) changes default values
+  Wire.begin(SDAPIN, CLKPIN); // (SDA, SCL)
   hdc.begin(&logger);
   hdc.SetMQTT(&mqtt, SF("THConnected"), SF("Temperature"), SF("Humidity"), SF("Heater"));