Merge pull request #199 from Gambalunga/master

Added calibration method example PCF8523

examples/pcf8523/pcf8523.ino

@@ -44,6 +44,29 @@ void setup () {
   // to be restarted by clearing the STOP bit. Let's do this to ensure
   // the RTC is running.
   rtc.start();
+  
+   // The PCF8523 can be calibrated for:
+  //        - Aging adjustment
+  //        - Temperature compensation
+  //        - Accuracy tuning
+  // The offset mode to use, once every two hours or once every minute.
+  // The offset Offset value from -64 to +63. See the Application Note for calculation of offset values.
+  // https://www.nxp.com/docs/en/application-note/AN11247.pdf
+  // The deviation in parts per million can be calculated over a period of observation. Both the drift (which can be negative)
+  // and the observation period must be in seconds. For accuracy the variation should be observed over about 1 week.
+  // Note: any previous calibration should cancelled prior to any new observation period.
+  // Example - RTC gaining 43 seconds in 1 week
+  float drift = 43; // seconds plus or minus over oservation period - set to 0 to cancel previous calibration.
+  float period_sec = (7 * 86400);  // total obsevation period in seconds (86400 = seconds in 1 day:  7 days = (7 * 86400) seconds )
+  float deviation_ppm = (drift / period_sec * 1000000); //  deviation in parts per million (μs)
+  float drift_unit = 4.34; // use with offset mode PCF8523_TwoHours
+  // float drift_unit = 4.069; //For corrections every min the drift_unit is 4.069 ppm (use with offset mode PCF8523_OneMinute)
+  int offset = round(deviation_ppm / drift_unit);
+  // rtc.calibrate(PCF8523_TwoHours, offset); // Un-comment to perform calibration once drift (seconds) and observation period (seconds) are correct
+  // rtc.calibrate(PCF8523_TwoHours, 0); // Un-comment to cancel previous calibration
+
+  Serial.print("Offset is "); Serial.println(offset); // Print to control offset
+
 }
 
 void loop () {