Gambalunga branch 1 (read the PCF8523 offset register)

examples/pcf8523_calibrate/README.txt

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+  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. A positive offset makes the clock slower.
+  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.
+  Recommendation: 
+	Syncronise host PC time.
+  	run this sketch cancelling any previous calibration,
+  	record the output including timestamp,
+  	after several days again syncronise host PC time,
+	run sketch again record the output including timestamp,
+  	calculate period of observation in seconds, and drift in seconds.
+  	Run sketch with the calculated figures and uncomment rtc.calibrate line as required.
+  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 (?s)
+  	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)
+  	int8_t 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_OneMinute, offset); // // Un-comment to perform calibration with offset mode PCF8523_OneMinute
+  	// rtc.calibrate(PCF8523_TwoHours, 0); // Un-comment to cancel previous calibration
+
+In order to provide a method of reading the offset register, which may contain an previous calibration
+two methods are provided; 1. rtc.readOffsetReg(), or 2. rtc.getOffsetMode() and rtc.getOffset()
+See the example sketch: pcf8523_calibrate.ino
+
+Hint:
+	Once the calibration Offset mode and Offset are known a line can be entered in the setup of the operating project sketch
+	to re-establish the offset register after a battery replacement or clock reset. Note that your sketch will still require a method
+	to insert the actual date and time.
+	In the case of the above sample the line to insert in setup() would be:
+  	rtc.calibrate(PCF8523_TwoHours, 16); // re-insert previously calculated calibration after clock reset. 

examples/pcf8523_calibrate/pcf8523_calibrate.ino

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+// Date and time functions using a PCF8523 RTC connected via I2C and Wire lib
+#include "RTClib.h"
+
+RTC_PCF8523 rtc;
+
+char daysOfTheWeek[7][12] = { "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday" };
+
+void setup() {
+  Serial.begin(57600);
+  delay(500);
+  while (!Serial)
+    ;  // wait for serial port to connect. Needed for native USB
+
+  if (!rtc.begin()) {
+    Serial.println("Couldn't find RTC");
+    Serial.flush();
+    while (1) delay(10);
+  }
+
+  if (!rtc.initialized() || rtc.lostPower()) {
+    Serial.println("RTC is NOT initialized, let's set the time!");
+    // When time needs to be set on a new device, or after a power loss, the
+    // following line sets the RTC to the date & time this sketch was compiled
+    rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
+    // This line sets the RTC with an explicit date & time, for example to set
+    // January 21, 2014 at 3am you would call:
+    // rtc.adjust(DateTime(2014, 1, 21, 3, 0, 0));
+    //
+    // Note: allow 2 seconds after inserting battery or applying external power
+    // without battery before calling adjust(). This gives the PCF8523's
+    // crystal oscillator time to stabilize. If you call adjust() very quickly
+    // after the RTC is powered, lostPower() may still return true.
+  }
+
+  // When time needs to be re-set on a previously configured device, the
+  // following line sets the RTC to the date & time this sketch was compiled
+  // rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
+  // This line sets the RTC with an explicit date & time, for example to set
+  // January 21, 2014 at 3am you would call:
+  // rtc.adjust(DateTime(2014, 1, 21, 3, 0, 0));
+
+  // When the RTC was stopped and stays connected to the battery, it has
+  // 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.
+  // Recommendation: Syncronise host PC time.
+  // run this sketch cancelling any previous calibration,
+  // record the output including timestamp,
+  // run sketch again after several days,
+  // calculate period of observation in seconds, and drift in seconds.
+  // Run sketch with the calculated figures and uncomment rtc.calibrate line as required.
+  // 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)
+  int8_t 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_OneMinute, offset); // // Un-comment to perform calibration with offset mode PCF8523_OneMinute
+  // rtc.calibrate(PCF8523_TwoHours, 0); // Un-comment to cancel previous calibration
+
+  Serial.println();
+  Serial.print("Calculated Offset for calibration is: ");
+  Serial.println(offset);  // Print to control calculated offset
+
+  // read offset register  *******************************
+  Serial.println("Read RTC PCF8523 Offset Register");  // Print to control offset
+
+  // Method 1 ****************************
+  int8_t OffsetReg = rtc.readOffsetReg();
+  Serial.print("Offset mode is: ");
+  if bitRead (OffsetReg, 7) {
+    Serial.println("PCF8523_OneMinute");
+  } else {
+    Serial.println("PCF8523_TwoHours ");
+  }
+  offset = OffsetReg;
+  bitWrite(offset, 7, bitRead(OffsetReg, 6));
+  Serial.print("Offset is: ");
+  Serial.println(offset);  // Print to control offset
+
+  // Method 2 ****************************
+  String OffsetMode = String(rtc.getOffsetMode());
+  Serial.print("Offset mode is: ");
+  Serial.println(OffsetMode);
+
+  offset = rtc.getOffset();
+  Serial.print("Offset is: ");
+  Serial.println(offset);  // Print to control offset
+                           // End read offset register  *******************************
+
+  DateTime now = rtc.now();
+
+  Serial.print(now.year(), DEC);
+  Serial.print('/');
+  Serial.print(now.month(), DEC);
+  Serial.print('/');
+  Serial.print(now.day(), DEC);
+  Serial.print(" (");
+  Serial.print(daysOfTheWeek[now.dayOfTheWeek()]);
+  Serial.print(") ");
+  Serial.print(now.hour(), DEC);
+  Serial.print(':');
+  Serial.print(now.minute(), DEC);
+  Serial.print(':');
+  Serial.print(now.second(), DEC);
+  Serial.println();
+}
+
+void loop() {
+
+  // do nothing
+}