Added processing of fractional time, setting of currentEpoch, sanity checking of received timestamps, and new example.

NTPClient.cpp

@@ -102,19 +102,27 @@ bool NTPClient::forceUpdate() {
     timeout++;
   } while (cb == 0);
 
-  this->_lastUpdate = millis() - (10 * (timeout + 1)); // Account for delay in reading the time
-
   this->_udp->read(this->_packetBuffer, NTP_PACKET_SIZE);
 
   unsigned long highWord = word(this->_packetBuffer[40], this->_packetBuffer[41]);
   unsigned long lowWord = word(this->_packetBuffer[42], this->_packetBuffer[43]);
   // combine the four bytes (two words) into a long integer
   // this is NTP time (seconds since Jan 1 1900):
   unsigned long secsSince1900 = highWord << 16 | lowWord;
-
-  this->_currentEpoc = secsSince1900 - SEVENZYYEARS;
-
-  return true;  // return true after successful update
+  /// return early if received timestamp is invalid
+  if (secsSince1900 == 0) return false;
+
+  unsigned long unixEpoch = secsSince1900 - SEVENZYYEARS;
+  /// compute the fractional time. we will use it to sync to the nearest millis() 
+  highWord = word(this->_packetBuffer[44], this->_packetBuffer[45]);
+  lowWord = word(this->_packetBuffer[46], this->_packetBuffer[47]);
+  unsigned long fractionPart = highWord << 16 | lowWord;
+  unsigned long milliseconds = ((unsigned long long) fractionPart * 1000) >> 32;
+
+  this->_lastUpdate = (unsigned long) (millis() - milliseconds);
+  this->_currentEpoc = unixEpoch;
+
+  return true;
 }
 
 bool NTPClient::update() {
@@ -131,9 +139,24 @@ bool NTPClient::isTimeSet() const {
 }
 
 unsigned long NTPClient::getEpochTime() const {
+  unsigned long t = millis();
+  /// if millis() has rollover, get difference between lastUpdate and maximum value of uint32_t.
+  /// then add it to current value of millis() and add 1 for when millis() was 0.
+  if (t < this->_lastUpdate) {
+    t = (((unsigned long) -1) - this->_lastUpdate) + t + 1; 
+  } else {
+    t = t - this->_lastUpdate;   
+  }
   return this->_timeOffset + // User offset
-         this->_currentEpoc + // Epoch returned by the NTP server
-         ((millis() - this->_lastUpdate) / 1000); // Time since last update
+         this->_currentEpoc + // Epoc returned by the NTP server
+         (t / 1000); // Time since last update
+}
+
+unsigned long NTPClient::setEpochTime(unsigned long localTime) {
+    this->_currentEpoc = localTime - this->_timeOffset;
+    this->_lastUpdate = millis();
+    /// Return Unix epoch
+    return (this->_currentEpoc);
 }
 
 int NTPClient::getDay() const {

NTPClient.h

@@ -18,7 +18,7 @@ class NTPClient {
     unsigned int  _port           = NTP_DEFAULT_LOCAL_PORT;
     long          _timeOffset     = 0;
 
-    unsigned long _updateInterval = 60000;  // In ms
+    unsigned long _updateInterval = 3600000;  // In ms
 
     unsigned long _currentEpoc    = 0;      // In s
     unsigned long _lastUpdate     = 0;      // In ms
@@ -107,6 +107,14 @@ class NTPClient {
      */
     unsigned long getEpochTime() const;
 
+    /**
+     * Set internal _currentEpoch from the localTime.  Useful when a hardware
+     * RTC is used to set the initial unix time of NTPClient.
+     * @param localTime, current local time in seconds since Jan. 1, 1970
+     * @return unix epoch in seconds (GMT) since Jan. 1, 1970
+     */
+    unsigned long setEpochTime(unsigned long localTime);
+
     /**
      * Stops the underlying UDP client
      */

examples/Advanced_with_RTC/Advanced_with_RTC.ino

@@ -0,0 +1,91 @@
+/* Advanced NTPClient With RTC example 
+ *  Copyright (c) 2021 by Anthony F. Dela Paz
+ *  
+ * Featuring:
+ *  + an RTC (RTCLib can use hardware RTC by simply changing 'RTC_Millis' with appropriate declaration).
+ *  + DateTime facility of the RTCLib to facilitate epochtime conversion
+ *  + separately setting the ntp server, timezone offset, and update interval
+ *  + syncing the internal software RTC of the NTPClient with the RTC time
+ *  
+ * This example code demonstrates NTPClient's new capability of syncing with NTP time server
+ * down to millisecond accuracy.  It's best to upload this code to at least two ESP-12E modules
+ * to see how both their blue LED blinks simultaneously upon synchronization with NTP server.
+ *  
+ * The MIT License (MIT) 
+ * 
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#include <ESP8266WiFi.h>
+#include <WiFiUdp.h>
+#include <NTPClient.h>
+#include "RTClib.h"
+
+const char *ssid     = "<SSID>"; // change the SSID and PASSWORD
+const char *password = "<PASSWORD>";
+
+WiFiUDP ntpUDP;
+NTPClient timeClient(ntpUDP);
+
+RTC_Millis rtc; // this should be a hardware RTC but that won't work as a quick example
+
+const byte ledBlue = 2;  // Blue LED on the ESP-12E module
+unsigned long oldTick, blinkMillis;
+
+void setup() {
+
+  pinMode(ledBlue, OUTPUT);
+
+  Serial.begin(115200);
+
+  WiFi.begin(ssid, password);
+
+  while ( WiFi.status() != WL_CONNECTED ) {
+    delay ( 500 );
+    Serial.print ( "." );
+  }
+
+  // following line sets the RTC to the date & time this sketch was compiled
+  rtc.begin(DateTime(F(__DATE__), F(__TIME__)));
+  // This line sets the RTC with an explicit local date & time, for example to set
+  // June 9, 2021 at 12:52:36 PM, you would call:
+  // rtc.adjust(DateTime(2021, 6, 9, 12, 52, 30));
+
+  timeClient.begin();
+  timeClient.setTimeOffset((-4) * 3600); // in seconds, Time zone in New York, NY, USA (GMT-4)
+  timeClient.setPoolServerName("europe.pool.ntp.org");
+  timeClient.setUpdateInterval(3600); // seconds
+  timeClient.setEpochTime(rtc.now().unixtime()); // set the timeClient's internal software rtc time.
+}
+
+void loop() {
+  timeClient.update();
+
+  // Synchronize the blinking LED with transition of seconds.
+  if (timeClient.getEpochTime() != oldTick) {
+    oldTick = timeClient.getEpochTime();
+    blinkMillis = millis();
+    Serial.println(timeClient.getFormattedTime());
+  }
+
+  // blink the LED once per second with 500ms ON-time
+  if ((millis() - blinkMillis) % 1000 > 500) {
+    digitalWrite(ledBlue, HIGH);
+  } else {
+    digitalWrite(ledBlue, LOW);
+  }
+}

keywords.txt

@@ -22,3 +22,4 @@ getEpochTime	KEYWORD2
 setTimeOffset	KEYWORD2
 setUpdateInterval	KEYWORD2
 setPoolServerName	KEYWORD2
+setEpochTime    KEYWORD2