Implement RTC_Micros with tunable drift

RTClib.cpp

@@ -337,17 +337,55 @@ void RTC_DS1307::writenvram(uint8_t address, uint8_t data) {
 ////////////////////////////////////////////////////////////////////////////////
 // RTC_Millis implementation
 
-long RTC_Millis::offset = 0;
+// Alignment between the milis() timescale and the Unix timescale. These
+// two variables are updated on each call to now(), which prevents
+// rollover issues. Note that lastMillis is **not** the millis() value
+// of the last call to now(): it's the millis() value corresponding to
+// the last **full second** of Unix time.
+uint32_t RTC_Millis::lastMillis;
+uint32_t RTC_Millis::lastUnix;
 
 void RTC_Millis::adjust(const DateTime& dt) {
-    offset = dt.unixtime() - millis() / 1000;
+  lastMillis = millis();
+  lastUnix = dt.unixtime();
 }
 
+// Note that computing (millis() - lastMillis) is rollover-safe as long
+// as this method is called at least once every 49.7 days.
 DateTime RTC_Millis::now() {
-  return (uint32_t)(offset + millis() / 1000);
+  uint32_t elapsedSeconds = (millis() - lastMillis) / 1000;
+  lastMillis += elapsedSeconds * 1000;
+  lastUnix += elapsedSeconds;
+  return lastUnix;
 }
 
 ////////////////////////////////////////////////////////////////////////////////
+// RTC_Micros implementation
+
+// Number of microseconds reported by micros() per "true" (calibrated)
+// second.
+uint32_t RTC_Micros::microsPerSecond = 1000000;
+
+// The timing logic is identical to RTC_Millis.
+uint32_t RTC_Micros::lastMicros;
+uint32_t RTC_Micros::lastUnix;
+
+void RTC_Micros::adjust(const DateTime& dt) {
+  lastMicros = micros();
+  lastUnix = dt.unixtime();
+}
+
+// A positive adjustment makes the clock faster.
+void RTC_Micros::adjustDrift(int ppm) {
+  microsPerSecond = 1000000 - ppm;
+}
+
+DateTime RTC_Micros::now() {
+  uint32_t elapsedSeconds = (micros() - lastMicros) / microsPerSecond;
+  lastMicros += elapsedSeconds * microsPerSecond;
+  lastUnix += elapsedSeconds;
+  return lastUnix;
+}
 
 ////////////////////////////////////////////////////////////////////////////////
 // RTC_PCF8563 implementation

RTClib.h

@@ -121,15 +121,34 @@ class RTC_PCF8523 {
 };
 
 // RTC using the internal millis() clock, has to be initialized before use
-// NOTE: this clock won't be correct once the millis() timer rolls over (>49d?)
+// NOTE: this is immune to millis() rollover events
 class RTC_Millis {
 public:
     static void begin(const DateTime& dt) { adjust(dt); }
     static void adjust(const DateTime& dt);
     static DateTime now();
 
 protected:
-    static long offset;
+    static uint32_t lastUnix;
+    static uint32_t lastMillis;
+};
+
+// RTC using the internal micros() clock, has to be initialized before
+// use. Unlike RTC_Millis, this can be tuned in order to compensate for
+// the natural drift of the system clock. Note that now() has to be
+// called more frequently than the micros() rollover period, which is
+// approximately 71.6 minutes.
+class RTC_Micros {
+public:
+    static void begin(const DateTime& dt) { adjust(dt); }
+    static void adjust(const DateTime& dt);
+    static void adjustDrift(int ppm);
+    static DateTime now();
+
+protected:
+    static uint32_t microsPerSecond;
+    static uint32_t lastUnix;
+    static uint32_t lastMicros;
 };
 
 #endif // _RTCLIB_H_

keywords.txt

@@ -10,6 +10,7 @@ DateTime	KEYWORD1
 TimeSpan	KEYWORD1
 RTC_DS1307	KEYWORD1
 RTC_Millis	KEYWORD1
+RTC_Micros	KEYWORD1
 Ds1307SqwPinMode	KEYWORD1
 
 #######################################
@@ -27,6 +28,7 @@ secondstime	KEYWORD2
 unixtime	KEYWORD2
 begin	KEYWORD2
 adjust	KEYWORD2
+adjustDrift	KEYWORD2
 isrunning	KEYWORD2
 now	KEYWORD2
 readSqwPinMode	KEYWORD2