Prior to the actual doomsday, we plan to use DOOMSDAY as a race timer for mountain bike races.  To that end we need precision time keeping.  We have a 1 ms accuracy goal relative to an absolute GPS time reference.  The GPS module provides time data in NEMA format as well as a one pulse per second (1pps) reference signal.  When the GPS signal goes out of view, a real time clock chip keeps track of time and provides its own 1 Hertz signal: a 1 Hertz square wave.

In order to sync the 1pps signals to the real time clock square wave, I made some modifications to two libraries: Time, and TinyGPS (by Mikal Hart).  Precision timing is made possible by adding a 1Hz reference to the existing library.  Millisecond accuracy is obtained by keeping track of the last 1 Hz transistion and the 1 Hz duration.  So in addition to the standard year(), month(), day(), hour(), minute(), and second() functions, I added millisecond() which returns an integer between 0 and 999.

Some other functions were added to support the 1Hz reference:

/* TJS: one Hertz interrupt to be called on rising edge of one Hz square wave. 
 *      Used to sync with GPS clock or other 1Hz source to get millisecond time accuracy
 *      trigger is one of LOW, CHANGE, RISING, or FALLING
 */
void set_1Hz_ref(time_t current_time, int interrupt_pin, void(*cb_ptr)(), int trigger);

/*
 * TJS: stop counting ticks.  Used to sync with absolute time.
 */
void pause_1Hz();

/*
 * TJS: start counting ticks.  Used to sync with absolute time.
 */
void unpause_1Hz();

The TinyGPS library also needed to be modified to support precision time synchronization.  The key enabler is a fix notification scheme.  At the heart is a user provided function that gets called when a new fix message arrives.   This function was added to support this:

/*
 * TJS: Add a callback routine to be called when a new fix arrives.
 */
void TinyGPS::add_callback(fix_cb_t fct_ptr);

// Example fix callback signature
void grab_datetime(unsigned long _date,
                   unsigned long _time,
                   long lat,
                   long lon,
                   long alt,
                   unsigned long speed,
                   unsigned long course);