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Input References

An input reference is an interface to a time reference. While a time reference can take many forms its fundamental responsibilities are to:

  • Provide a 1PPS on-time-point
  • Provide a time-of-day and date input (usually), and
  • Provide a control and status protocol to the time source (sometimes).

The input references can also provide other time related data such as Leap Second indication and number of seconds, Julian date, day of year, day of week, week of year, status, sync indication, accuracy indication and other fields unrelated to time.

The TSync Time Code Processor architecture’s input reference subsystem is designed to support multiple possible input references, while allowing only a single time reference and 1PPS reference to discipline the local clock subsystem at any given time. The user can choose to use the factory default priority list for input references, or may define a proprietary priority list. This user priority list can be created to combine different time and 1PPS sources (such as GNSS time coupled with the external 1PPS). The user is also provided with the means to enable or disable the selection of a specified input reference allowing them to disregard the list and synchronize to a specific output.

Built-in References

The TSync Time Code Processor card provides built-in references that support specialized user applications.

Input Reference Monitor

The input reference monitor subsystem maintains the reference priority table and determines which input reference(s) are selected to synchronize the clock subsystem.

Three tables are maintained by the system:

  • A default table, which provides the default reference pairings in timing accuracy priority
  • A working table, which is the table used for selecting reference inputs
  • A user table, which can be stored persistently and, if present, will be loaded into the working table at startup.

Example default table

Enable Priority Time Ref 1PPS Ref
en 1 gps0 gps0
en 2 ird0 ird0
en 3 ira0 ira0
en 4 hst0 epp0
en 5 hst0 self
dis 6 self self
dis 0    
dis 0    

Legend: gps = GPS Reference, ird = IRIG DCLS Reference, ira = IRIG AM Reference, epp = External 1PPS Reference, hst0 = Host Reference, self = Self Reference

 

Entries can be added to and deleted from the working table. In addition, individual entries can be enabled or disabled. Their priorities can be changed at any time.  Any changes to the table will cause the reference monitor to reevaluate the best reference to use for synchronization.  The working table can be saved to the user table and persisted, or it can be reset to the default table or an already existing user table at any time.

At any given time, the highest priority enabled entry in the table that has both a valid time and a valid 1PPS reference will be used as the best reference for synchronization. The power of the reference monitor is in its ability to generate any combination of time and 1PPS references in any priority.  For example, if a user has a high precision 1PPS source, this can serve as the provided external 1PPS reference and can be paired with a GNSS time reference.

The reference tables, the currently selected best reference, and the current validity states of all input references can be requested from the card.