Where: YYYY: Year (Example: "2017"), and ZZZ: Day of year (Example: "050" for 19-Febru-
ary)
3.4 Holdover Mode
When input references have been supplying input to SecureSync and input from all the ref-
erences has been lost, SecureSync will not immediately declare loss of time synchronization,
but first will go into Holdover mode. While the unit is in Holdover mode, the time outputs are
derived from the internal 10MHz oscillator incrementing the System Time, but the oscillator is
not disciplined/steered by the external reference e.g., GNSS.
Because of the stability of the internal oscillator, accurate time can still be derived even after all
the primary references are no longer valid or present. The more stable the oscillator is without
an external reference, the longer this holdover period can be and have it still maintain very
accurate outputs. The benefit of Holdover is that time synchronization and the availability of the
time outputs is not immediately lost when input references are no longer available.
While SecureSync is in Holdover, the only difference is the Holdover and associated Minor
alarm are asserted. There are no changes to NTP or any of the other outputs, i.e. while in Hol-
dover mode, NTP inside SecureSync continues to be at the same Stratum level it was at before
going into Holdover mode (such as Stratum1 when synced to GPS). Should the Holdover
period expire, however, or the unit is rebooted, the NTP Stratum will go to 16, preventing any
clients from being able to sync with SecureSync until GPS or another reference has been
restored.
How long will the unit remain in Holdover mode?
SecureSync will remain in Holdover mode until either:
a.
Any enabled and valid input reference becomes available again: If one or more ref-
erences return and are declared valid before the Holdover period has expired (even
momentarily, i.e. for at least one second), SecureSync exits the Holdover mode and
returns to its fully synchronized state.
b.
The Holdover Timeout period expires. In this case, SecureSync will declare loss of syn-
chronization.
Note that Holdover mode does not persist through reboots or power cycles. If a reboot or
power cycle occurs while SecureSync is in Holdover mode, it will power-up and remain in a
“not synchronized” state until at least one valid Time and 1PPS input reference becomes avail-
able again. While in this state, NTP will be Stratum15 and outputs will not be usable. If the
input references are restored and then lost or declared not valid again, SecureSync will then
go back into Holdover mode.
What is "Holdover Timeout"?
Holdover Timeout is the user-configurable allowable time period in which SecureSync remains
in Holdover mode before it declares loss of synchronization. Holdover Timeout can be
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CHAPTER 3 • SecureSync User Reference Guide Rev. 26
3.4 Holdover Mode
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