STC3500 Datasheet, PDF (38/48 Pages) Connor-Winfield Corporation – INTEGRATED - STRATUM 3 TIMING SOURCE

Application Notes continued
Hold Over History Accumulation and Maintenance continued
Whenever Hold Over is entered, it is the Active Holdover History that is used to determine the Hold Over frequency.
The History_Cmd register allows the application to issue three Hold Over history control commands:
1) Save the Active Holdover History to the Backup History.
2) Restore a Backup History to the Active.
3) Flush the active History as well as the accumulation register. The Backup history remains intact.
Both the Active and the Backup Hold Over histories are loaded with the calibrated Free Run synthesizer control data on
reset/power-up.
The application might use the “save to backup” in a situation where, for example, the primary reference is known to be
of higher quality than any secondary references, in which case it may be desirable to save and then restore the Hold
Over history accumulated on the primary reference if the primary reference is lost and Hold Over is entered upon loss
of a secondary reference. Users can restore the history from backup any time, even while operating in Holdover mode.
The frequency transient will be smooth and continuous. It is the responsibility of application software to keep track of
the age and viability of the Hold Over backup history. Given time and temperature effects on oscillator aging, the
application may wish to periodically perform a “Save” of the Active history to keep the backup current.
When switching to a new reference, the active Hold Over history will remain intact and marked as “Holdover Available”
(if it was available before the reference switch) until a new history is accumulated on the new reference (Typically 15
minutes after lock has been achieved). During the new history accumulation, the “Holdover Build Complete” bit is reset.
Once the new history accumulation is complete, it is transferred to the Active History and the “Holdover Build Complete”
bit is set. The active history will then continue to be updated to track the reference.
The History_Policy register allows the application to control how a new history is built. When set to “Rebuild”:
1) History accumulation begins when lock is achieved on the new reference.
2) The Hold Over history is rebuilt (taking about 15 minutes). The Active History remains untouched until it is
replaced when the build is complete.
When the policy is set to “Continue”:
1) If there is no “Available” Active History, a new build occurs, as under the “Rebuild” policy.
2) If there is an “Available” Active History, it will continue, the accumulation register will be loaded from the Active
History, and the “Build” process is essentially completed immediately following lock on the new reference.
The “Continue” policy may be used by the application if, for example, it is known that the reference switched to may be
traced to the same source and therefore likely has no frequency offset from the prior reference. In that case, the
“Continue” policy avoids the delay of rebuilding the Hold Over history. If the switch is likely to be between references
with known or unknown frequency offset, then it is preferable to use the “Rebuild” policy.
The time since the Hold Over state was entered may be read from the Hold Over_Time register. Values are from 0 to
255 hours, limited at 255, and reset to 0 when not in the Hold Over state.
Boundary Scan – The STC3800 provides a standard IEEE 1149.1 JTAG boundary scan interface via the TMS, TCK,
TDI, TDO, and TRST pins. Boundary scan may be used to verify proper device I/O connectivity and functionality.
Preliminary Data Sheet: TM060 Page 38 of 48 Rev: P06 Date: 11/22/04
© Copyright 2001 The Connor-Winfield Corp. All Rights Reserved Specifications subject to change without notice