SI5348 Datasheet, PDF (14/54 Pages) Silicon Laboratories – Network Synchronizer for SyncE/ 1588 PTP Telecom Boundary (T-BC) and Slave (T-SC) Clocks

Si5348 Rev D Data Sheet
Functional Description
3.8.2 XA/XB LOS Detection
A LOS monitor is available to ensure that the external crystal or reference clock is valid. By default the output clocks are disabled when
XAXB_LOS is detected. This feature can be disabled such that the device will continue to produce output clocks when XAXB_LOS is
detected.
3.8.3 OOF Detection
Input clocks IN0, IN1, IN2 are monitored for frequency accuracy with respect to an OOF reference, which it considers as its “0_ppm”
reference. Since a TCXO or OCXO will be connected to the REF input, most applications will declare the REF input to be the OOF
reference. The final OOF status is determined by the combination of both a precise OOF monitor and a fast OOF monitor as shown in
the figure below. An option to disable either monitor is also available. The live OOF register always displays the current OOF state and
its sticky register bit stays asserted until cleared.
Monitor
en
OOF
Precision
Fast
en
OLOOFS
Live
Sticky
Figure 3.12. OOF Status Indicator
3.8.4 Precision OOF Monitor
The precision OOF monitor circuit measures the frequency of all input clocks to within ±1/16 ppm accuracy with respect to the selected
OOF frequency reference. A valid input clock frequency is one that remains within the OOF frequency range, which is register configu-
rable up to ±500 ppm in steps of 1/16 ppm. A configurable amount of hysteresis is also available to prevent the OOF status from tog-
gling at the failure boundary. An example is shown in the figure below. In this case, the OOF monitor is configured with a valid frequen-
cy range of ±6 ppm and with 2 ppm of hysteresis. An option to use one of the input pins (IN0 – IN2) as the 0 ppm OOF reference
instead of the REF/REFb pins is available. This option is register-configurable. XA/XB can also be used as the 0 ppm reference.
OOF Declared
OOF Cleared
Hysteresis
-6 ppm
(Set)
-4 ppm
(Clear)
0 ppm
OOF Reference
fIN
Hysteresis
+4 ppm
(Clear)
+6 ppm
(Set)
Figure 3.13. Example of Precise OOF Monitor Assertion and De-assertion Triggers
3.8.5 Fast OOF Monitor
Because the precision OOF monitor needs to provide 1/16 ppm of frequency measurement accuracy, it must measure the monitored
input clock frequencies over a relatively long period of time. This may be too slow to detect an input clock that is quickly ramping in
frequency. An additional level of OOF monitoring called the Fast OOF monitor runs in parallel with the precision OOF monitors to quick-
ly detect a ramping input frequency. The Fast OOF monitor asserts OOF on an input clock frequency that has changed by greater than
±4000 ppm.
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