SI5347_16 Datasheet, PDF (12/55 Pages) Silicon Laboratories – Dual/Quad DSPLL Any-Frequency, Any-Output Jitter Attenuators

Si5347/46 Rev D Data Sheet
Functional Description
3.7.3 OOF Detection
Each input clock is monitored for frequency accuracy with respect to an OOF reference, which it considers as its “0_ppm” reference.
This OOF reference can be selected as either:
• XA/XB pins
• Any input clock (IN0, IN1, IN2, IN3)
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 3.9. OOF Status Indicator
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 – IN3) as the 0 ppm OOF reference
instead of the XA/XB pins is available. This option is register-configurable.
OOF Declared
OOF Cleared
Hysteresis
-6 ppm
(Set)
-4 ppm
(Clear)
0 ppm
OOF
Reference
fIN
Hysteresis
+4 ppm
(Clear)
+6 ppm
(Set)
Figure 3.10. Example of Precise OOF Monitor Assertion and De-assertion Triggers
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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