SI5380 Datasheet, PDF (13/50 Pages) Silicon Laboratories – Ultra-Low Phase Noise, 12-output JESD204B Clock Generator

Si5380 Data Sheet
Functional Description
3.4.3 OOF Detection
Each input clock is monitored for frequency accuracy with respect to a OOF reference which it considers as its “0_ppm” reference. This
OOF reference can be selected as either: XAXB, IN0, IN1, IN2 or IN3. IN3 is only available as the OOF reference when not in ZDM.
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
3.4.4 Precision OOF Monitor
The Precision OOF monitor circuit measures the frequency of all input clocks to within ±1 ppm accuracy with respect to the frequency at
the XA/XB pins. The OOF monitor considers the frequency at the XA/XB pins as its 0 ppm OOF reference. A valid input frequency is
one that remains within the OOF frequency range which is register configurable from ±2 ppm to ±500 ppm in steps of 2 ppm. A configu-
rable amount of hysteresis is also available to prevent the OOF status from toggling at the failure boundary. An example is shown in the
figure below. In this case the OOF monitor is configured with a valid frequency 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 config-
urable.
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
3.4.5 Fast OOF Monitor
Because the precision OOF monitor needs to provide 1 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 frequen-
cy. An additional level of OOF monitoring called the Fast OOF monitor runs in parallel with the precision OOF monitors to quickly detect
a ramping input frequency. The Fast OOF monitor asserts OOF on an input clock frequency that has changed by 1,000 to 16,000 ppm.
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