SI5347 Datasheet, PDF (34/59 Pages) Silicon Laboratories – DUAL/QUAD DSPLL ANY-FREQUENCY, ANY-OUTPUT JITTER ATTENUATORS

Si5347/46
5.7.2. 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.
5.7.3. 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 Figure 19. 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 19. OOF Status Indicator
5.7.3.1. Precision OOF Monitor
The precision OOF monitor circuit measures the frequency of all input clocks to within ±1 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 configurable from ±2 ppm to ±500 ppm in steps of 2 ppm. A configurable amount
of hysteresis is also available to prevent the OOF status from toggling at the failure boundary. An example is shown
in Figure 20. 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-configurable.
OOF Declared
OOF Cleared
Hysteresis
-6 ppm
(Set)
-4 ppm
(Clear)
0 ppm
OOF
Reference
f_IN
Hysteresis
+4 ppm
(Clear)
+6 ppm
(Set)
Figure 20. Example of Precise OOF Monitor Assertion and De-assertion Triggers
5.7.3.2. 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 frequency. 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 greater than ±4000 ppm.
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Rev. 1.1