Warning:
Any attempt to read or write any register other than DEVICE_READY before DEVICE_READY reads as 0x0F may corrupt
the NVM programming and may corrupt the register contents, as they are read from NVM. Note that this includes accesses to the
PAGE register.
4.4 Free Run Mode
Once power is applied to the Si5397/96 and initialization is complete, if valid input is not present, the DSPLL will automatically enter
freerun mode, generating the frequencies determined by the NVM. The frequency accuracy of the generated output clocks in freerun
mode is entirely dependent on the frequency accuracy of the crystal or reference clock on the XA/XB pins. For example, if the crystal
frequency is ±100 ppm, then all the output clocks will be generated at their configured frequency ±100 ppm in freerun mode. Any drift of
the crystal frequency will be tracked at the output clock frequencies. A TCXO or OCXO is recommended for applications that need bet-
ter frequency accuracy and stability while in freerun or holdover modes. Because there is little or no jitter attenuation from the XAXB
pins to the clock outputs, a low-jitter XAXB source will be needed for low-jitter clock outputs.
4.5 Lock Acquisition Mode
Each of the DSPLLs independently monitors its configured inputs for a valid clock. If at least one valid clock is available for synchroni-
zation, a DSPLL will automatically start the lock acquisition process. If the fast lock feature is enabled, a DSPLL will acquire lock using
the Fastlock Loop Bandwidth setting and then transition to the DSPLL Loop Bandwidth setting when lock acquisition is complete. Dur-
ing lock acquisition the outputs will generate a clock that follows the VCO frequency change as it pulls-in to the input clock frequency.
4.6 Locked Mode
Once locked, a DSPLL will generate output clocks that are both frequency and phase locked to their selected input clocks. At this point
any XTAL frequency drift will not affect the output frequency. DSPLL has its LOL pin and status bit to indicate when lock is achieved.
See Section
5.3.3 Loss of Lock (LOL) Fault Monitoring
for more details on the operation of the loss of lock circuit.
Si5397/96 Reference Manual
Modes of Operation
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