4.7 Holdover Mode
The
DSPLL programmed for holdover mode automatically enters holdover when the selected input clock becomes invalid (i.e. when
either OOF or LOS are asserted) and no other valid input clocks are available for selection. The DSPLL calculates a historical average
of the input frequency while in locked mode to minimize the initial frequency offset when entering the holdover mode.
The averaging circuit for the DSPLL stores up to 120 seconds of historical frequency data while locked to a valid clock input. The final
averaged holdover frequency value is calculated from a programmable window with the stored historical frequency data. The window
size determines the amount of holdover frequency averaging. The delay value is used to ignore frequency data that may be corrupt just
before the input clock failure. Both the window size and the delay are programmable as shown in the figure below.
Programmable delay
Clock Failure
and Entry into
Holdover
time
0s
Historical Frequency Data Collected
Programmable historical data window
used
to determine the final holdover value
120s
1s,10s, 30s, 60s
30ms, 60ms, 1s,10s, 30s, 60s
Figure 4.4. Programmable Holdover Window
When entering Holdover, the DSPLL will pull its output clock frequency to the calculated averaged holdover frequency. While in Hold-
over,
the output frequency drift is entirely dependent on the external crystal or external reference clock connected to the XAXB pins. If
the clock input becomes valid, the DSPLL will automatically exit the Holdover mode and re-acquire lock to the new input clock. This
process involves pulling the output clock frequency to achieve frequency and phase lock with the input clock. These options are register
programmable.
The recommended mode of exit from holdover is a ramp in frequency. Just before the exit begins, the frequency difference between the
output frequency while in holdover and the desired, new output frequency is measured. It is likely that the new output clock frequency
will not be the same as the holdover output frequency because the new input clock frequency might have changed and the XTAL drift
might have changed the output frequency. The ramp logic calculates the difference in frequency between the holdover frequency and
the new, desired output frequency. Using the user selected ramp rate, the correct ramp time is calculated. The output ramp rate is then
applied for the correct amount of time so that when the ramp ends, the output frequency will be the desired new frequency. Using the
ramp, the transition between the two frequencies is smooth and linear. The ramp rate can be selected to be very slow (0.2 ppm/sec),
very fast (40,000 ppm/sec) or any of approximately 40 values that are in between. The loop bandwidth values do not limit or affect the
ramp rate selections and vice versa. CBPro defaults to ramped exit from holdover. Ramped exit from holdover is also used for ramped
input clock switching. See Section 5.2.4 Ramped Input Switching for more information.
As shown in Figure 4.1 Modes of Operation on page 13, the Holdover and Freerun modes are closely related. The device will only enter
Holdover if a valid clock has been selected long enough for the holdover history to become valid. If the clock fails before the combined
holdover history length and holdover history delay time has been met, then holdover history won't be valid and the device will enter
Freerun mode instead. Reducing the holdover history length and holdover history delay times will allow Holdover in less time, limited by
the source clock failure and wander characteristics. Note that the Holdover history accumulation is suspended when the input clock is
removed and resumes accumulating when a valid input clock is again presented to the DSPLL.
Si5395/94/92 Reference Manual
Modes of Operation
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