SI5344H-42H Datasheet, PDF (31/56 Pages) Silicon Laboratories – HIGH-FREQUENCY,

Si5344H/42H
4.6.6. Synchronizing to Gapped Input Clocks
The DSPLL supports locking to an input clock that has missing periods. This is also referred to as a gapped clock.
The purpose of gapped clocking is to modulate the frequency of a periodic clock by selectively removing some of
its cycles. Gapping a clock severely increases its jitter so a phase-locked loop with high jitter tolerance and low
loop bandwidth is required to produce a low-jitter periodic clock. The resulting output will be a periodic non-gapped
clock with an average frequency of the input with its missing cycles. For example, an input clock of 100 MHz with
one cycle removed every 10 cycles will result in a 90 MHz periodic non-gapped output clock. This is shown in
Figure 13. For more information on gapped clocks, see “AN561: Introduction to Gapped Clocks and PLLs”.
Gapped Input Clock
100 MHz clock
1 missing period every 10
Periodic Output Clock
90 MHz non-gapped clock
100 ns
100 ns
1 2 3 4 5 6 7 8 9 10
DSPLL
123456789
10 ns
Period Removed
11.11111... ns
Figure 13. Generating an Averaged Clock Output Frequency from a Gapped Clock Input
A valid gapped clock input must have a minimum frequency of 10 MHz with a maximum of two missing cycles out
of every 8. Locking to a gapped clock will not trigger the LOS, OOF, and LOL fault monitors. Clock switching
between gapped clocks may violate the hitless switching specification in Table 8 when the switch occurs during a
gap in either input clock.
4.7. Fault Monitoring
The input clocks (IN0, IN1) are monitored for loss of signal (LOS) and out-of-frequency (OOF) as shown in
Figure 14. The reference at the XA/XB pins is also monitored for LOS since it provides a critical reference clock for
the DSPLL. There is also a Loss Of Lock (LOL) indicator which is asserted when the DSPLL loses synchronization.
XA XB
OSC
IN0
IN0
÷P₀
LOS
OOF
Precision
Fast
IN1
IN1
÷P₁
LOS
OOF
Precision
Fast
LOS
LOL
DSPLL
PD LPF
÷M
Figure 14. Fault Monitors
Rev. 1.0
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