Si5345-44-42-D-RM
26
Rev. 1.0
5.2.5. Synchronizing to Gapped Input Clocks
The DSPLL supports locking to an input clock that has missing clock 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, truly 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. A valid
gapped clock input must have a minimum frequency of 10 MHz with a maximum of 2 missing cycles out of every 8.
When properly configured, 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 of up to 1.5 ns for a maximum
phase transient, when the switch occurs during a gap in either input clocks. Figure 8 shows a 100 MHz clock with
one cycle removed every 10 cycles, which results in a 90 MHz periodic non-gapped output clock.
Figure 8. Generating an Averaged Non Gapped Output Frequency from a Gapped Input
5.3. Fault Monitoring
The four clocks (IN0, IN1, IN2, IN3/FB_IN) are monitored for loss of signal (LOS) and out-of-frequency (OOF).
Note that 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 asserted when the DSPLL loses synchronization within
the feedback loop. Figure 9 shows the fault monitors for each input path going into the DSPLL, which includes the
crystal input as well as IN0-3.
Figure 9. Si5342/44/45 Fault Monitors
DSPLL
100 ns
100 ns
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
9
100 MHz clock
1 missing period every 10
90 MHz non-gapped clock
10 ns
11.11111... ns
Gapped Input Clock
Periodic Output Clock
Period Removed
DSPLL
LPF
PD
÷M
IN0
IN0
Precision
Fast
OOF
LOS
÷P
0
IN1
IN1
Precision
Fast
OOF
LOS
÷P
1
IN3/FB_IN
IN3/FB_IN
Precision
Fast
OOF
LOS
÷P
3
IN2
IN2
Precision
Fast
OOF
LOS
÷P
2
LOL
XB
XA
OSC
LOS
Si5345/44/42