November 2006
rev 0.3
in this section should be adequate to eliminate power
supply noise related problems in most designs.
PCS5I9658
Using the PCS5I9658 in zero-delay applications
Nested clock trees are typical applications for the
PCS5I9658. Designs using the PCS5I9658, as LVCMOS
PLL fanout buffer with zero insertion delay will show
significantly lower clock skew than clock distributions
developed from CMOS fanout buffers. The external
feedback option of the PCS59658 clock driver allows for
its use as a zero delay buffer. The PLL aligns the
feedback clock output edge with the clock input reference
edge resulting a near zero delay through the device (the
propagation delay through the device is virtually
eliminated). The maximum insertion delay of the device in
zero-delay applications is measured between the
reference clock input and any output. This effective delay
consists of the static phase offset, I/O jitter
(phase or long-term jitter), feedback path delay and the
output-to-output skew error relative to the feedback
output.
Figure 4. PCS5I9658 max device-to-device skew
Due to the statistical nature of I/O jitter a RMS value (1σ)
is specified. I/O jitter numbers for other confidence factors
(CF) can be derived from Table 8.
Table 8: Confidence Factor CF
Probability of clock edge within the
CF
distribution
± 1σ
± 2σ
± 3σ
± 4σ
± 5σ
± 6σ
0.68268948
0.95449988
0.99730007
0.99993663
0.99999943
0.99999999
Calculation of part-to-part skew
The PCS5I9658 zero delay buffer supports applications
where critical clock signal timing can be maintained
across several devices. If the reference clock inputs of
two or more PCS5I9658 are connected together, the
maximum overall timing uncertainty from the common
PCLK input to any output is:
t
SK(PP)
= t
(φ)
+ t
SK(O)
+ t
PD
, LINE(FB) + t
JIT(φ)
.CF
This maximum timing uncertainty consist of 4
components: static phase offset, output skew, feedback
board trace delay and I/O (phase) jitter:
The feedback trace delay is determined by the board
layout and can be used to fine-tune the effective delay
through each device. In the following example calculation
a I/O jitter confidence factor of 99.7% (±3σ) is assumed,
resulting in a worst case timing uncertainty from input to
any output of -214pS to 224pS relative to PCLK
(f
REF
= 100MHz,
f
VCO
= 400MHz):
FB=÷4,
t
jit(φ)
=8pS
RMS
at
t
SK(PP)
= [–70pS...80pS] + [–120pS...120pS] +
[(8Ps . –3)...(8pS . 3)] + t
PD
, LINE(FB)
t
SK(PP)
= [–214pS...224pS] + t
PD
, LINE(FB)
Due to the frequency dependence of the I/O jitter, Figure
5. can be used for a more precise timing performance
analysis.
PCLK
Common
-t
(Ø)
QFB
Device 1
t
JIT(Ø)
t
PD,LINE (FB)
Any Q
Device 1
+t
SK(O)
+t
(Ø
QFB
Device 2
t
JIT(Ø)
Any Q
Device 2
Max. skew
Figure 5. Maximum I/O Jitter versus frequency
+t
SK(O)
t
SK(PP)
3.3V 1:10 LVCMOS PLL Clock Generator
Notice: The information in this document is subject to change without notice.
8 of 15
PULSECORE [ PulseCore Semiconductor ]
