Advanced Clock Drivers Device Data
Freescale Semiconductor 7
MPC9653A
Calculation of Part-to-Part Skew
The MPC9653A 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
MPC9653As are connected together, the maximum overall
timing uncertainty from the common PCLK input to any output
is:
t
SK(PP)
= t
(
∅
)
+ t
SK(O)
+ t
PD, LINE(FB)
+ t
JIT(
∅
)
⋅ CF
This maximum timing uncertainty consist of 4 components:
static phase offset, output skew, feedback board trace delay
and I/O (phase) jitter:
Figure 4. MPC9653A Maximum Device-to-Device Skew
Due to the statistical nature of I/O jitter a RMS value (1 σ)
is specified. I/O jitter numbers for other confidence factors
(CF) can be derived from Table 8.
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 –197 ps to 297 ps (at 125 MHz reference frequency)
relative to PCLK:
t
SK(PP)
= [-17ps...117ps] + [-150ps...150ps] +
[(10ps @ -3)...(10ps @ 3)] + t
PD, LINE(FB)
t
SK(PP)
= [-197ps...297ps] + 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.
Figure 5. Maximum I/O Jitter versus Frequency
Driving Transmission Lines
The MPC9653A clock driver was designed to drive high
speed signals in a terminated transmission line environment.
To provide the optimum flexibility to the user the output
drivers were designed to exhibit the lowest impedance
possible. With an output impedance of less than 20 Ω the
drivers can drive either parallel or series terminated
transmission lines. For more information on transmission
lines the reader is referred to Freescale Semiconductor
application note AN1091. In most high performance clock
networks point-to-point distribution of signals is the method of
choice. In a point-to-point scheme either series terminated or
parallel terminated transmission lines can be used. The
parallel technique terminates the signal at the end of the line
with a 50 Ω resistance to V
CC
÷ 2.
This technique draws a fairly high level of DC current and
thus only a single terminated line can be driven by each
output of the MPC9653A clock driver. For the series
terminated case however there is no DC current draw, thus
the outputs can drive multiple series terminated lines.
Figure 5, illustrates an output driving a single series
terminated line versus two series terminated lines in parallel.
When taken to its extreme the fanout of the MPC9653A clock
driver is effectively doubled due to its capability to drive
multiple lines.
Table 8. Confidence Factor CF
CF Probability of clock edge within the distribution
± 1σ 0.68268948
± 2σ 0.95449988
± 3σ 0.99730007
± 4σ 0.99993663
± 5σ 0.99999943
± 6σ 0.99999999
t
PD,LINE(FB)
t
JIT(∅)
+t
SK(O)
—t(ý)
+t
(∅)
t
JIT(∅)
+t
SK(O)
t
SK(PP)
Max. skew
PCLK
Common
QFB
Device 1
Any Q
Device 1
QFB
Device2
Any Q
Device 2
30
20
10
0
25 35 45 55 65 75 85 95 105 115 125
FB = ÷ 8FB=÷ 4
3I/O Jitter [ps] RMS
Reference Frequency [MHz]
