Design Guide
57
System Bus Routing Guidelines
The package trace lengths for the MCH are available in the I
ntel
®
E7500 Chipset Memory
Controller Hub (MCH) Datasheet.
The package trace lengths for the Intel Xeon processor with 512
KB L2 cache are available in the matching spreadsheet contained in the
Intel
®
Xeon™ Processor
with 512 Cache Signal Integrity Models
.
When length matching, every signal’s pad to pad length is set equal to each other (± 25 mils). This
yields the following equation:
CPU0
pkg_len
(Signal 1)
+ CPU
pad
-to-CPU
pad
(Signal 1) + 0.78 * CPU1
pkg_len
(Signal 1)
=
CPU0
pkg_len
(Signal 2)
+ CPU
pad
-to-CPU
pad
(Signal 2)
+ 0.78 * CPU1
pkg_len
(Signal 2)
To length match Signal 1 and Signal 2, hold one of the signals constant, and vary the second signal
until the equation is satisfied. Since all the pkg_len values are constant, we can solve for Signal 2:
CPU
pad
-to-CPU
pad
(Signal 2)
= CPU0
pkg_len
(Signal 1)
+ CPU
pad
-to-CPU
pad
(Signal 1)
+ 0.78 * CPU1
pkg_len
(Signal 1)
- CPU0
pkg_len
(Signal 2)
- 0.78 * CPU1
pkg_len
(Signal 2)
Generally, when length matching a group of signals, a designer will first layout all signals to the
shortest length possible allowed by specification. Then, keeping the longest signal as the constant
value (Signal 1), lengthen all the other signals so that the pad to pad lengths are all equal.
Trace Length Matching Example
Consider two signals, DSTBP0 and HD4, from the same group. Assume a nominal PCB length of
4.00". Calculate CPU to CPU length:
CPU
pin
-to-CPU
pin
(HD4)
(motherboard trace from Processor 0 to Processor1)
= 4.000"
CPU
pkg_len
(DSTBP0)
(strobe package trace length)
= 0.150"
CPU
pkg_len
(HD4)
(HD4 package trace length)
= 0.350"
CPU1
pkg_comp
(DSTBP0) = 0.78 * CPU
pkg_len
(DSTBP0) = 0.78 * 0.150" = 0.117"
CPU1
pkg_comp
(HD4) = 0.78 * CPU
pkg_len
(HD4) = 0.78 * 0.350" = 0.273"
CPU
pin
-to-CPU
pin
(DSTBP0) = CPU
pkg_len
(HD4) + CPU
pin
-to-CPU
pin
(HD4)
+ CPU1
pkg_comp
(HD4) – CPU
pkg_len
(DSTBP0) – CPU1
pkg_comp
(DSTBP0)
= 0.350 + 4.000 + 0.273 – 0.150 – 0.117
CPU
pin
-to-CPU
pin
(DSTBP0) = 4.356"
Figure 5-2. Trace Length Matching for the Dual Processor System Bus
Processor 0
Processor 1
MCH
CPU
pin
-to-CPU
pin
CPU
pkg_len
MCH
pkg_len
CPU1
pkg_comp
= 0.78 * CPU
pkg_len
CPU
pin
-to-MCH
pin
Summary of Contents for Xeon
Page 24: ...Introduction 24 Design Guide This page is intentionally left blank ...
Page 30: ...Component Quadrant Layout 30 Design Guide This page is intentionally left blank ...
Page 52: ...Platform Clock Routing Guidelines 52 Design Guide This page is intentionally left blank ...
Page 66: ...System Bus Routing Guidelines 66 Design Guide This page is intentionally left blank ...
Page 118: ...Intel 82870P2 P64H2 118 Design Guide This page is intentionally left blank ...
Page 146: ...I O Controller Hub 146 Design Guide This page is intentionally left blank ...
Page 148: ...Debug Port 148 Design Guide This page is intentionally left blank ...
Page 210: ...Schematic Checklist 210 Design Guide This page is intentionally left blank ...
Page 220: ...Layout Checklist 220 Design Guide This page is intentionally left blank ...
Page 222: ...Schematics 222 Design Guide This page is intentionally left blank ...