78
Hardware Design Guide
IXP28XX Network Processor
QDR SRAM
Table 31
provides routing guidelines for the QDR control BWE# signal group.
Figure 41
illustrates routing for control BWE# signal trace width/spacing.
Figure 40.
QDR Control BWE# Signals Routing Topology
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Table 31.
QDR Control BWE# Signal Group Routing Guidelines
Parameter
Routing Guideline
Signal Group
CONTROL BWE#
Topology
Daisy Chain or Balanced-T topology
Reference Plane
Ground
Characteristic Trace Impedance
33
Ω
±10%
RTT
35
Ω
±1%
Nominal Trace Width
5 mils
Nominal Trace Separation
15 mils or more
Group Spacing
Isolation from all other signals is 20 - 25mils.
Trace length P
1
+A to SRAMs
1.
P refers to the package length.
Should be matched to K-Clk trace length minus 0.5 inches.
Maximum = 11.0 inches
Trace length B
As short as possible.
Maximum = 0.4 inches
Trace length C
Maximum = 0.5 inches
Trace length D
As short as possible; Maximum = 1.0 inch
Maximum via count per signal
As small as possible; Maximum = 7 vias
Length tuning method
All CONTROL signals matched within ±25 mils, where
length includes package length compensation (P+A)
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