
74
Hardware Design Guide
IXP28XX Network Processor
QDR SRAM
It is important to mention that the above two formulas are not independent but rather interrelated
and must be satisfied simultaneously. If adjustments are made to the lengths or topologies of these
signals then it is strongly recommended that simulations are performed to ensure that the signals
are properly aligned.
4.5.6
QDR SRAM C, C#, CIN, CIN# Clock Topologies
This output clock pair provides a user-controlled means of tuning SRAM device output data. The
rising edge of C is used as the output timing reference for first output data. The rising edge of C# is
used as the output reference for second output data. Ideally, C# is 180 degrees out of phase with C.
Figure 36
illustrates the routing topology for QDR C, C#, CIN, and CIN#.
Table 27
provides routing guidelines for the QDR C, C#, CIN, and CIN# signal groups.
Figure 36.
QDR C, C#, CIN, and CIN# Routing Topology
B3957-01
A
Intel
®
IXP2800 Driver
Cout - Clock
C, C#
C-Clk signal
C, C# and CIN, CIN#
Top SRAM
Bottom SRAM
B
Intel
®
IXP2800 Receiver
Cin - Clock
CIN, CIN#
On-Die Termination
Table 27.
QDR C, C#, CIN, and CIN# Signal Group Routing Guidelines (Sheet 1 of 2)
Parameter
Routing Guideline
Signal Group
C, C#, CIN, CIN#
Topology
Point-to-Point
Reference Plane
Ground
Characteristic Trace Impedance
50
Ω
±10%
RTT
50
Ω
±1% on-die termination at IXP28XX receiver
Nominal Trace Width
5 mils
Nominal Trace Separation
20 - 25 mils
Group spacing
Isolation from all other signals is 20 mils
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