68
Hardware Design Guide
IXP28XX Network Processor
QDR SRAM
Table 20
lists the QDR address stack-up signal cross-section details.
4.5.3
QDR SRAM D (Data Out) Topology
For Data Bus to work successfully at 233MHz only x9 SRAM parts need to be used and not x18
parts for multiple loads applications. Thus the data bus (READ and WRITE) should be split in two
halves each with a 9bit including the parity bit. The loads for each Data signal must be maintained
at 2 SRAMs only that are clam-shelled together so that they electrically constitute one load only.
As mentioned in section 3 on SRAM clam-shelling, some signals are not fully mirrored in the
SRAM part. Data-OUT is an example of these un-mirrored signals. Therefore, for Data-OUT
signals a daisy-chain configuration would be used for its topology. A better choice, however, is to
use a T-topology for Data-OUT whenever it is possible.
Figure 30
illustrates the routing topology for QDR SRAM D (Data Out).
Table 21
provides routing guidelines for the QDR D signal group.
Table 20.
QDR Address Stack-up Signal Cross-section Details
Parameter
QDR
Signal
Trace
Width (W)
[mils]
Trace
Thickness
(Tsignal)
[mils]
Trace
Spacing
(S) [mils]
D1
Thickness
(TD1)
[mils]
D2
Thickness
(Td2)
[mils]
Er(D1) Er(D2)
Spacing
between
signal
groups
[mils]
Value
Address
Main
Trunk = 10
0.5
15 - 20
5.0
5.7
3.5
3.8
20 - 25
Branches
= 3.5
Figure 30.
D (Data Out) Routing Topology
B3954-01
B
C
D
Bottom
SRAM
A
Intel
®
IXP2800 Driver
Data-Out, Parity-Out
Top
SRAM
V
TT
=0.75V
R
TT
= 50
Ω
Daisy-Chained
Data-Out (D0 - D7, D9 - D16)
and Parity-Out (D8, D17)
Signals
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