Clamshell Configuration Of Srams - Intel IXP28XX Manual

Table 18.
IXP28XX Signals
QDRn_Q_H[16:9]
QDRn_Q_H[8]
QDRn_Q_H[7:0]
QDRn_Q_H[8]
QDRn_D_H[16:9]
QDRn_D_H[17]
QDRn_D_H[7:0]
QDRn_D_H[8]
QDRn_RPS_L[0]
QDRn_WPS_L[0]
QDRn_RPS_L[1]
QDRn_WPS_L[0]
QDRn_BWS_L[0]
QDRn_BWS_L[1]
V = 1.5
DDQ
V =1.3 V
DD_Core
QDRn_ZQ[1:0]
VREF_QDRn
1. Termination is not required for all topologies. Signal Integrity simulations should be performed to determine if the termination is required.
4.4.4
Hardware Design Guide
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QDR SRAM Signal Mapping (Sheet 2 of 2)
Top SRAM 1 Bottom SRAM 2
(2Mx9) (2Mx9)
– –
– –
Q[7:0] Q[7:0]
Q[8] Q[8]
– –
– –
D[7..0] D[7..0]
D[8] D[8]
R# –
W# –
– R#
– W#
BW# BW#
– –
V =1.5 V V =1.5 V
DDQ DDQ
V = V =
DD_Core DD_Core
1.8 V 1.8 V
Z =250Ω to Z =250Ω to
Q Q
GND GND
V =0.75 V =0.75
ref ref

ClamShell Configuration of SRAMs

When it is necessary to use more than one SRAM device to achieve a certain memory size capacity,
the SRAMs can be arranged in either a Width-Expansion configuration or a Depth-Expansion
configuration. Clam-shelling—that is, locating two devices on opposite sides of the PCB—is
possible in either case of width-expansion or depth-expansion. However, it is important to check
whether the particular signal addressed is properly mirrored or no in the SRAM part used so that it
can be routed properly. If it is not mirrored then the signal can be routed as is where one SRAM is
Termination Top SRAM 3
V =0.75 V (2Mx9)
TT
– Q[7:0]
– Q[8]
On-die at
IXP28XX –
Receiver
On-die at
IXP28XX –
Receiver
– D[7:0]
– D[8]
PU50Ω –
PU50Ω –
– R#
– W#
– –
– –
PU35Ω –
– BW#
– V =1.5 V
DDQ
V =
DD_Core
–
1.8 V
Z =250Ω to
Q
–
GND
– V =0.75
ref
IXP28XX Network Processor
QDR SRAM
Bottom SRAM 4 Termination
(2Mx9) V =0.75 V
TT
On-die at
Q[7:0] IXP28XX
Receiver
On-die at
Q[8] IXP28XX
Receiver
– –
– –
D[7:0] PU50Ω
D[8] PU50Ω
– –
– –
PU35Ω Center
– Termination at
T-junction
PU35Ω Center
– Termination at
T-junction
PU35Ω Center
R# Termination at
T-junction
W# PU35Ω
– –
BW# PU35Ω
V =1.5 V –
DDQ
V =
DD_Core
–
1.8 V
Z =250Ω to
Q
–
GND
V =0.75 –
ref
63