
Hardware Design Guide
63
IXP28XX Network Processor
QDR SRAM
4.4.4
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
QDRn_Q_H[16:9]
–
–
–
Q[7:0]
Q[7:0]
On-die at
IXP28XX
Receiver
QDRn_Q_H[8]
–
–
–
Q[8]
Q[8]
On-die at
IXP28XX
Receiver
QDRn_Q_H[7:0]
Q[7:0]
Q[7:0]
On-die at
IXP28XX
Receiver
–
–
–
QDRn_Q_H[8]
Q[8]
Q[8]
On-die at
IXP28XX
Receiver
–
–
–
QDRn_D_H[16:9]
–
–
–
D[7:0]
D[7:0]
PU50
Ω
QDRn_D_H[17]
–
–
–
D[8]
D[8]
PU50
Ω
QDRn_D_H[7:0]
D[7..0]
D[7..0]
PU50
Ω
–
–
–
QDRn_D_H[8]
D[8]
D[8]
PU50
Ω
–
–
–
QDRn_RPS_L[0]
R#
–
–
R#
–
PU35
Ω
Center
Termination at
T-junction
QDRn_WPS_L[0]
W#
–
–
W#
–
PU35
Ω
Center
Termination at
T-junction
QDRn_RPS_L[1]
–
R#
–
–
R#
PU35
Ω
Center
Termination at
T-junction
QDRn_WPS_L[0]
–
W#
–
–
W#
PU35
Ω
QDRn_BWS_L[0]
BW#
BW#
PU35
Ω
–
–
–
QDRn_BWS_L[1]
–
–
–
BW#
BW#
PU35
Ω
V
DDQ
= 1.5
V
DDQ
=1.5 V
V
DDQ
=1.5 V
–
V
DDQ
=1.5 V
V
DDQ
=1.5 V
–
V
DD_Core
=1.3 V
V
DD_Core
=
1.8 V
V
DD_Core
=
1.8 V
–
V
DD_Core
=
1.8 V
V
DD_Core
=
1.8 V
–
QDRn_ZQ[1:0]
Z
Q
=250
Ω
to
GND
Z
Q
=250
Ω
to
GND
–
Z
Q
=250
Ω
to
GND
Z
Q
=250
Ω
to
GND
–
VREF_QDRn
V
ref
=0.75
V
ref
=0.75
–
V
ref
=0.75
V
ref
=0.75
–
1.
Termination is not required for all topologies. Signal Integrity simulations should be performed to determine if the termination is required.
Table 18.
QDR SRAM Signal Mapping (Sheet 2 of 2)
IXP28XX Signals
Top SRAM 1
(2Mx9)
Bottom SRAM 2
(2Mx9)
Termination
V
TT
=0.75 V
Top SRAM 3
(2Mx9)
Bottom SRAM 4
(2Mx9)
Termination
V
TT
=0.75 V
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