
Hardware Design Guide
35
IXP28XX Network Processor
RDRAM
3.2
Rambus* Channel Design
The following sections provide a high-level overview of Rambus* channel design. For complete
design and layout details please refer to the Rambus* design guidelines which can be found on the
Rambus* website at www.rambus.com.
3.2.1
RSL Trace Requirements and Recommendations
The following is a list of RSL (Rambus* Signaling Level) trace requirements and
recommendations for Rambus* designs:
•
All signals must be length-matched (RSL and clock signals) from the IXP28XX network
processor to each RDRAM pin. Signals are then length-matched for RDRAM to RDRAM pin.
•
Lines should be matched to within 10 mils.
•
No length-match requirement is necessary from the last RDRAM pin to the termination island.
•
Length matching must compensate for controller package substrate routing. These lengths are
provided in
Section 3.7
.
— Note that the flight time of the package is 154ps/in and is typically 180ps/in on a standard
FR4 PCB. Hence the trace lengths provided in
Section 3.7
must be adjusted to reflect the
flight delay of the target PCB. An example of this is provided in
Section 3.7
.
•
For impedance control, use the following guidelines:
— 34
Ω
for a short channel and 28
Ω
for a long channel.
— Trace widths for the desired impedance should be obtained from the manufacturer of the
PCB as field solvers may not be accurate.
— Device loading is compensated by narrowing (necking) the trace width in the vicinity of
RDRAM.
— The recommended electrical pitch is:
15 mm - 18 mm for a 128-Mbyte RDRAM and
12 mm - 18 mm for a 256-Mbyte RDRAM
•
Post layout simulations are strongly recommended to ensure proper signal integrity.
— Fine tuning of signal lengths may be required to meet timing and signal integrity
requirements.
Dual-channel RIMM*
(eight devices per channel)
768 Mbytes
1.5 Gbytes
•
Vendors: Elpida*, Samsung*
Quad-channel RIMM*
(four devices per channel)
384 Mbytes
768 Mbytes
•
Vendor: Samsung*.
•
Rambus* is working with vendors to
establish a standard.
Table 12.
RDRAM Subsystem Implementation Options (Sheet 2 of 2)
Option
Memory per NPU
Comments
Using 256-
Mbyte part
Using 512-
Mbyte part
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