34
Hardware Design Guide
IXP28XX Network Processor
RDRAM
The multi-tiered NexMod* module contains all of the circuitry for an entire Rambus* channel in a
single module. The NexMod* module can be attached either to the main board with pin grid array
(PGA) connectors or soldered with ball grid array (BGA) technology. The module incorporates
termination resistors, the Direct Rambus* Clock Generator (DRCG), and a Voltage Regulator
Module (VRM) on the same modular subsystem as the memory chips.
In summary, its features are:
•
Density up to 256 Mbytes per channel
•
Built-in termination and clock generation
•
With its 1.1 in. x 2.0 in. size, NexMod* is very space-efficient (compared to SO-RIMM and
RIMM solutions)
•
Stacked assembly
The NexMod* modules are also available from Samsung and Viking. The IXMB2800 layout
details are provided in
Section 3.5
.
3.1.1
RDRAM Subsystem Implementation Options
Table 12
describes RDRAM subsystem implementation options.
Table 12.
RDRAM Subsystem Implementation Options (Sheet 1 of 2)
Option
Memory per NPU
Comments
Using 256-
Mbyte part
Using 512-
Mbyte part
Short channel with two
devices
192 Mbytes
384 Mbytes
•
Clam-shell configuration helps in layout.
•
Observability of clam-shell components is
an issue.
•
Must follow all design rules defined in the
Rambus* Short Channel Design Guide
.
Short channel with four
devices
384 Mbytes
768 Mbytes
•
Clam-shell configuration helps in layout.
•
Observability of clam-shell components is
an issue.
•
Must follow all design rules defined in the
Rambus* Short Channel Design Guide.
HCD NexMOD*
(eight devices per channel)
768 Mbytes
1.5 Gbytes
•
HCD module contains VRM and terminators
on the module, making it very space-
efficient.
•
HCD also simplifies channel design since
many of the Rambus* design rules are
handled on the module.
SO-RIMM*
(eight devices per channel)
768 Mbytes
1.5 Gbytes
•
SO-RIMM* mounting is horizontal.
•
Must follow all design rules defined in the
Rambus* Long Channel Design Guide
.
Standard RIMM*
(eight devices per channel)
768 Mbytes
1.5 Gbytes
•
RIMM* mounting is vertical, but the
connector is very long.
•
Board placement for a dual-NPU design is
challenging.
•
Vendors: Samsung*, Elpida*
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