1 matching the reference planes, 2 adding additional decoupling capacitor – Intel 440GX User Manual

Page 47

Intel

440GX AGPset Design Guide

2-23

Motherboard Layout and Routing Guidelines

There are also “population” rules which need to be observed. To properly adjust memory timings
for 100 MHz operation, it is asked of the OEM and end user to populate the motherboard starting
with the DIMM located the furthest from the 82443GX.

2.9.1.1

Matching the Reference Planes

Providing a good return path for the AC currents induced on the power and ground planes is critical
to reducing signal noise. The best way to provide a low inductance return path is to “match” the
BGA and motherboard reference planes for a given signal. For example, MD0 is routed on the
BGA next to the ground plane. To “match” the reference planes, MD0 should be routed on the
Motherboard such that it is closest to the motherboard ground plane. Routing the memory signals
in this manner will provide the best possible path for the return currents.

2.9.1.2

Adding Additional Decoupling Capacitor

Another way to provide a low inductance path for return currents is to provide additional
decoupling capacitors next to signal vias. It is not possible to route all the MD lines on a single
layer. As a result, some of the MD lines will transition between signal layers through vias. The
return currents associated with these signals also require a low inductance path between Vcc and
ground. This low inductance path is provided by decoupling capacitors between Vcc and ground.
These decoupling capacitors should be placed as close as possible to the signal vias.

Figure 2-17. Registered SDRAM DIMM Example

SDR

DRAM

SDR

PLL

Data

Control

Clock

Table 2-17. MDx lines Reference Planes Routing

Memory Data Line

82443GX BGA

Reference Layer

Motherboard

Reference Plane

MD0,MD1,MD2,MD3, MD4, MD7, MD11, MD14,

MD15, MD16, MD17, MD19, MD20, MD21, MD22,

MD23, MD27, MD28, MD29, MD31, MD33, MD36,

MD37, MD38, MD40, MD41, MD42, MD43, MD 44,

MD45, MD48, MD49, MD52, MD53,MD55, MD56,

MD 57, MD 58, MD61, MD62, MD63, MECC[6:0]

GND layer

GND plane

MD5, MD6, MD8, MD9, MD10, MD12, MD13, MD18,

MD24, MD25, MD26, MD30, MD32, MD34, MD35,

MD39, MD46, MD47, MD50, MD51, MD54, MD59,

MD60, MECC7

3.3v VCC layer

3.3v power plane