Design Guide

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Integrated Graphics Display Port

160 Intel

855GM/855GME Chipset Platform Design Guide

Equation 1.

)2.73(*32

)4/(

Vbg

Ireference

Vreference

REFSET ==

A 127 Ω 1% precision resistor value is the recommend value to use. See Figure 77 for the

recommended Rset placement.

Note: When using 855GME platform with external graphics only, Rset resistor is not needed.

Figure 77. Refset Placement

Large via or multiple

vias straight down to

ground plane

IREF ball

Refset

Resistor

Solder

Pads

Top Side of

Motherboard

Short, wide route

connecting the

resistor to the IREF

ball

No toggling signals

should be routed near this

reference resistor

127Ω, 1%, 1/16W

SMT metal film

resistor

Intel 855GM

Chipset

8.1.3. RAMDAC Board Design Guidelines

Care should be taken when routing the analog RAMDAC signals. This is especially true to successfully

support high display resolution where pixel frequency can be as high as 350 MHz. Intel recommends

that each analog R, G, B signal be routed single-endedly. The analog RGB signals should be routed with

an impedance of 37.5 Ω. Intel recommends that these routes be routed on an inner routing layer and that

it be shielded with VSS planes, if possible. Spacing between DAC channels and to other signals should

be maximized; 20-mil spacing is recommended. The RGB signals require pi filters that should be placed

near the VGA connector. It consists of two 3.3-pF caps with a 75 Ω ferrite bead at 100-MHz between

them. The RGB signals should have a 75-Ω, 1% terminating pull-down resistor. The complement signals

(R#, G#, and B#) should be grounded to the ground plane.

Note: When using 855GME platform with external graphics only, the RGB 75-Ω termination resistors are not

needed.

Intel recommends that the pi filter and terminating resistors be placed as close as possible to the VGA

connector. After the 75-Ω termination resistor, the RGB signals routing to the pi-filters and the VGA

connector should ideally be routed with 75-Ω impedance (~ 5 mil traces), or as close to 75-Ω

impedance as possible.