Design Guide 181
Intel
®
855GME Chipset and Intel
®
82801DB ICH4 Embedded Platform Design Guide
AGP Port Design Guidelines
The recommended AGP pull-up/pull-down resistor value is 8.2 kΩ.
7.2.8 AGP VDDQ and VCC
AGP specifies two separate power planes: VCC and VDDQ. VCC is the core power for the
graphics controller and VDDQ is the interface voltage.
7.2.9 VREF Generation for AGP 2.0 (2X and 4X)
7.2.9.1 1.5 V AGP Interface (2X/4X)
The voltage divider networks consist of AC and DC elements. The reference voltage that should be
supplied to the Vref pins of the GMCH and the graphics controller is ½ * VDDQ. Two 1 k
Ω ± 1%
resistors can be used to divide VDDQ down to the necessary voltage level.
The Vref divider network should be placed as close to the AGP interface as is practical to get the
benefit of the common mode power supply effects. However, the trace spacing around the Vref
signals must be a minimum of 25 mils to reduce crosstalk and maintain signal integrity.
7.2.10 AGP Compensation
The 855GME chipset GMCH AGP interface supports resistive buffer compensation. For PCBs
with characteristic impedance of 55
Ω, tie the GRCOMP pin to a 40.2 Ω ± 1% pull-down resistor (to
ground) via a 10-mil wide, very short (
≈ 0.5 inches) trace.
AGP Link:
http://www.intel.com/technology/agp/info.htm
AGP StressTool Link:
http://www.intel.com/technology/agp/downloads/agp_stress.htm
7.2.11 PM_SUS_CLK/AGP_PIPE# Design Consideration
The following design consideration provides the option to support both AGP and DVO devices
with one ADD Connector. Refer to Figure 84 and customer reference schematics for more detail.
The GMCH expects either the PM_SUS_CLK signal from the ADD connector when there is a no
AGP device or the AGP_PIPE# signal when there is an AGP device. The AGP_TYPEDET# signal
is driven high when no AGP card is detected, allowing DPMS_CLK to be driven by
PM_SUS_CLK. In the case where an AGP card is detected, AGP_TYPE# signal goes high which
allows DMPS_CLK to be driven by AGP_PIPE#.
Table 65. AGP 2.0 Pull-up Resistor Values
Rmin Rmax
4 k
Ω 16 kΩ
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