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8.
Integrated Graphics Display Port
The GMCH contains four display ports: an analog CRT port, a dedicated LVDS port, and two 12-bit
Digital Video Out (DVO) ports. Section 8.1 will discuss the CRT and RAMDAC routing requirements.
Section 8.2 will discuss the dedicated LVDS port. Section 0 will discuss the DVOB and DVOC design
guideline. Section 8.3.4 provides recommendations for a flexible modular design guideline for DVOB
and DVOC muxed interfaces. Section 8.5 provides recommendations for the GPIO signal group.
8.1.
Analog RGB/CRT Guidelines
8.1.1.
RAMDAC/Display Interface
The GMCH integrated graphics/chipset design interfaces to an analog display via a RAMDAC. The
RAMDAC is a subsection of the graphics controller display engine and consists of three identical 8-bit
digital-to-analog converter (DAC) channels, one for the display's red, green, and blue electron guns.
Each RGB output is doubly terminated with a 75-Ω resistance: One 75-Ω resistance is connected from
the DAC output to the board ground, and the other termination resistance exists within the display. The
equivalent DC resistance at the output of each DAC is 37.5 Ω. The current output from each DAC flows
into this equivalent resistive load to produce a video voltage, without the need for external buffering.
There is also a pi-filter on each channel that is used to reduce high-frequency noise and to reduce EMI.
In order to maximize the performance, the filter impedance, cable impedance, and load impedance
should be matched.
Since the DAC operates at pixel frequencies up to 350 MHz, special attention should be paid to signal
integrity and EMI. RGB routing, component placement, component selection, cable and load impedance
(monitor) all play a large role in the analog display's quality and robustness. This holds true for all
resolutions, but especially for those at 1600x1200 resolutions or higher.
8.1.2.
Reference Resistor (REFSET)
A reference resistor, Rset, is used to set the reference current for the DAC. This resistor is an external
resistor with a 1% tolerance that is placed on the circuit board. A reference resistor can be selected from
a range between 124 Ω to 137 Ω (1%). Based on board design, DAC RGB outputs may be measured
when the display is completely white. If the RGB voltage value is between 665 mV to 770 mV, then the
video level is within VESA specification and the resistor value that was chosen will be optimal for board
design.
A reference voltage is generated on the GMCH from a bandgap voltage reference circuit. The bandgap
reference voltage level is approximately 1.2 V and this voltage is divided by four to generate the
reference voltage. The VESA video standard defines the LSB current for each DAC channel. The
RAMDAC reference current is designed on-die to be equal to 32LSB. Therefore, the external reference
resistor value is defined as:
®
Intel
855GM/855GME Chipset Platform Design Guide
Integrated Graphics Display Port
159
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