Intel 6300ESB ICH, Design Manual

Page: 153 / 320

January 2007 153
Intel
®
855GME Chipset and Intel
®
6300ESB ICH Embedded Platform Design Guide
Integrated Graphics Display Port
Integrated Graphics Display Port
6
The GMCH contains four display ports: an analog CRT port, a dedicated LVDS port, and two
12-bit Digital Video Out (DVO) ports.
Section 6.1 discusses the CRT and RAMDAC routing
requirements. Section 6.2 discusses the dedicated LVDS port. Section 6.3 discusses the DVOB and
DVOC design guideline. Section 6.4 provides recommendations for a flexible modular design
guideline for DVOB and DVOC muxed interfaces. Section 6.5 provides recommendations for the
GPIO signal group.
6.1 Analog RGB/CRT Guidelines
6.1.1 RAMDAC/Display Interface
The GMCH integrated graphics/chipset design interfaces to an analog display using 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 an LC 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 shall be matched.
Since the DAC runs at speeds up to 350 MHz, special attention shall be paid to signal integrity and
EMI. RGB routing, component placement, component selection, cable and load impedance
(monitor). They all play a large role in the analog display’s quality and robustness. This holds true
for all resolutions, but especially for those at 1600 x 1200 resolutions or higher.
6.1.2 Reference Resistor (RSET)
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 may be
selected from a range between 124
Ω
to 137
Ω
(1 percent). Based on board design, DAC RGB
outputs may be measured when the display is completely white. When the RGB voltage value is
between 665 mV and 770 mV, the video level is within VESA specification and the resistor value
that was chosen is optimal for board design.
Refer to Figure 76 for the recommended Rset
placement.
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 32 LSB.