![](http://datasheet.mmic.net.cn/340000/82865G_datasheet_16452471/82865G_176.png)
Intel
82865G/82865GV GMCH Datasheet
177
Functional Description
Integrated RAMDAC
The display function contains a RAM-based Digital-to-Analog Converter (RAMDAC) that
transforms the digital data from the graphics and video subsystems to analog data for the CRT
monitor. GMCH’s integrated 350 MHz RAMDAC supports resolutions up to 2048 x 1536 @
75 Hz. Three 8-bit DACs provide the RED, GREEN, and BLUE signals to the monitor.
Sync Signals
HSYNC and VSYNC signals are digital and conform to TTL signal levels at the connector. Since
these levels cannot be generated internal to the device, external level shifting buffers are required.
These signals can be polarity adjusted and individually disabled in one of the two possible states.
The sync signals should power up disabled in the high state. No composite sync or special flat
panel sync support is included.
VESA/VGA Mode
VESA/VGA mode provides compatibility for pre-existing software that set the display mode using
the VGA CRTC registers. Timings are generated based on the VGA register values and the timing
generator registers are not used.
DDC (Display Data Channel)
DDC is a standard defined by VESA. Its purpose is to allow communication between the host
system and display. Both configuration and control information can be exchanged allowing plug-
and-play systems to be realized. Support for DDC 1 and 2 is implemented. The GMCH uses the
DDCA_CLK and DDCA_DATA to communicate with the analog monitor. The GMCH generates
these signals at 2.6 V. External pull-up resistors and level shifting circuitry should be implemented
on the board.
The GMCH implements a hardware GMBus controller that can be used to control these signals.
This allows higher speed transactions (up to 400 kHz) on theses lines than previous software
centric ‘bit-bashing’ techniques.
5.5.2
Digital Display Interface
The GMCH has several options for driving digital displays. The GMCH contains two DVO ports
that are multiplexed on the AGP interface. When an external AGP graphics accelerator is not
present, the GMCH can use the multiplexed DVO ports to provide extra digital display options.
These additional digital display capabilities may be provided through an ADD card that is designed
to plug in to a 1.5 V AGP connector.
5.5.2.1
Digital Display Channels – Intel
DVOB and Intel
DVOC
The GMCH has the capability to support digital display devices through two DVO ports
multiplexed with the AGP signals. When an external graphics accelerator is used via AGP, these
DVO ports are not available. Refer to
Section 2.5.6
for a detailed description of the shared DVO
signals.
The shared DVO ports each support a pixel clock up to 165 MHz and can support a variety of
transmission devices. When using a 24-bit external transmitter, it will be possible to pair the two
DVO ports in dual-channel mode to support a single digital display with higher resolutions and
refresh rates. In this mode, the GMCH is capable of driving a pixel clock up to 330 MHz.