Browse Prior Art Database

Liquid Crystal Display Support Bus for XGA Adapters

IP.com Disclosure Number: IPCOM000115080D
Original Publication Date: 1995-Mar-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 6 page(s) / 192K

Publishing Venue

IBM

Related People

Borroghs, S: AUTHOR [+5]

Abstract

Disclosed are concepts and an implementation of an Liquid Crystal Display (LCD) Support Bus for an SPD3.5 (Serializer Palette Digital-to-analog convertor) chip for a PS/2* XGA* display adapter and for other PS/2 portable products which may be used with LCD display panels.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 36% of the total text.

Liquid Crystal Display Support Bus for XGA Adapters

      Disclosed are concepts and an implementation of an Liquid
Crystal Display (LCD) Support Bus for an SPD3.5 (Serializer Palette
Digital-to-analog convertor) chip for a PS/2* XGA* display adapter
and for other PS/2 portable products which may be used with LCD
display panels.

      In an LCD Support Mode, the SPD3.5 chip supports LCD display
devices by providing data from Digital-to-Analog Convertor (DAC)
inputs, so that all display data is available in a digital form.  A
digital output of the DAC inputs is provided, along with Clock and
Blanking signals.  For the PASS1 version of the SPD3.5 chip, this
facility is offered as an alternative to the VFB output, which uses
the same pins on the chip.  For the PASS2 version of the chip, this
facility is offered in addition to the VFB output, requiring
physically separate pins on the chip.

      Conventionally, in order to provide a display on LCD panels, an
Liquid Crystal Display Controller (LCDC) is connected to the VFB
output of the display subsystem.  Thus, the LCDC is required to
duplicate much of the function of the SPD device.  Even duplicating
the palette, together with the  SPD hardware sprite circuits and
storage for the creation of small images to be moved on the screen
independently of other images, such an LCDC is unable to operate when
the XGA subsystem is running applications in direct color, since the
VFB is unable to handle such data.

      From the SPD3.5 device, the "DigiDac" output, which is more
properly called an LCD Support Bus eliminates the need for such
duplication.  An Liquid Crystal Display Interface (LCDI) chip, which
is much simpler than the LCDC chip, is still needed to provide a
display on an LCD panel.  Connection to an LCDC chip is not
supported.

      Ideally, the LCD Support Bus would provide an 888RGB data
output, using the full eight bits from the inputs of each of the Red,
Green, and Blue DAC's.  However, this output requires a large number
of pins, and severe simultaneous switching problems can be expected
when 24 outputs are switched at pixel speeds.  A typical LCD panel
can display up to eight different intensities, requiring three bits
of information, for each color, providing 512 different screen
colors.  Using spatial and temporal dithering techniques, it is
possible to extend the apparent number of displayable colors beyond
this limit, at a cost of either reduced resolution or a reduced
effective frame rate.

      Fig. 1 schematically shows the LCD Support Control Register for
the PASS1 version of the SPD3.5 chip, while Fig. 2 schematically
shows this register for the PASS2 version of this device.  The
difference between these registers reflects the change to using
separate pins, in PASS2 for the VFB output.  This register can be
written and read from the SPD3.5 chip.  Bits 7:3, which cannot be
written, return zeroes when read.  Thus, write data should have...