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Extended Display Identification Data Programming of Microprocessor-Based Display Data Channel Design

IP.com Disclosure Number: IPCOM000115065D
Original Publication Date: 1995-Mar-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 4 page(s) / 100K

Publishing Venue

IBM

Related People

Dalgleish, SN: AUTHOR

Abstract

Monitors compatible with the Video Electronics Standards Association (VESA) Display Data Channel (DDC) Standard contain 128 bytes of Extended Display Identification Data (EDID), at least 8 bytes of which (Product Code, Serial Number, Week of Manufacture, Year of Manufacture) require to be different from monitor to monitor. This means that EDID information cannot be hard-coded (masked) into ROM for a microprocessor implementation of DDC; some non- volatile RAM or EEPROM requires to hold this data. As such, most DDC designs will hold all 128 bytes of EDID in a single area of EEPROM.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Extended Display Identification Data Programming of Microprocessor-Based
Display Data Channel Design

      Monitors compatible with the Video Electronics Standards
Association (VESA) Display Data Channel (DDC) Standard contain 128
bytes of Extended Display Identification Data (EDID), at least 8
bytes of which (Product Code, Serial Number, Week of Manufacture,
Year of Manufacture) require to be different from monitor to monitor.
This means that EDID information cannot be hard-coded (masked) into
ROM for a microprocessor implementation of DDC; some non- volatile
RAM or EEPROM requires to hold this data.  As such, most DDC designs
will hold all 128 bytes of EDID in a single area of EEPROM.

There are two possible methods of programming EDID Data:
  1.  Programming of the microprocessor device, prior to assembly of
it
       into the DDC card and subsequent card/box test.
  2.  Programming of the microprocessor device, after card
       assembly/test and during box test.

      Method 1 has the disadvantage that the monitor serial number is
not known at assembly stage.  Also, it is not possible to change EDID
data without box disassembly and device removal, etc.  Furthermore,
if the monitor/circuit card fails card test(s), then this serial
number may be "lost".  Still furthermore, monitor circuit cards may
be manufactured on a different date to monitor assembly due to use of
Vendors and stock pipe-lining etc.

      Method 2 has the advantage that the monitor serial number is
added after good test and is known at this stage.  Also, EDID data,
including serial number, date of manufacture, etc. can all be
changed, on demand.  Furthermore, a Service Repair Strategy can be
generated, whereby any updates in EDID content can be implemented,
without the requirement to disassemble box, remove device,
re-program, replace part and reassemble the box.

      However, Method 2 has the disadvantage that enabling
"write-capability" of EDID via the DDC IIC bus is potentially
dangerous, in that a user/customer could inadvertently overwrite his
EDID data.  The DDC microprocessor software cannot distinguish
between
a valid manufacturing write command and an invalid user write
command.

      In a preferred solution to the above problems, a "SECRET" IIC
command is incorporated in the monitor to Enable Write Capability.
Until this "secret" command is supplied the monitor will be
WRITE-PROTECTED.  This "secret" command will be known by
manufacturing
only and not by customers/users.

      Write capability is not enabled on power-up. ...