Browse Prior Art Database

Automatic Correction of CRT Control Parameters

IP.com Disclosure Number: IPCOM000107843D
Original Publication Date: 1992-Mar-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 6 page(s) / 258K

Publishing Venue

IBM

Related People

Butler, ND: AUTHOR [+2]

Abstract

In graphics systems it is desirable to be able to change between monitors using interlaced and non-interlaced scanning, while continuing to run existing software unchanged. For systems where the software is written to include the setup of the CRT control (CRTC) parameters, e.g., for the VGA or XGA* displays, this requires the CRTC parameters to remain unchanged, imposing an impossible restriction on new monitor designs. Disclosed is a design that allows a new CRTC to accept the old CRTC parameters but modifies them internally to produce new timings for the monitor, allowing more cost effective monitor designs, without migration problems for the software.

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Automatic Correction of CRT Control Parameters

       In graphics systems it is desirable to be able to change
between monitors using interlaced and non-interlaced scanning, while
continuing to run existing software unchanged.  For systems where the
software is written to include the setup of the CRT control (CRTC)
parameters, e.g., for the VGA or XGA* displays, this requires the
CRTC parameters to remain unchanged, imposing an impossible
restriction on new monitor designs.  Disclosed is a design that
allows a new CRTC to accept the old CRTC parameters but modifies them
internally to produce new timings for the monitor, allowing more cost
effective monitor designs, without migration problems for the
software.  CRTC parameter adjustment values can be loaded at power-on
time to suit different monitor characteristics;  this can be done by
simple device-driver software which can be changed if another monitor
with different timings is later incorporated.

      New standards for the front-of-screen quality of monitors
require higher display refresh rates to reduce perceived flicker,
thus requiring faster monitors.  Market pressures are forcing
suppliers to provide non-interlaced monitors instead of interlaced
while users demand that their existing software will stay compatible.
Changes in VGA modes can probably be achieved without changing CRTC
parameters up to a certain monitor refresh rate, but above that
changes are required.  Changes to XGA mode cannot be achieved without
some changes to the CRTC parameters.  Refer to the Appendix and Figs.
1 to 5 for detail of the problems.

      In prior art the change to the CRTC parameters would need to be
made by changing the software that writes the CRTC registers, so
existing software cannot run unaltered. This disclosure allows the
CRTC to make the required adjustment automatically, and in a way
transparent to the user.  A timing adjustment register and a number
of adders are included so that the correct adjustment can be added in
to the appropriate parameters.  Fig. 6 shows how a conventional CRTC
uses the three relevant horizontal CRTC parameter registers.  The
value in the register is compared with the value of a horizontal
counter, and when the two are equal, the appropriate action is taken,
e.g., turning off the horizontal sync pulse.  Fig. 7 shows a CRTC
utilizing the invention.  The value in each CRTC parameter remains
unchanged, but before being used in the comparison the contents of
the timing adjust register are added, thus creating the desired
effect of stretching the flyback time. The timing adjust register is
set by the display initialization code according to what type of
monitor is attached and the amount of stretching required.  The CRTC
uses a clock that is related to the pixel clock rate, so that
original CRTC parameters, and hence the adjust register, are based on
numbers of pixels and not absolute time.  Note that the adjust
register is written by initia...