Browse Prior Art Database

Digital Contrast and Brightness Controls for a Liquid Crystal Display

IP.com Disclosure Number: IPCOM000106282D
Original Publication Date: 1993-Oct-01
Included in the Prior Art Database: 2005-Mar-20
Document File: 4 page(s) / 155K

Publishing Venue

IBM

Related People

Hortensius, PD: AUTHOR [+2]

Abstract

Disclosed is the use of a dual digital potentiometer to set the contrast and brightness of a Liquid Crystal Display (LCD) under the control of a microprocessor, the service processor. Binary switches are operated by the user, providing inputs periodically polled by the service processor, to change contrast and brightness levels. These levels can also be changed by the operating system or an application program.

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Digital Contrast and Brightness Controls for a Liquid Crystal Display

      Disclosed is the use of a dual digital potentiometer to set the
contrast and brightness of a Liquid Crystal Display (LCD) under the
control of a microprocessor, the service processor.  Binary switches
are operated by the user, providing inputs periodically polled by the
service processor, to change contrast and brightness levels.  These
levels can also be changed by the operating system or an application
program.

      As shown in Fig. 1, switches 1 and 2 provide UP and DOWN
contrast inputs, respectively, while switches 3 and 4 provide UP and
DOWN brightness inputs, respectively, to service processor 5.  When a
switch is depressed, the associated input is tied to electrical
ground, providing a low level; otherwise the associated input is
pulled to a high level through a 10K ohm pullup resistor 6.

      Microcode within service processor 5 polls these switches every
25 msec to determine their state, and accordingly alters the
brightness and contrast values in a dual digital potentiometer 7.
New values are loaded into potentiometer 7 by clocking in a 17-bit
code, in which eight bits represent the contrast resistance value,
eight bits represent the brightness value, and one bit is a control
bit.  When the CHIP SELECT input of potentiometer 7 is activated, the
newly loaded values of contrast and brightness take effect.

      Service processor 5 uses a number of variables and flags stored
within its own on-board RAM to control contrast and brightness.  The
present contrast potentiometer value is stored as the variable CPOT.
Upon power up, the system processor (not shown) sends to service
processor 5 a value for contrast, which the service processor loads
into potentiometer 7.  Upon power down, the system processor reads,
from service processor 5, the latest value of CPOT, which is then
saved in system non-volatile storage, such as CMOS RAM with a battery
backup.  In this way, the most recent contrast value set by the user
is stored before power down.

      Every time the user depresses contrast UP button 1 or DOWN
button 2, service processor 5 steps CPOT upward or downward to the
next value in a CONTRAST TABLE, which is a table of 32 preset
resistance values spanning the contrast spectrum, each chosen to
represent the smallest possible perceptible change in contrast.

      The brightness potentiometer value BPOT is used to control
brightness, being changed in manner similar to that described above
for CPOT, whenever power is brought up or down, or whenever the user
depresses brightness UP button 3 or DOWN button 4.  A BRIGHTNESS
TABLE is similarly provided, with 32 resistance values spanning the
brightness spectrum.

      The value TYPEMATIC_TIMER represents a count-down timer for
each of the four switches 1 through 4.  Service processor 5 starts
this timer every time it detects a switch depression.  If the switch
is still depre...