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Method and device for stabilizing a display against temperature
dependent contrast variations.
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The invention relates to a method and device for stabilizing
a display against temperature dependent contrast variations,
said display being controlled by display drive signals including
a temperature dependent contrast control signal generated
by a display controller being timed with an output
signal of a local oscillator, such display in particular being
a liquid crystal display (LCD).
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Devices using such methods are known in various forms of implementation,
e.g. in accordance with applications of the
Universal LCD Driver for Low Multiplex Rates Integrated Circuit
type PCF 8576C as described in Philips Semiconductor
Data Handbook dated October 2001, in accordance with applications
of the Column Row Driver LSI for Dot Matrix Graphic LCD
type T6K14 as described in Toshiba Data Handbook dated February
26, 2001 and/or in applications of Epson's LCD-Controller/Driver
With Integrated Temperature Sensor types
SED1575 and SED157A published in Epson Newsletter dated 14-16
February, 2001.
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In general, displays such as LCDs, typically have a contrast
input voltage that is used to vary the contrast in order to
allow adjustment at a wanted predetermined contrast setpoint.
However, the voltage required to obtain such contrast setting
depends amongst others on the display temperature. To prevent
variations in the display temperature from varying said contrast,
it is on itself known to generate a contrast control
signal which varies with an output signal of a temperature
sensor, such that contrast stabilization at the wanted setpoint
is obtained, at least within a certain temperature
range. In conventional displays, such temperature sensor may
be positioned in direct contact with or integrated into the
display itself, such as known from European Patent Application
0 244 510. This allows on the one hand for an accurate
temperature measurement, but requires on the other hand special
and therewith costly production and handling facilities.
A more cost effective, but less accurate, method of temperature
measurement is applied in other conventional display
systems, in which a temperature sensor is located on a
printed circuit board separated from the display.
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In consequence, amongst other things, it is an object of the
present invention to provide a method and device as recited
suprea allowing for an accurate and cost effective detection
of the display temperature, which is easy to implement.
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Now therefore, according to one of its aspects, the invention
is characterized by said contrast control signal varying with
temperature dependent frequency variations of said local oscillator
output signal.
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On itself it is known from published German patent application
DE 42 39 522 to measure the temperature of a circuit
element in the vicinity of an integrated microprocessor circuit
by detection of a temperature dependent electric parameter
of said circuit element. The invention, however, is based
on the recognition that the use of a display controller in
display drivers to generate display drive signals is common
practice and therewith also the use of a local oscillator
providing a time base for, also being referred to as "timing",
the internal logic of the display controller and its
drive signals. The timing organizes the internal data flow of
the device as such, securing proper synchronization of said
display drive signals. Such local oscillator may be built
into the display controller itself or may be located outside
the display controller. The local oscillator varies in its
oscillation frequency with temperature and along therewith
the frequency of all clocksignals derived from the oscillator
output signal. This normally unwanted phenomenon is now used
in accordance with the invention to determine the display
temperature. By applying the invention, the display temperature
is determined by measuring or detecting the actual frequency
of the local oscillator output signal directly, or indirectly
through one of the clocksignals derived therefrom.
This not only removes the need for a separate dedicated temperature
sensor such as used in the above conventional LCD
drivers but also allows for a cost effective and simple implementation,
in particular when said frequency detection is
applied to a low frequency clock signal.
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Therefore, a method according to the invention is preferably
characterized by said contrast control signal varying with a
clocksignal being derived from said local oscillator output
signal through frequency division.
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Such frequency division may already be applied to obtain
proper synchronization in generating display drive signals,
such as display column and row control signals or master-slave
synchronization. This removes the need for an extra
dedicated frequency divider.
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According to another aspect, the invention is characterized
by said contrast control signal being derived from a temperature
dependent frequency reference curve being calibrated by
actual measurement of display temperature and frequency of
said local oscillator output signal. This allows for an optimization
in the accuracy of temperature measurement, which
takes into account device-to-device tolerance deviation dependent
frequency variations of the local oscillator output
signal.
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To increase the detection gain, i.e. the slope in the temperature
dependent frequency curve of the local oscillator,
said local oscillator is preferably being implemented as an
RC oscillator.
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The invention also relates to a display driving device being
arranged for implementing a method as claimed in Claim 1.
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Further advantageous aspects of the invention are recited in
dependent Claims.
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These and further features, aspects and advantages of the invention
will be discussed more in detail hereinafter with
reference to the disclosure of preferred embodiments of the
invention, and in particular with reference to the appended
Figures that illustrate:
- Figure 1,
- a block diagram of a display system including a
display driving device implementing the method
according to the invention;
- Figure 2,
- a detailed functional diagram of a part of the
display controller showing a local oscillator
terminal providing a local oscillator output signal
for clock synchronization as well as temperature
detection according to the invention;
- Figure 3,
- a general block diagram of an LCD driver circuit
showing in broader context the location of the
local oscillator terminal of Figure 2;
- Figure 4,
- signal timing diagrams showing various clocksignals
derived from said local oscillator output
signal suited for temperature detection according
to the invention;
- Figure 5,
- a signal plot showing a temperature dependent
frequency reference curve for calibration by actual
temperature-frequency measurements;
- Figure 6,
- a flow chart of a method for stabilizing a display
against temperature dependent contrast
variations according to the invention;
- Figure 7,
- a graphical presentation of a calibration set up
for calibration of the temperature dependent frequency
reference curve as shown in Figure 5 to
the specifics of an LCD driving device.
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In the following description, well known circuits have been
shown in block diagram form in order not to obscure the present
invention in unnecessary detail. For the most part, details
concerning timing and processing considerations and the
like have been omitted inasmuch as such details are not necessary
to obtain a complete understanding of the present invention
and are within the skill of persons of ordinary skill
in the relevant art.
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Reference will now be made to the drawings, wherein depicted
elements are not necessarily shown to scale and wherein like
or similar elements are designated by the same reference numeral
through the several views.
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Figure 1 shows a a block diagram of a display system 1-5 including
a display driving device 1-4 and providing display
drive signals to a display 5 for proper operation thereof,
such display preferably being a liquid crystal display
(LCD).The display driving device 1-4 comprises a display controller
1 using a microcontroller, which in generating said
display drive signals, is clock synchronized with an output
signal of a local oscillator, as will be further clarified
with reference to Figure 2. The display controller 1 is provided
with a contrast control input receiving through a contrast
control signal line 4, a contrast control signal VLCD
from a contrast control signal generating circuit 3, for adjusting
the display contrast. The contrast control signal
VLCD varies with temperature such, that temperature dependent
deviations from a predetermined display contrast setting are
compensated, therewith obtaining dynamic contrast stabilisation.
The display system of Figure 1 described so far corresponds
in function and operation to conventional type display
systems such as the above cited LCD drivers. For further details
in this respect, reference is made to these known LCD
drivers.
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Unlike the conventional LCD drivers, the display driving device
1-4 of Figure 1 implements the method according to the
invention in that the temperature dependent contrast control
signal VLCD is generated without using a dedicated temperature
sensor to measure the display temperature, but a frequency
detector. In accordance with the invention, the display
temperature is measured by detecting the frequency of
the local oscillator output signal itself, i.e. directly, or
indirectly by detecting the frequency of one of the clock
signals which are derived from said local oscillator output
signal. Any change in display temperature will cause the frequency
of the local oscillator to change and along therewith
also the output signal of said frequency detector, as will be
explained in more detail with reference to Figure 5.
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In the embodiment of the invention as shown in Figure 1, the
frequency detector is included in the contrast control signal
generating circuit 3 and is being supplied with said local
oscillator output signal or said one of the clocksignals from
the display controller 1 through signal line 2. The frequency
detector may be implemented by means of a microcontroller detecting
or "reading" the frequency of the local oscillator
output signal or said one of the clocksignals when being supplied
to its frequency signal input. The frequency detector
or microcobtroller provides a detection signal varying with
temperature dependent frequency variations of said local oscillator
output signal (fOSC2), which in accordance with the
invention is used to vary said contrast control signal VLCD
with the detected frequency variations such, that an appropriate
contrast stabilisation against display temperature
variations is obtained.
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Figure 2 shows a detailed functional diagram of a part of the
display controller 1 of Figure 1, showing a master-slave configuration
which on itself is known from the above conventional
LCD drivers. In this known configuration a master device
6 comprises an internal local RC oscillator 8 having a
frequency determining resistor Rf coupled externally to this
master device 6 through oscillator terminals OSC2 and OSC1.
In general, the oscillator characteristics and oscillator
frequency of RC oscillators depend strongly of the temperature,
therewith making such oscillator type in particular
suitable to implement the invention. In the embodiment shown
in this Figure 2, the oscillator terminal OSC2 is coupled
through a local oscillator signal line to oscillator terminal
OSC2' of a slave device 7 to supply thereto the local oscillator
output signal fOSC2. For further details on function and
operation of this configuration and its various elements,
such as those identified with indications such as VDD, VSS,
M/S, LSI and others, reference is made to the above cited
publications of these conventional LCD drivers. The invention
is applied to this known master-slave configuration by using
said local oscillator output signal fOSC2 for the purpose of
display temperature detection through frequency detection.
Therefore, this local oscillator output signal fOSC2 is being
coupled through signal line 2 to the abovementioned frequency
detector 9 included in the contrast control signal generating
circuit 3.
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Figure 3 shows a general block diagram of a conventional
large scale integrated (LSI) LCD driver circuit showing in
broader context locations and/or terminals carrying the local
oscillator output signal or clocksignals derived from said
local oscillator output signal. Knowledge of function and operation
of this LCD driver circuit is not necessary for understanding
the invention, reason for which further details
thereof are ignored.
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The LSI LCD driver circuit includes amongst others, a display
timing generator DTG, providing at its interface connections
easy external access to the local oscillator output signal
fOSC2, the frame synchronisation signal FR or any other synchronisation
signal derived from said local oscillator output
signal.
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Figure 4 shows timing diagrams illustrating a frame synchronisation
signal FR derived by frequency division from a Common
Master timing signal MC and a Common Slave timing signal
SC clock signal, which in their turn are clocksynchronized
with the local oscillator output signal fOSC2. A selection can
be made for the frame signal between a duty of 1/16 or 1/32
of the duty cycle of said common timing signals. The frame
synchronisation signal FR and the Common Master and Common
Slave timing signal MC and SC, respectively, are fully synchronous
with the local oscillator output signal fOSC2 and allow
to use a simple frequency detector for frequency detection
due to their relatively low frequency.
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Figure 5 shows a signal plot illustrating with curve I a
typical temperature dependent frequency variation of a display
controller clock signal within a temperature range from
- 40° C to + 80° C. Curve I shows a decrease in frequency at
increasing temperature, which is approximately linear and
shows a relative variation in frequency between minimum and
maximum limits within this temperature range, exceeding 20%.
Curve II shows the variation in frequency of a crystal oscillator
output signal relative to a nominal frequency within
the same temperature range, which is almost flat.
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Curve I may be obtained by measuring the actual frequency of
a local oscillator output signal or a clocksignal derived
therefrom, for reason of simplicity in the aggregate also being
referred to as local oscillator output signal fOSC2, at
various temperatures of the display and may be used as reference
curve for calibration by actual frequency measurement at
predetermined reference temperature values, as will be explained
hereinafter with reference to Figure 6.
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Figure 6 shows a flow chart of a method for calibrating display
temperature stabilisation according to the invention in
a series of display driving devices, or more in particular
LCD drivers, which are subject to device-to-device tolerance
deviations. Herein, the contrast control signal is being derived
from a temperature dependent frequency reference curve
being calibrated by actual frequency measurement of said local
oscillator output signal fOSC2 at predetermined reference
temperature values as follows.
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In block 10, curve I of a first display driving device according
to the invention, hereinafter also indicated as "reference
display driving device" is measured as describe above
and the temperature frequency correlation of this reference
curve I within a practically chosen temperature range is being
stored in the form of a fixed table in a non-volatile
memory of a second display driving device , which is to be
calibrated in its automatic contrast control. Such non volatile
memory may be an EEPROM positioned on the printed circuit
board next to the main micro controller. Due to device-to-device
tolerance deviations, the temperature frequency
correlation of this second display driving device deviates
from that of the reference display driving device. To quantify
this deviation, indicated in Figure 6 with Δ, actual
measurements of display temperature and corresponding frequency
of the local oscillator output signal (or frequency of
a clocksignal derived from said local oscillator output signal)
of this second display driving device are being made in
block 11, e.g. by measuring the actual frequency occurring at
one or more predetermined display temperature values within
the last mentioned temperature range, and used in block 12
to compare the same with the corresponding temperature frequency
correlation values of the reference curve I as being
stored in the fixed table of the non-volatile memory of the
second display driving device. The so quantified deviation Δ
is being supplied to block 13, in which the curve I as provided
for in block 10 is parallel shifted and therewith precisely
matched or calibrated to the specific signal processing
properties of the second display driving device.
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In block 14, the temperature frequency correlation of this
calibrated curve I is being stored in a fixed table of the
non-volatile memory of the second display driving device,
e.g. by overwriting the originally stored temperature frequency
correlation of the reference curve I, or by using a
further fixed table. This table is used in operation as a
frequency-temperature conversion table allowing to converse
each frequency value f within the temperature range of interest
immediately into its corresponding temperature value T.
This temperature value T is further processed to obtain in on
itself known manner a contrast control signal stabilising appropriately
the display contrast against display temperature
variations.
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Figure 7 is a graphical presentation of a calibration set up
for calibration of the temperature dependent frequency reference
curve I as shown in Figure 5 to the specifics of an LCD
driving device, in the foregoing being referred to as second
display driving device. The LCD driving device comprises an
LCD controller with internal local oscillator 9 positioned at
or onto the LCD and being connected to a microcontroller 3,
also being referred to as main microcontroller, mounted on a
printed circuit board (PCB) 16. Also mounted on the PCB 16 is
a non-volatile memory 15 containing initially the above mentioned
temperature dependent frequency reference curve I.
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The calibration set up includes a temperature sensor 18, such
as an infrared thermometer, measuring the actual display temperature
and supplying this temperature data through line 20
to a host computer 17. The microcontroller 3 detects or measures
the frequency of the output signal of the internal local
oscillator 9 occurring at the measured actual display temperature
and supplies this frequency data through line 20 to
the host computer 17. The host computer 17 compares the so
measured temperature-frequency correlation with the corresponding
temperature-frequency correlation of the temperature
dependent frequency reference curve I. Any deviation between
the measured actual values on the one hand and the corresponding
reference values on the other hand will cause the
host computer to shift or offset the temperature dependent
frequency reference curve I such that the initial deviation
is fully cancelled. The so obtained new curve is fully
matched to the specific deviation spread of this LCD driving
device and is stored in the non-volatile memory 15 for immediate
conversion of frequency data into temperature data,
needed for temperature stabilization of display contrast in
accordance with the invention.
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Now, the present invention has hereabove been disclosed with
reference to preferred embodiments thereof. Persons skilled
in the art will recognize that numerous modifications and
changes may be made thereto without exceeding the scope of
the appended Claims. In consequence, the embodiments should
be considered as being illustrative, and no restriction
should be construed from those embodiments, other than as
have been recited in the Claims.
