Method and device for stabilizing a display against temperature dependent contrast variations

Abstract

 Method for stabilizing a display (5) against temperature dependent contrast variations, said display (5) 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 (f_OSC2). To avoid the use of a dedicated temperature sensor the display temperature is being derived in accordance with the invention by measuring temperature dependent frequency variations of said local oscillator output signal (f_OSC2).

Description

[0001] Method and device for stabilizing a display against temperature dependent contrast variations.

[0002] 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).

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] The invention also relates to a display driving device being arranged for implementing a method as claimed in Claim 1.

[0013] Further advantageous aspects of the invention are recited in dependent Claims.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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 (f_OSC2), 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.

[0020] 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 f_OSC2. For further details on function and operation of this configuration and its various elements, such as those identified with indications such as V_DD, V_SS, 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 f_OSC2 for the purpose of display temperature detection through frequency detection. Therefore, this local oscillator output signal f_OSC2 is being coupled through signal line 2 to the abovementioned frequency detector 9 included in the contrast control signal generating circuit 3.

[0021] 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.

[0022] 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 f_OSC2, the frame synchronisation signal FR or any other synchronisation signal derived from said local oscillator output signal.

[0023] 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 f_OSC2. 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 f_OSC2 and allow to use a simple frequency detector for frequency detection due to their relatively low frequency.

[0024] 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.

[0025] 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 f_OSC2, 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.

[0026] 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 f_OSC2 at predetermined reference temperature values as follows.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

Claims

1. Method for stabilizing a display (5) against temperature dependent contrast variations, said display (5) 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 (f_OSC2), characterized by said contrast control signal varying with temperature dependent frequency variations of said local oscillator output signal (f_OSC2).

2. Method according to claim 1, characterized by said contrast control signal varying with a clocksignal (FR, MC, SC) being derived from said local oscillator output signal (f_OSC2) through frequency division.

3. Method according to claim 2, characterized by said clocksignal (FR) being used for synchronizing column and row control signals.

4. Method according to claim 2, characterized by said clocksignal (MC, SC) being used for master-slave synchronization.

5. Method according to one of claims 1 to 4, characterized by said contrast control signal being derived from a temperature dependent frequency reference curve (I) being calibrated by actual measurement of display temperature and frequency of said local oscillator output signal (f_OSC2).

6. A display driving device being arranged for implementing a method as claimed in Claim 1.

7. A display driving device according to claim 6, comprising a display controller generating display drive signals being timed with an output signal of a local oscillator (f_OSC2), characterized by a frequency detector (9) providing a detection signal varying with temperature dependent frequency variations of said local oscillator output signal (f_OSC2).

8. A display driving device according to claim 6 or 7, characterized by a frequency divider deriving a clocksignal (FR, MC, SC) from said local oscillator output signal (f_OSC2) through frequency division, said clock signal (FR, MC, SC) being supplied to said frequency detector (9).

9. A display driving device according to one of claims 6 to 8, characterized by memory means for storing a temperature dependent frequency reference curve (I), as well as processing means (13) for calibrating said temperature dependent frequency reference curve (I) by actual measurement of display temperature and frequency of said local oscillator output signal (f_OSC2) and for deriving said contrast control signal from said calibrated temperature dependent frequency reference curve.

10. A display driving device according to one of claims 6 to 9 for driving a liquid crystal display.

11. A display driving device according to one of claims 6 to 10, characterized by said local oscillator being implemented as an RC oscillator.

Drawing