Abstract:
An LCD driver does not require an extra step for contrast adjustment of an LCD in the assembly process of an appliance such as a portable phone that is fitted with an LCD; nor does it require the microcomputer to which it is connected to perform extra operations for contrast adjustment. This LCD driver drives a dot-matrix LCD in accordance with display data fed from a microcomputer, and includes a rewritable nonvolatile data-storage circuit for storing data concerning an electronic variable resistor for setting contrast of the LCD.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an LCD driver for driving an LCD (liquid crystal display) in accordance with display data fed from a separately provided microcomputer, and particularly to an LCD driver for driving a dot-matrix LCD. 
     2. Description of the Prior Art 
     FIGS. 6A and 6B illustrate how a conventional LCD driver is used. As shown in FIG. 6A, a conventional LCD driver  73 , when used in a portable phone or other appliance that incorporates a microcomputer  72  for controlling the entire appliance and an LCD  74 , is placed between the microcomputer  72  and the LCD  74 , so that the LCD driver  73 , by being fed with display data from the microcomputer  72 , drives the LCD  74 . 
     Such an appliance is also provided with an nonvolatile data-storage device  71  such as an EEPROM (electrically erasable programmable read-only memory) for storing setting data so that a predetermined degree of contrast can be readily obtained in accordance with such factors as the type and use of the appliance. In the assembly process of the appliance, the contrast of the image displayed on the LCD  74  is adjusted to the desired degree, and the setting data for an electronic variable resistor at that time is stored in the data-storage device  71 . 
     When this appliance is in actual use, the contrast of the LCD  74  is adjusted, as shown in FIG. 6B, by letting the microcomputer  72  read the setting data from the data-storage device  71  and feed it to the LCD driver  73 . This helps prevent different LCDs  74  from exhibiting unduly different contrast as a result of variations in their characteristics that inevitably occur in their production. 
     However, storing the setting data for the electronic variable resistor in the data-storage device provided as a separate component requires the assembly process of the appliance such as a portable phone to include a step for adjusting the contrast of the display, that is, it necessitates an extra step in the assembly process. In addition, letting the microcomputer  72  read the setting data for the electronic variable resistor from the data-storage device  71  and feed it to the LCD driver  73  increases the operational burden to be borne by the microcomputer  72 . 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an LCD driver that does not require an extra step for contrast adjustment in the assembly process of an appliance such as a portable phone that is fitted with an LCD. Another object of the present invention is to provide an LCD driver that does not require a microcomputer to perform extra operations for contrast adjustment. 
     To achieve the above objects, according to one aspect of the present invention, an LCD driver for driving a dot-matrix LCD in accordance with display data fed from a microcomputer is provided with a rewritable nonvolatile data-storage circuit for storing data concerning an electronic variable resistor for setting the contrast of the LCD. 
     With this LCD driver, it is only after the LCD to be driven has been determined that contrast is adjusted and the setting data for the electronic variable resistor is stored in the nonvolatile data-storage circuit of the LCD driver. As a result, it is no longer necessary to adjust contrast while an appliance such as a portable phone is assembled from components including the LCD driver and the microcomputer. Nor is it necessary to provide as a separate component a data-storage device for storing the setting data for the electronic variable resistor. 
     In this LCD driver, the above-mentioned nonvolatile data-storage circuit may be realized as an EEPROM. In that case, it is possible, when the LCD driver is manufactured, to integrate the EEPROM readily into the same chip as the LCD driver simply by slightly modifying the manufacturing process of CMOS (complementary metal-oxide semiconductor). In addition, it is also possible to use the voltage step-up circuit provided in the LCD driver both for driving the LCD and for effecting the writing of data to the EEPROM. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This and other objects and features of the present invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanied drawings in which: 
     FIG. 1 is a diagram showing the construction of an LCD driver embodying the invention; 
     FIG. 2 is a circuit diagram of the portion around the voltage step-up circuit of the LCD driver of the invention; 
     FIG. 3 is a circuit diagram of the portion around the reference voltage producing circuit of the LCD driver of the invention; 
     FIG. 4 is a diagram showing examples of the waveforms of the signals outputted from the LCD driver of the invention; 
     FIG. 5 is a diagram showing a pattern displayed on an LCD driven by the LCD driver of the invention in accordance with the signals as shown in FIG. 4; 
     FIG. 6A is a diagram showing the construction of an appliance that employs a conventional LCD driver and the flow of data and instructions in its manufacturing process: and 
     FIG. 6B is a diagram showing the construction of the appliance that employs the conventional LCD driver and the flow of data and instructions in its actual use. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to FIGS. 1 to  5 . FIG. 1 is a block diagram illustrating the construction of an LCD driver  10  embodying the invention and the data flow therein. The LCD driver  10  is connected to a microcomputer  1  and to an LCD  11 . 
     In an appliance such as a portable phone, the microcomputer  1  controls the entire appliance, and it also controls the LCD driver  10 , by feeding it with data in accordance with signals fed from switches (not shown) for example, so that the LCD driver  10  drives the LCD  11  to effect display. The LCD driver  10  is formed on a single chip as a separate unit from the microcomputer  1 . The LCD  11  is a dot-matrix LCD capable of black-and-while display, and the contrast of each of its dots depends on the voltage of the signal that is fed from the LCD driver  10  to turn on that dot. 
     When the microcomputer  1  sends out display data, the LCD driver  10  receives it via an interface  3  provided therein. The received display data is then processed by a logic circuit  5  composed of decoders, registers, and the like, and is then stored in a display data RAM (random access memory)  6 . While the LCD driver  10  is performing its own processing, it keeps outputting a busy flag to the microcomputer  1 . 
     In accordance with the display data stored in the RAM  6 , a segment driver  9  produces signals that have waveforms as described later. On the other hand, a common driver produces signals having predetermined waveforms at regular intervals. The signals outputted from the common driver  8  and the segment driver  9  are fed to the LCD  11 . The common driver  8  and the segment driver  9  receive a voltage from a voltage step-up circuit. 
     The LCD driver  10  is provided with an input terminal  12  for receiving setting data for an electronic variable resistor that is to be fed to an EEPROM  4 . Here, the “electronic variable resistor” refers to the circuit for varying for contrast adjustment the voltage used as a reference with respect to which the voltage supplied from the voltage step-up circuit to the common driver  8  and the segment driver  9  is determined. The received setting data is stored in the EEPROM  4  that is provided within the LCD driver  10 . 
     The LCD driver  10  also has a power-on detection circuit  13  for detecting the presence of supplied power by means of a switching device for example, so that the LCD driver  10 , when turned on, can send a signal to the EEPROM  4 . This signal causes the setting data for the electronic variable resistor stored in the EEPROM  4  to be fed to the voltage step-up circuit  7 , which then supplies the common driver  8  and the segment driver  9  with a voltage that is determined in accordance with the received setting data. The voltage obtained from the voltage step-up circuit  7  is used also to erase the data stored in the EEPROM  4 . 
     With the LCD driver  10  of this embodiment, it is possible to adjust contrast and store the setting data for the electronic variable resistor in the EEPROM  4  as soon as a decision has been made as to the model of the LCD  11  with which the LCD driver  10  is to be combined to produce an LCD module or the like. This eliminates the need to transfer the setting data from the microcomputer to the LCD driver as with the conventional LCD driver  73  (see FIGS.  6 A and  6 B). The setting data for the electronic variable resistor consists of six bits for example, in which case contrast can be adjusted in 64 steps of gradation. The LCD driver  10  also has an oscillation circuit  2  for producing display-synchronizing and other signals. 
     FIG. 2 shows a circuit diagram of an example of the voltage step-up circuit  7 . The setting data for the electronic variable resistor is fed from the EEPROM  4  to an electronic variable resistor control circuit  51 , which controls the voltage produced by a reference voltage producing circuit  50 . An operational amplifier  52  has its non-inverting input terminal (+) connected through the reference voltage producing circuit  50  to a power source voltage VDD, and has its inverting input terminal (−) connected through a resistor R 1  to the power source voltage VDD. The operational amplifier  52  has its output terminal connected through a resistor R 2  to its inverting input terminal (−), thereby to achieve feedback. 
     Between the output terminal of the operational amplifier  52  and the power source voltage VDD, resistors R 3 , R 4 , . . . , R 7  are connected in series. From the individual junctions between the resistors R 3 , R 4 , . . . , R 7 , voltages V 1 , V 2 , V 3 , and V 4  are extracted through buffers  54 ,  55 ,  56 , and  57 , respectively. In addition, the voltage step-up circuit  7  also outputs the power source voltage VDD through a buffer  53 , and outputs the output of the operational amplifier  52  through a buffer  58  as a voltage V 5 . In this way, in accordance with the setting data stored in the EEPROM  4 , the voltage step-up circuit  7  outputs voltages V 1  to V 5  in five steps to the common driver  8  and the segment driver  9 . 
     Next, a description will be given as to how a reference voltage is produced in the reference voltage producing circuit  50 . FIG. 3 shows a circuit diagram of an example of the portion around the reference voltage producing circuit  50 . Between the power source voltages VDD and VSS, seven resistors R 10 , R 11 , . . . , R 16  are connected in series, with the junction between the resistors R 15  and R 16  connected to the non-inverting input terminal (+) of the operational amplifier  52 . In addition, six FETs (field-effect transistors)  60  to  65  are connected individually in parallel with the resistors R 10  to R 15 , respectively. 
     The FETs  60  to  65  have their gate connected to the electronic variable resistor control circuit  51  so that the former will be turned on and off by the latter. The resistors R 10  to R 15  have different resistances so that, as the FETs  60  to  65  are individually switched between the on and off states, the operational amplifier  52  will receive varying voltages. Stored in the EEPROM  4  is data that indicates whether the individual FETs  60  to  65  are in the on or off state. 
     The LCD driver  10  drives the LCD  11  by line-sequential scanning. FIG. 4 shows examples of the waveforms of the signals that the common driver  8  and the segment driver  9  output by use of the power source voltage VDD as well as the voltages V 1  to V 5  outputted from the voltage step-up circuit  7 . When these signals have the waveforms as shown in FIG. 4, they display the pattern as shown in FIG. 5 on a part of the LCD  11 . The signal FR is a signal with a period T outputted from the oscillation circuit  2 , and is used as a display-synchronizing signal. 
     COM 0 , COM 1 , . . . represent signals outputted from the common driver  8 ; SEG 0 , SEG 1 , . . . represent signals outputted from the segment driver  9 . SEG 0 -COM 0  represents the voltage that is applied to the intersection between SEG 0  and COM 0  of the LCD  11 ; SEG 1 -COM 0  represents the voltage that is applied to the intersection between SEG 1  and COM 0  of the LCD  11 . In FIG. 4, the power source voltage VDD is assumed to be equal to the reference level (0 V). 
     COM 0 , COM 1 , . . . are outputted from the common driver  8  with a period T, each having a waveform with selection points delayed by a length of time t relative to the previous one. SEG 0 , SEG 1 , . . . vary with the pattern displayed. For example, in the period  20 . SEG 0  equals the power source voltage VDD, and this corresponds to the fact that those points where SEGO intersects COM 0 , COM 1 , and COM 2  are turned on to form a display pattern in FIG. 5; in the period  21 , SEG 0  equals the voltage V 5 , and thus it exhibits a symmetrical waveform. In the period  22 , SEG 1  equals the power source voltage VDD; in the next period  23 , it equals the voltage V 2 ; in the next period  24 , it equals the power source voltage VDD. 
     As a result, as indicated by the waveform of SEG 0 -COM 0 , the intersection between SEG 0  and COM 0  receives the voltage V 5  in the period  25 , and thereafter receives voltages varying in the vicinity of the voltage VDD; it receives a voltage −V 5  in the period  26 , and thereafter again receives voltages varying in the vicinity of the voltage VDD. This waveform is repeated with a period T as long as there is no change in the displayed pattern. Every half the period, this intersection is selected with the polarity of the voltage applied thereto inverted every time. This is to prevent deterioration of the liquid crystal of the LCD  11  by canceling the direct-current components contained in the voltage applied thereto. 
     The intersection between SEG 1  and COM 0  (as indicated by SEG 1 -COM 0 ) receives the voltage V 5  in the period  27 , and thereafter receives voltages varying in the vicinity of voltage VDD: it receives the voltage −V 5  in the period  28 , and thereafter again receives voltages varying in the vicinity of VDD. This intersection is also turned on like the intersection between SEG 0  and COM 0 . Although not shown in the figure, a dot that is not turned on such as the intersection between SEG 1  and COM 1  does not receive neither the voltage V 5  or the voltage −V 5 , as will be clear when SEG 1 -COM 1  is calculated from the above-mentioned SEG 1  and COM 1 . In this way, every intersection is selected once within every period T. 
     As described above, in this embodiment, when the LCD driver  10  starts receiving power, the voltages V 1  to V 5  outputted from the voltage step-up circuit  7  are set in accordance with the setting data for the electronic variable resistor stored in the EEPROM  4 , and, in accordance with those voltages V 1  to V 5 , the voltage of the signal supplied from the common driver  8  and the segment driver  9  to the LCD  11 . In this way, the contrast of the LCD  11  is adjusted. 
     Thus, there is no need to feed the setting data for the electronic variable resistor from the microcomputer  1  to the LCD driver  10 , and this helps reduce the operational burden to be borne by the microcomputer  1  a s compared with the conventional LCD driver  73  (see FIGS.  6 A and  6 B). Moreover, since it is possible to adjust contrast beforehand as soon as the LCD driver  10  is combined with a specific model of the LCD  11  to produce an LCD module, it is not necessary to adjust contrast when an appliance such as a portable phone i s assembled from components including such an LCD module and a microcomputer  1 . Even in case it is necessary to adjust contrast, the setting data can be fed to the LCD driver  10  via the terminal  12 . 
     Furthermore, whereas the conventional LCD driver  73 , as shown in FIGS. 6A and 6B. requires a data-storage device  71  for storing the setting data for the electronic variable resistor, the LCD driver  10  of this embodiment does not need any. This helps reduce the scale of the circuit as a whole, and thus contributes to the miniaturization of appliances such as portable phones. The voltage step-up circuit  7  is used not only for the common driver  8  and the segment driver  9 , but also for rewriting the data stored in the EEPROM  4 . This makes efficient use of the circuit components possible, and thus prevents the circuit from becoming unduly large. The EEPROM  4  can readily be integrated into the same chip as the LCD driver simply by slightly modifying the manufacturing process of CMOS. 
     Note that the setting data may be outputted from the EEPROM  4  any time other than when the LCD driver is turned on; for example, it may be outputted when display is started in accordance with the data fed from the microcomputer  1 . Note also that, although the voltage step-up circuit  7  outputs voltages V 1  to V 5  in five steps in the above-described embodiment as shown in FIG. 2, the voltages may be outputted in any number of steps other than five. In that case, the signals outputted from the common driver  8  and the segment driver  9  will naturally exhibit waveforms different from those shown in FIG.  4 . 
     Moreover, although the reference voltage producing circuit  50  achieves voltage division by use of resistors in the example shown in FIG. 3, it is also possible to use, for example, voltage-regulating diodes or voltage-regulating circuits for the same purpose. On the other hand, the EEPROM  4  may be replaced with a rewritable nonvolatile memory such as a ferroelectric memory. The setting data for the electronic variable resistor may consist of any number of bits other than six in accordance with the number of actually required steps of gradation. 
     As described above, according to the present invention, an LCD driver is provided with a nonvolatile data-storage circuit for storing setting data for an electronic variable resistor. As a result, it is possible to adjust contrast as soon as the LCD driver is combined with an LCD to produce an LCD module. This eliminates the need to adjust contrast when an appliance such as a portable phone is assembled. Moreover, it is not necessary to provide as a separate component a data-storage device for storing the above setting data. This helps reduce the circuit scale, and thus contributes to the miniaturization of appliances such as portable phones. Furthermore, it is no longer necessary to feed the contrast setting data from a microcomputer to the LCD driver. This helps reduce the operational burden to be borne by the microcomputer. 
     In addition, when the LCD driver is manufactured, it is possible to integrate an EEPROM readily into the same chip as the LCD driver simply by slightly modifying the manufacturing process of CMOS. The EEPROM can, as a voltage step-up circuit it needs for the rewriting of data, use the voltage step-up circuit provided in the LCD driver. This makes efficient use of the circuit components possible. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described.