JPH08115061A - Driving method for liquid crystal display device - Google Patents

Abstract

PURPOSE: To enable gradation display of a frame modulation system in even a liquid crystal display device having no gradation display function by corresponding display data stored in a display memory to a gradation level and rewriting plural frames as one period with a frame unit. CONSTITUTION: In a liquid crystal display device in which a LCD control circuit 31 and display memory 32 and the like are incorporated in one chip IC 36, a LCD control circuit 31 copes with only binary display. Then, a character to be displayed at some position on a liquid crystal panel is written in an address corresponding to the display memory 32 as 'A' in the first frame period. Next, a character to be displayed at the same position in the second frame period is written in the same address as a space. Repeating alternately these operation, display data is transferred to a signal electrode driving circuit 34. Since this transferred data is transferred thinning display data at the rate of one time per two frames, 1/2 gradation level can be realized as display.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of driving a liquid crystal display device. More specifically, the present invention relates to a gradation display method for a liquid crystal display device or a color control method for a multi-color liquid crystal display device using birefringence modulation (hereinafter referred to as ECB effect) of a liquid crystal layer.

[0002]

2. Description of the Related Art In order to perform gradation display on a liquid crystal display panel, there is a difference between an effective voltage of a drive voltage waveform corresponding to display ON and an effective voltage of a drive voltage waveform corresponding to display OFF.
Several effective voltages must be applied to the liquid crystal display panel in correspondence with the gradation level.

Further, in the multi-color liquid crystal display panel using the ECB effect, there is a difference that the effective voltage corresponds to the color, compared with the effective voltage corresponding to the gradation level in the normal liquid crystal display panel. The same applies in that the effective voltage of three values or more is applied to the liquid crystal display panel to control the display.

Therefore, although only the gradation display will be described below, if the same technique is applied to a multi-color liquid crystal display device using the ECB effect, color control can be performed. Well-known techniques such as a pulse width modulation method (PWM), a pulse height modulation method (PHM), and a frame modulation method (FRC) are known as gradation display methods for liquid crystal display panels. The frame modulation method is most widely adopted because it can be easily realized by using an LCD driver IC.

In the frame modulation system, the data transferred to the signal electrode drive circuit is thinned out in frame units according to the gradation level thereof, that is, the data is compulsorily turned off so as to correspond to display OFF. An average effective voltage between display ON and display OFF is obtained as an average value over the period. This data processing is performed by the LCD control circuit which is one of the components of the liquid crystal display device.

[0006]

Therefore, a conventional LCD control circuit having a data thinning function, which is different from the LCD control circuit for binary display, is conventionally required to perform the gradation display of the frame modulation method. It was essential.

[0007]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, the present invention provides a liquid crystal display device having a built-in LCD control circuit for binary display, which does not have a gradation display function. The purpose is to enable gradation display. The following measures have been taken in order to achieve this object.

In general, the liquid crystal display device comprises a liquid crystal display panel 21, a scanning electrode drive circuit 22, a signal electrode drive circuit 23, an LCD control circuit 24, a display memory 25, etc., as shown in the block diagram of FIG. The display data is input from a CPU circuit or the like connected to the outside. Normally, the display data is transferred from the CPU circuit to the display memory only when the display content is updated, or a control command for setting an area used for display in the display memory is input from the CPU circuit to the LCD control circuit. To do. And these timings have nothing to do with the display timings.

On the other hand, according to the first means of the present invention, even if the display content is not changed, the data is periodically transferred from the CPU circuit to the display memory in synchronization with the display timing.
That is, the display data stored in the display memory in a liquid crystal display device having a built-in LCD control circuit for binary display corresponds to a gradation level, and a plurality of frames are divided into one.
By rewriting as a cycle in frame units, data transfer equivalent to thinning out transfer data to the signal electrode drive circuit in frame units using a special LCD control circuit is realized, and gradation display is enabled. .

Further, according to the second means of the present invention, the display memory for a plurality of screens is stored with data corresponding to the gradation levels, and the area used for display is controlled by the CPU circuit to display the contents. Switch regularly even if there is no change in. This switching is performed frame by frame in synchronization with the display timing with a plurality of frames as one cycle, so that in a liquid crystal display device having a built-in LCD control circuit for binary display, a signal electrode is used by using a special LCD control circuit. We realized a data transfer equivalent to thinning out the transfer data to the drive circuit in frame units, and enabled gradation display.

[0011]

In the gradation display method according to the present invention, the display data is transferred from the CPU circuit to the liquid crystal display device by a method different from the conventional one, or the display memory area used for display from the CPU circuit is set. Even if a liquid crystal display device having a built-in LCD control circuit for binary display is used, it is possible to perform gradation display by a frame modulation method only by performing a method different from the conventional method. Therefore, there is no need to change the hardware of the existing binary liquid crystal display device, or
With only a slight change, gradation display is possible by modifying the software of the CPU circuit.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail below with reference to the drawings. (Embodiment 1) FIG. 3 is a circuit block diagram of a character display liquid crystal display device. In this liquid crystal display device, an LCD control circuit 31, a display memory 32, a character generator 33, a signal electrode drive circuit 34, a scan electrode drive circuit 35, etc. are built in a one-chip IC 36. LC
The D control circuit only supports binary display.
In reality, a commercially available LCD controller / driver IC, HD44780 (Hitachi) was used.

A character code of a character to be displayed is stored in a display memory of the liquid crystal display device, and a character designated by the character code is displayed at a predetermined position. The character font data is stored in advance in the character generator. In the case of normal binary display, the character code of the character to be displayed may be written only once at an appropriate address in the display memory. The LCD control circuit automatically reads the display memory in synchronization with the display timing and continuously transfers the display data for one screen (character font data) to the signal electrode drive circuit once per frame. Display is performed on the liquid crystal panel.

A method of performing gradation display in this character display liquid crystal display device will be described below. For example, the character to be displayed at a specific position on the liquid crystal panel during the period of the first frame is "A", and the character code of "A" is written in the corresponding address of the display memory. Then, the character to be displayed at the same position is set as "space" in the period of the next second frame, and the character code of "space" is written at the same address. By repeating this alternately, the display data (character font data) is transferred to the signal electrode drive circuit so that "A" is displayed in the odd frames and "Space" is displayed in the even frames. This data transfer ends up being 2
Since the display data is thinned out and transferred once per frame, the display on the liquid crystal panel
A half gradation level can be realized. This is illustrated in
In the display example 2 of FIG. 4, the character of "A" of 1/2 gradation level is displayed on the background of the OFF level.

If a character code is written so that "A" is displayed in odd frames and "all ON" is displayed in even frames, as in display example 3 of FIG. The character "A" of the ON level is displayed on the background. When displaying the characters of "A" at the ON level on the background of the OFF level, that is, the same display as the binary display as in the display example 1 of Fig. 4, the character code of "A" is set for each frame. It does not have to be written or written once after writing the character code of "A".

The transfer of the display data in the display example 2 of FIG. 4 is shown in more detail in the timing chart of FIG. In this liquid crystal display device, data is written in the display memory at the falling timing of the write enable signal b which is one of the control signals input from the CPU circuit to the LCD control circuit. When focusing on only one character to be displayed at a specific position on the liquid crystal panel, a write enable signal pulse corresponding to that character is generated once per frame as shown in FIG. 1b. The data written to the display memory at that time is switched to the character code of the character "A" and the character code of "space" for each frame as shown in FIG. Then, the character code stored in a specific address of the display memory is, as shown in FIG. 1c, alternating between the character code of the character "A" and the character code of "space" for each frame. It will be.

On the other hand, the transfer of the display data to the signal electrode driving circuit is performed in synchronization with the display timing after the character code stored in the display memory is expanded into the character font data via the character generator. That is,
The character font data for one character is transferred to the signal electrode drive circuit so as to be applied as the drive waveform of the signal electrode during the selection period of the plurality of scan electrodes corresponding to the display position. Considering collectively the data transfer for one character, as shown in d of FIG. 1, it is performed once per frame.

From the above, the transfer data to the signal electrode drive circuit is such that the font data of the character "A" and the font data of "space" are alternately repeated for each frame, and therefore 1 / It is possible to display at two gradation levels. Also, in the examples so far, the display data is rewritten at the cycle of 2 frames to realize the display at the 1/2 gradation level. However, as easily inferred, the display data is rewritten at the cycle of 3 frames. 1 /
It is possible to display 3 gradation levels and 2/3 gradation levels. Generally, n frame periods (n = 2, 3, 4, ...
・) If you rewrite the display data with
1 / n, 2 / n, ..., (n-1) / n gradation levels can be displayed. However, the larger n is, the more flicker occurs on the liquid crystal display screen. There is a limit to this. In this embodiment, LC with a response speed of 300 ms
Using the D panel, it was confirmed that at a frame frequency of 100 Hz, it was at a practical level up to about n = 8.

(Embodiment 2) In Embodiment 1, the display data is rewritten on a frame-by-frame basis. However, if the display memory has a sufficient capacity, the following procedure may be performed without writing the display data for each frame. Thus, gradation display can be realized.

That is, when the display memory 32 of the liquid crystal display device for character display shown in FIG. 3 has a capacity of two or more display screens, for example, as shown in FIG. As display data corresponding to
The character code of the character "A" is displayed in the display memory area of the first screen, and "space" is displayed in the display memory area of the second screen.
Memorize the character code of. Then, the control command for setting the correspondence between the position on the display screen and the address of the display memory is set once per frame in the external CPU.
Input from the circuit to the LCD control circuit, so that the display memory area of the first screen is used for display in odd frames and the display memory area of the second screen is used in display in even frames.

Then, in the character font data transferred to the signal electrode drive circuit, the characters "A" and "space" are alternately repeated for each frame, which is the same as rewriting the data in frame units. In addition, it is possible to display a half gradation level. Generally, a display memory area for m screens is prepared, data corresponding to gradation levels is stored, and an area used for display is set for each frame on a first screen, a second screen, ... If you switch between screen, 1st screen, 2nd screen, ..., 1 / m, 2 as halftone
/ M, ..., (m-1) / m gradation levels can be displayed.

(Embodiment 3) FIG. 6 is a circuit block diagram of a liquid crystal display device for graphic display. In this liquid crystal display device, an LCD control circuit (A) 61 and a scan electrode driving circuit 62 are built in a one-chip IC 63, and an LCD control circuit (B) 64 and a display memory 6 are provided.
5 and the signal electrode drive circuit 66 are different one-chip ICs 6
Built in 7. LCD control circuit (A)
, Has a main function of generating a display timing signal, and the LCD control circuit (B) has a main function of controlling a display memory. The LCD control circuits (A) and (B) only support binary display. In reality, a commercially available LCD driver IC, HD6620
3 and HD66202 (both Hitachi) were used.

In this liquid crystal display device, one bit of the display memory corresponds to one dot of the display screen. When the display data is "0", the display is off, and when the display data is "1", the display is on. In the case of normal binary display, if the display data in which ON / OFF of the graphic pattern to be displayed corresponds to “1” / “0” is written only once to an appropriate address in the display memory, the LCD control circuit (A )
The LCD control circuit (B) automatically reads out the display memory in synchronism with the display timing generated by, the display data is continuously transferred to the signal electrode drive circuit, and the display is performed on the liquid crystal panel. This data transfer is performed once per frame for display data for one screen.

Therefore, the difference between the character display liquid crystal display device of the first embodiment and the graphic display liquid crystal display device of the present embodiment is that the display data stored in the display memory is character code data or display pattern data. Therefore, also in the present embodiment, similarly to the first embodiment, by rewriting the display data in units of frames, it is possible to realize the gradation display of the frame modulation method.

For example, as shown in FIG. 7, the bits of the display memory corresponding to the display dots A, B, C and D are a, b, c and d, respectively. At this time, a, b, c,
As shown in FIG. 7, when the data of d has 3 frames as a cycle,
(A, b, c, d) = (0,1,1,1), (0,0,
Each frame is rewritten into three types (1, 1) and (0, 0, 0, 1). Then, the display dot A is at the OFF level,
Display dot B is 1/3 gradation level and display dot C is 2 /
3 gradation levels, and the display dot D is an ON level display.

(Embodiment 4) When the display memory 65 of the liquid crystal display device for graphic display of FIG. 6 has a capacity for a plurality of screens, gradation display can be realized by the same method as in Embodiment 2. . That is, the data corresponding to the gradation level is stored in each display memory area corresponding to the plurality of display screens. Then, by switching the region used for display for each frame with a plurality of frames as one cycle, gradation display by the frame modulation method can be performed.

[0027]

As described above, according to the present invention,
A plurality of frames is set as one cycle, and data changed in frame units is written to the display memory from the outside in a frame unit in synchronization with the display timing, or a plurality of frames is set as one cycle for display in the display memory. By switching the area to be used in frame units in synchronization with the display timing, even in a liquid crystal display device having an LCD control circuit that supports only binary display, it is possible to perform gray scale display by a frame modulation method. It has an excellent effect.

[Brief description of drawings]

FIG. 1 is a timing diagram of display data transfer according to the present invention.

FIG. 2 is a block diagram of a general liquid crystal display device.

FIG. 3 is a circuit block diagram of a general character display liquid crystal display device.

4 is a display example for realizing the present invention in the display device of FIG.

5 is an example of display data for realizing the present invention in the display device of FIG.

FIG. 6 is a circuit block diagram of a general graphic display liquid crystal display device.

7 is an example of display data for realizing the present invention in the display device of FIG.

[Explanation of symbols]

 21 LCD panel 22 Scan electrode drive circuit 23 Signal electrode drive circuit 24 LCD control circuit 25 Display memory 31 LCD control circuit 32 Display memory 33 Character generator 34 Signal electrode drive circuit 35 Scan electrode drive circuit 36 IC 37 LCD panel 38 External CPU circuit 39 Data bus 40 Control signal 61 LCD control circuit (A) 62 Scan electrode drive circuit 63 IC 64 LCD control circuit (B) 65 Display memory 66 Signal electrode drive circuit 67 IC 68 Liquid crystal panel 69 External CPU circuit 70 Data bus 71 Control signal

Claims (3)

[Claims] 1. A liquid crystal display panel in which a liquid crystal layer is held between scan electrode groups and signal electrode groups to provide pixels in a matrix, a scan electrode drive circuit for applying a voltage to the scan electrode groups, and From a signal electrode drive circuit that applies a voltage to a signal electrode group, a display memory that stores display data corresponding to display contents, an LCD control circuit that controls the scan electrode drive circuit, the signal electrode drive circuit, and the display memory In the method for driving a liquid crystal display device, the liquid crystal display is characterized in that data changed in frame units with a plurality of frames as one cycle is externally written in the display memory in frame units in synchronization with display timing. Device driving method. 2. A liquid crystal display panel in which a liquid crystal layer is held between scan electrode groups and signal electrode groups to provide pixels in a matrix, a scan electrode driving circuit for applying a voltage to the scan electrode groups, A signal electrode drive circuit that applies a voltage to a signal electrode group, a display memory that stores display data corresponding to display contents for a plurality of screens, an LCD that controls the scan electrode drive circuit, the signal electrode drive circuit, and the display memory In a method of driving a liquid crystal display device including a control circuit, a plurality of frames are set as one cycle, and an area used for display in the display memory is switched frame by frame in synchronization with display timing. Driving method. 3. The method for driving a liquid crystal display device according to claim 1, wherein the liquid crystal display panel is a multi-color liquid crystal display panel using a birefringence index modulation of a liquid crystal layer.