EL field display technology and its application in portable electronic systems

    Abstract: This article introduces the EL field display technology and the principles and methods of how to construct an EL display control module to control the display of the EL screen. The design block diagram and actual circuit connection diagram of the control module are given. Finally, the software design flow chart and initialization program of this interface technology are given.

    Keywords: EL LCD display control electromagnetic interference interface technology

1 Introduction

As a key device for human-computer interaction, the importance of the display screen is self-evident. Due to limitations of its basic structure, traditional CRT display screens often have unsatisfactory performance in terms of size and power consumption. Then the LCD liquid crystal display appeared. Its compact shape and low power consumption immediately attracted people's attention. Currently, LCDs are widely used in many fields. However, LCD's shortcomings in brightness, visual range, and response time have become the biggest obstacles to its continued promotion. The introduction of EL field-induced display screens has largely made up for the shortcomings of CRT and LCD, and thus represents the development trend of the first generation display screens.

2 EL field display principle

The electron source of field-emission display relies on the electric field to be generated from the tip of the cathode emitter material, which makes the energy distribution range of the field-emission electron beam narrower than that of the traditional thermal electron beam, so it has a higher brightness. Different from the light-receiving mode of LCD, EL screen uses a method similar to traditional CRT, which emits light by bombarding the phosphor on the screen with electron beams. Since CRT has three electron guns inside the picture tube, in order to achieve sufficient deviation of the electron beam, the picture tube must be made a certain distance away, so the CRT monitor is large, thick, and heavy. The EL screen places thousands of tiny protrusion-like electron emitters less than 3mm behind each fluorescent point. Therefore, EL display technology combines the brightness and clarity of CRT cathode ray tubes with liquid crystal displays. It is a new type of display device that combines lightness and thinness. It has the thickness of an LCD display and the fast response characteristics of a CRT display, and can achieve much greater brightness than an LCD display. The built-in tens of millions of redundant electron emitters of the EL screen make it have a wider viewing angle than the LCD, and there will be no situation where a damaged transistor will be clearly shown on the display like the LCD. Even if one is broken, there are thousands more that can be replaced.

3 Main technical parameters of EL display screen

3.1 Temperature range

At present, EL display screens are mainly used in industrial site control and field operations, so they have higher requirements for the temperature range. Under normal circumstances, EL screens can operate normally within the temperature range of -40 to +85°C, while the operating temperature range of TFTLCD is only 0 to 55°C.

3.2 Visible range

The EL screen has a brighter viewing angle range in both horizontal and vertical directions, and its viewing range is ≥160°C.

3.3 EMI indicators

EL screens not only have lower EMI sensitivity, but also can effectively suppress the generation of EMI. The lower EMI sensitivity makes the EL display less susceptible to EMI interference from adjacent power lines and other equipment during operation; while the better EMI suppression characteristics make the EL screen not interfere with its periodic electronic equipment during operation.

3.4 Power consumption

With the further development of miniaturization and miniaturization of modern instruments and meters, the power consumption of electronic devices has become a very important technical indicator. The low power consumption of EL screens makes it possible to use EL screens as human-computer interaction devices in battery-powered portable applications. At the same time, the simple input power requirements also greatly simplify the user's design process.

3.5 Response time

The EL screen has a fast response time and can meet the requirements of real-time display. Its response time is less than 1ms; while the response time of TFTLCD requires 40ms. In applications that have high requirements for display speed, EL screens are the best choice.

4 Application of EL screen in portable systems

In most portable devices, the display options are currently only LCD and EL. But in situations where real-time requirements are high, LCD seems to be somewhat inadequate. The following will focus on the interface application issues between the system and the EL screen based on the design of the ultrasonic digital flaw detector.

Since in ultrasonic flaw detection equipment, it is often necessary to display the collected echo signals in real time, and the display refresh speed must be quite fast, so in this case, the response speed of the LCD is far from meeting the design requirements. Here we An EL320×240.36 EL screen from the American PLANAR company (www.planar.com) was selected. The screen not only has extremely high response speed (less than 1ms), but also has low power consumption (2.4W).

In fact, the interface and compatibility with LCD have been taken into consideration when designing the EL screen, so its interface standard fully matches the 4-bit LCD control signal. Control of EL screens generally requires the following signals:

(1) VS: frame signal;

(2) HS: data latch signal;

(3) VCLK: displacement clock signal of column driver;

(4) VID0~VID3: column driver data lines;

(5) VCC: 5V power input;

(6) VH: 12V power input;

(7) GND: Ground.

However, only these few control signals are far from being able to display text and graphics on the EL screen. Generally, it is necessary to build a display control module by yourself before displaying. Figure 1 shows the overall block diagram of the control module.

In this control module, SMOS133X is the core component. It is a display control chip (SED133X series) launched by SEIKOEPOSON. The chip has a powerful I/O buffer, rich instruction set and four-bit data parallel transmission capability. SED133X can interface with 8080 series and M6800 series MCUs. It consists of an oscillator, functional logic circuit, display RAM management circuit, character library management circuit and a timing generator that generates driving timing. Its oscillator can work in the range of 1MHz ~ 10MHz. SED133X can quickly interpret the instruction code sent by the MCU at a very high operating frequency, place the parameters into the corresponding registers, and trigger the corresponding logic function circuits to operate normally. The control part can manage the 64k display RAM, the built-in character generator and the external expanded character generator CGRAM or EXCGROM. SRAM can be used for display storage of this control module.

Figure 2 shows the actual interface circuit of the control module.

In order for the EL screen to work normally, a series of initialization work must be performed on the SED133X. Figure 3 is the specific process of initialization. The specific instructions are as follows:

(1) The system setting work mainly involves software initialization of the display module. If there is an error in this setting, the system will not display properly. The initial parameters of the system settings include the selection of internal and external character generators, and the character code range of the external character generator CGRAM. The character dot matrix format of the external character generator, the drive system configuration, the height and width of the displayed characters, the length of the effective display window, and the number of dot lines of the EL, etc.

(2) The display area setting is mainly to set the starting address of each display area and the number of displayed lines clicked.

(3) The composite display mode setting is used to specify the composite mode of overlapping display and the display attributes of the first and third display areas.

(4) Cursor address setting is mainly to set the cursor address in parallel. This address has two functions: one is as the current position of the cursor displayed on the display screen, and the other is as the current address pointer of the display buffer. If the cursor address value exceeds the address range corresponding to the display screen, the cursor will disappear, so special attention should be paid when programming. The cursor address will be modified automatically after reading or writing operations.

(5) Cursor form setting is used to set the display mode and shape of the cursor.

After initialization, text and images can be displayed on the display.

The following is the specific initialization program: the occupied registers are:

A, DPTR, R2, R3.

PR1: MOV DPTR, #0001H; command port

MOV A, #40H; SYSTEM SET instruction code

MOVX @DPTR, A; write instruction code

MOV R2, #08H

MOV R3, #00H

PR11: MOV DPTR, #TAB1; write SYSTEM SET parameters

MOV A, R3

MOVC A,@A+DPTR

MOV DPTR, #0000H; data port

MOVX @DPTR,A

INC R3

DJNZ R2, PR11

MOV DPTR, #0001H

MOV A, #44H; SCROLL instruction code

MOVX @DPTR, A; write SCROLL instruction code

MOV R2, #0AH

MOV R3, #00H

PR12: MOV DPTR, #TAB2; write SCROLL parameter

MOV A, R3

MOVC A,@A+DPTR

MOV DPTR, #0000H; data port

MOVX @DPTR,A

INC R3

DJNZ R2, PR12

MOV A, #5AH; HDOTSCR instruction code

MOV DPTR, #0001H

MOVX @DPTR,A

MOV DPTR, #0000H

MOV A, #00H; HDOTSCR P1=0

MOVX @DPTR,A

MOV DPTR, #0001H

MOV A, #5BH; OVERLAY instruction code

MOVX @DPTR,A

MOV DPTR, #0000H

MOV A, #00H; OVERLAY P1=0

MOVX @DPTR,A

MOV DPTR, #0001H

LCALL PR2; clear RAM subroutine

MOV A, #59H

MOVX @DPTR,A

MOV DPTR, #0000H

MOV A, #54H; open text and graphics display

MOVX @DPTR,A

RET

The subroutine for clearing and displaying the RAM area is as follows:

PR2: MOV DPTR, #0001H

MOV A, #4CH; cursor address pointer movement direction

MOVX @DPTR,A

MOV A, #46H; Cursor address pointer setting

MOVA @DPTR,A

MOV DPTR, #0000H

MOV A, #00H

MOVX @DPTR,A

MOVX @DPTR,A

MOV R2, #00H

MOV R3, #00H

MOV DPTR, #0001H

MOV A, #42H; Write data command

MOVX @DPTR,A

CLR A

PR21: MOVX @DPTR, A

DJNZ R3, PT21

DJNZ R2, PR21

RET

(Note: TAB1 and TAB2 are SYSTEM SET and SCROLL instruction parameters. Different tables can be made according to needs, such as TAB1: DB 30H, 87H...etc.)

5 Conclusion

As a new generation of flat-panel display screens, EL screens have become a new generation of display devices replacing LCD screens in industrial control sites and portable instruments due to their own technical advantages and simple display control functions.