Principle and application of four-digit serial segment LCD display EDM1190A

　　Compared with the seven-segment digital display LED (Light Emitting Diode), the liquid crystal display (LCD) is a display with extremely low power consumption. LCD is a flat film display device. In addition to low power consumption, it also has the advantages of beautiful appearance, low display operating voltage, strong anti-interference ability, and good electrical performance matching with CMOS circuits. Therefore, its application is very wide, from electronic watches to calculators, from pocket meters to portable microcomputers and some word processors all use LCD. At present, there are two types of LCD: segment type and dot matrix type. When it only involves data display and simple letter prompts, smart instruments usually use segment type LCD. The EDM1190A introduced in this article is a practical and beautiful four-digit serial segment LCD module. Compared with some existing parallel segment LCD modules, EDM1190A has fewer pins (nowadays, the general four-digit parallel segment LCD modules generally have up to 40 pins, while EDM1190A only has 4 pins), and is compatible with microcontrollers. The system has the advantages of simple connection and convenient programming.

1 Performance characteristics and pin description of EDM1190A

1.1 Characteristics of EDM1190A The
　　EDM1190A segmented liquid crystal display module consists of an LCD liquid crystal display, a drive circuit, and an 8-bit CPU interface circuit. It has the advantages of low power consumption, strong anti-interference, and wide temperature range; in addition, the input interface signal of EDM1190A is compatible with CMOS and TTL levels, and all four of its pins have electrostatic protection circuits. 1.2 Main technical parameters     (1) Power supply voltage: +5 V;     (2) Driving mode: static;     (3) Viewing angle: 6 points;     (4) Display capacity: 4 digits (with decimal point);     (5) Data transmission Mode: serial;     (6) Display mode: low level display;     (7) Working temperature: 0～+55℃;     (8) Storage temperature: -20～+70℃; 　　(9) Working time: >50 kh. 1.3 Pin introduction　　The appearance and pin diagram of EDM1190A are shown in Figure 1. 　　Figure 1: The appearance and pin diagram of EDM1190A. The viewport size is: 54.2 mm×24 mm, and the character height is: 9 mm×18 mm. The four pins from top to bottom on the far left side of the picture are pins 1, 2, 3, and 4 respectively. The pin functions of EDM1190A are shown in Table 1.　　















   

　　

2 Data transmission principle of EDM1190A

2.1 Data transmission direction
　　
　　Each digit in the seven-segment digital display is composed of seven segments. These seven segments are recorded as A, B, C, D, E, F, G and decimal point respectively. Recorded as DP. The two vertical dots between the second and third digits are marked as CLN. These two vertical dots are used to separate the hours and minutes when displaying the time.
　　
　　The data transmission direction of EDM1190A is shown in Figure 2. When displaying numbers, first input 8 numbers (0 or 1) from the D _IN pin (pin 2) in order to control whether CLN is lit (as long as the D6 bit is equal to 0, CLN will be lit, otherwise CLN will be off). Then input the segment code corresponding to each number from the D _IN pin in sequence, and perform shift transmission in the order of high bit first and then low bit. The segment code corresponding to each digit is shown in Figure 2. When the input signal is "0" (low level), the segment is lit. Note that in order to make the EDM1190A display the correct number, you must finally input another stop bit to the D _IN pin. This bit can be input as 0 or 1.

2.2 Timing of data transmission
　　
　　The timing of data transmission of EDM1190A is shown in Figure 3. The clock signal CLK (pin 4) is also the chip select signal of the LCD module. Data is input from the D _IN pin when the level is high , then the clock signal CLK changes to the low level, and then after a period of time, the input data is latched, thus completing the input of a data. According to this timing, you can Input the signal through the D _IN pin one after another . Each time a digit is displayed (including the decimal point), you only need to shift an 8-bit binary number into the D _IN pin.

3 Application of EDM1190A

3.1 Design of EDM1190A interface circuit
　　The interface circuit between EDM1190A and the microcontroller is shown in Figure 4.   

　　Two LCD display modules EDM1190A are used to display two 4-digit numbers respectively. (1) and (2) The data input pins D _IN of the two EDM1190A are connected to the P1.0 port of the microcontroller. P1.1 and P1.2 of the microcontroller are connected to (1) and (2) the clock signal pins CLK of the two EDM1190A respectively. The binary segment code of the pre-displayed number is output bit by bit from the P1.0 port. As long as the square wave signal is generated at the P1.1 and P1.2 ports in a time-shared manner, it can be controlled on (1) and (2) LCDs. Display numbers.
　　
　　The segment codes corresponding to the displayed numbers of EDM1190A are shown in Table 2.

3.2 Design of display program
　　
　　As shown in Figure 4, the display subroutine of the (1) LCD is used to illustrate the design method of the display program of EDM1190A. Note that no matter how many digits are displayed (1 to 4 digits), the binary number must be input to the D _IN pin of EDM1190A strictly in accordance with the format in Figure 2. The specific program and some comments are as follows:







　　
　　From the above program, it can be seen that the segment codes sent to EDM1190A from high to low are 0FFH, 0DDH, 83H, and 89H respectively. It can be seen from Table 2 that the number displayed on the first LCD at this time is "123".

4 Conclusion
　　EDM1190A is a beautiful and practical four-digit serial LCD display module with extremely low power consumption. Its interface circuit with the microcontroller is very simple, and it is especially suitable for portable instruments. It can not only display four digits, but also display decimal points, symbols, and time.　　

[1] Li Hua, Practical interface technology of MCS-51 series microcontroller［M］． Beijing: Beihang University Press, 2001.
[2] Weng Shaojie. Principle and application of four-digit seven-segment LCD driver ICM7211 [J]. Journal of South China Tropical Crops Institute, 1996, (6).