LCD (Liquid Crystal Diodes) is the abbreviation for Liquid Crystal Display. LCD displays have the characteristics of small size, light weight, extremely low power consumption, easy customization, and rich display content. They are more and more widely used in instrumentation, communication products, household appliances and other fields. According to different types of LCD, its driving mode is divided into static and dynamic two. Among them, the dynamic driving mode can reduce the lead wire of the LCD display and the corresponding driving circuit. It is suitable for more character display and dot matrix display. The dominant way of LCD display driving in the future. However, the dynamic driving and control of LCD are more complicated. In practical applications, a dedicated IC chip is usually used, or a single-chip microcomputer with a dynamic LCD driving interface is directly used. The former is generally more expensive and has poor versatility, while the latter requires a development system or development method for the corresponding single-chip microcomputer. All of these limit the wider application of LCD displays. This article introduces a simple method that can use a general parallel interface to cooperate with the microcontroller software to dynamically drive the LCD, so that anyone with any type of microcontroller development method can use the dynamically driven LCD screen
. As an example, this article uses the parallel port expanded by the serial port of the ATMEL 89C1051 microcontroller compatible with the MCS51 series to realize the drive of an LCD with 3 common back poles and 51 display segments. The display effect is good and the cost is extremely low. 2' General driving principle of dynamic LCD  Due to the electrochemical characteristics of LCD, the driving of LCD generally adopts AC driving. Figure 1 shows the basic LCD drive circuit and operating waveforms. In the figure, A is the display frequency signal, and C is the display control signal. It can be seen from this that when the voltage between the two poles of the LCD is zero, no display is displayed; and when the two ends of the LCD are alternating voltages, the LCD displays. The essence of the dynamic driving method is to use the matrix driving method to drive the display of the field. Here, the field lead is equivalent to the row lead, the common back lead is equivalent to the column lead, and each field of the character is equivalent to a point of the matrix. Because it is an AC drive, it is not possible to use a dynamic drive method like LED, that is, use the common electrode of the LCD as the switch control pole of the display; nor can the LCD drive line be suspended, otherwise the non-gating at the intersection of the floating line and the strobe line The dots will have a cross-display effect due to the capacitive characteristics of the liquid crystal, which will reduce the definition. The general practice is to add a voltage signal lower than the LCD display threshold to the non-gated point to eliminate the influence of the cross effect, such as the bias method. Figure 2 uses a 2×2 matrix as an example to illustrate the situation where only the intersection of the D and S lines are displayed when the bias method is used. The voltage applied on each line and its phase are shown in Figure 2(b), and the voltage of each display point is shown in Figure 2(c). It can be seen from this that there is a working voltage Vc at the display point, and the highest voltage at other points is only 1/2Vc. Therefore, when the display threshold voltage is greater than 1/2Vc but less than Vc, only dot display is displayed. It can be seen from the above that dynamic LCD drive and control are more complicated, so in practical applications, special integrated circuits are usually used, such as MC145000 and MC145001, or single-chip microcomputers with LCD dynamic drive interfaces. 3 The driving methods in this article all use AC square wave signals in the above driving methods, so they cannot be driven by ordinary parallel interfaces. Because the parallel interface signal is a single-level square wave signal, on the one hand, it cannot meet the driving requirements, on the other hand, its DC component will reduce the life of the LCD. If we consider changing the AC square wave signal to an AC sawtooth wave signal, we can use the parallel port and use the transient effect of the capacitor to drive. Figure 3(a) is a circuit that generates an approximate AC sawtooth wave by connecting the parallel port to the LCD. The size of the capacitor C should make the period of the sawtooth wave roughly match the frequency of the binary bit output from the parallel port. Due to the DC blocking and transient effects of the capacitor C, the single-level square wave signal output from the parallel port becomes an approximate AC sawtooth signal when it reaches the LCD electrode, as shown in Figure 3(b) and (c). The driving waveforms in Fig. 3(b) and (c) correspond to the standard 1/2 bias method driving waveforms in Fig. 2(b) and (c) respectively, indicating that the driving principle is completely reliable. From Figures 2 and 3, it can be seen that the LCD display threshold of the sawtooth wave drive will be higher than that of the square wave drive. Therefore, the voltage Vc (that is, the high level of the parallel port) should be adjusted according to the size of the original display threshold of the LCD. Compensate the decline in LCD screen contrast caused by the use of sawtooth waves. 4 Example Figure 4 is a circuit successfully implemented in this article to drive a dynamic LCD with three common back poles and 51 display segments using the 89C1051 serial port and two CD4094 expanded parallel ports. The LCD model is 5N321UF produced by Shanghai Vacuum Electronics, and the capacitance value is selected as 6800pF. Display contrast can be adjusted by the power supply voltage (89C1051 can work between 2.7 to 7 volts, CD4094 is wider), this article finally uses 3.0 volts. Figure 5 is a driving waveform diagram when the three segments connected to a certain pin Pin x of the LCD screen the first segment, and the first, second and third segments are all displayed. This waveform is completed by software in the timer interrupt service routine. The frequency of the timing interruption can be selected from 100 to 200 Hz, so that the display time of each segment is within the persistence of human vision. In the display program, a slice of LCD segment image area can be opened in the microcontroller memory, and the characters to be displayed are matched with the LCD segment through two look-up tables.
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