Segment Code LCD Drive Principle

- Jun 06, 2018-


I. Overview
 There are two types of displays commonly used in instrumentation. One is a light emitting diode (LED) and the other is a liquid crystal display (LCD). These two types of displays are low-cost, flexible in configuration, and easy to interface with a single-chip microcomputer, while the latter has a small drive current, low power consumption, long life, good-looking fonts, clear display, large viewing angle, flexible drive, and wide application [1]. However, the LCD on the control is more complicated because the DC voltage between the LCD electrodes must be 0 [2]. Otherwise, the LCD will be oxidized easily. Therefore, the LCD cannot be controlled by the level signal simply, but the waveform must be used. Wave sequence to control. LCD display has both static and time split
The former is simple, but requires more lines; the latter is complicated, but requires fewer lines, which are determined by the choice of electrode lead. The following is an example of a liquid crystal display of an electronic watch. The display panel is shown in (1). The high of the hour is also off or on. When the high of the minute is on the display of numbers 1 to 5, the top and bottom are also off or on. The two dot points are also on or off at the same time. The driving method is split driving with a bias ratio of 1/2. There are 11 segment electrodes and two common electrodes.



figure 1)
Second, LCD display principle
General substances can be divided into gas, liquid and solid. However, the properties of some substances do not belong to these three kinds. Liquid crystal is one of them. It is not a complete liquid, nor a complete solid. It can flow like a liquid and has solid crystals. In the natural state, the liquid crystal molecules are placed in very fine concavities, and the liquid crystal molecules are arranged in the direction of the grooves [3]. LCD monitors work by using these properties of liquid crystals. A liquid crystal material is added between the upper and lower electrodes of the LCD display. The liquid crystal molecules are arranged in parallel and have optical activity. The liquid crystal molecules are usually transparent. When a certain voltage is applied between the upper and lower electrodes, the liquid crystal molecules turn vertically and lose their optical rotation. Black [4]. In order to prevent the liquid crystal from oxidizing, it is required that the relative voltage DC average between the LCD electrodes must be zero [1], so the LCD cannot be driven by the level signal simply, but must be driven by a certain square wave sequence. The driving waveform is very particular, and the time division method with an offset ratio of 1/2 is taken as an example. Figure (2) shows the waveform that should be generated on the segment and common electrodes to make a stroke bright or off. From figure (2) we can see that B1 and COM2 are in the waveform direction, so B1 is bright; B3 and COM1 are in the same direction, so B3 is off [5]. (where B1 and B3 share a single SEG port)

figure 2)
Generally, the waveform of the COM port is always fixed. For the dynamic 1/2-hour division mode, the waveforms at the COM1 and COM2 sides are in opposite phases. To control the display and extinguishment of each stroke, appropriate waveforms must be generated on the corresponding electrodes. The realization of the waveform has the following characteristics: 1) It can be seen from the two common electrodes that the two common electrodes have three levels, which are three voltages of 0V, 1.5V and 3V respectively; 2) Two common electrodes COM1 and The COM2 waveform is directional; 3) The period of the common electrode and segment code driving waveform is the same, in which the common electrode changes every cycle four times, and the segment code changes twice each cycle, which is a square wave signal. Because of the characteristics of common electrode driving waveforms, in the industry, most of the microcontrollers and corresponding software are used to generate the common electrode driving waveforms. For the ASIC design, if the above method is used, a large chip area is occupied and the number of chips increases. Cost. Therefore, this article will introduce a practical digital and analog circuit as a segmented LCD driver.
Third, LCD display driver circuit design
1. COM1 and COM2 waveform generation circuit
Design points: As described in the Display Principle section, the waveforms of the two common electrodes are fixed. It has 3 levels, which are 0V, 1.5V, 3V, and each cycle changes 4 times. The waveforms of COM1 and COM2 are Directional. Figure (3) shows the solution. The circuit is composed of an NMOS transistor and a 3-state control gate. The frequency of DA is 2 times that of d3. The NMOS tube is connected to 1.5V and the 3-state gate is set to 3V. This can generate Each cycle changes 4 times, there are 3 levels of fixed common electrode waveforms. In order to be recognized by the human eye, the frequency of d3 is 10 Hz. The HSPICE waveform generated by this circuit is shown in (3-1) (using a 1.5V power supply and a 3V voltage generated by a peripheral voltage doubler circuit). To achieve this design requirement, in Figure (3), the W/L of the N-tube is 28uM/4uM, the W/L of the two P-tubes of the 3-state gate is 8uM/3uM, and the W/L of the two N-tubes is 4uM /3uM.

image 3)

Figure (3 -1)
2. SEG mouth circuit and waveform
Technical point: 11 segments and 2 common electrodes drive the display of the electronic watch, and the segment and common electrode cycles must remain the same. The solution is shown in figure (4). Figure (4) is a segment drive circuit consisting of an XOR gate and a NOT gate. In order to keep the common electrode and segment cycle consistent, the input signal d3 And d3 in COM circuit is the same signal, it is a periodic square wave with frequency of 10Hz; The signal of D1 is produced by the decoding circuit, it decides the electronic table reveals the digital decoding, the result produced by three types, constant is High level 1, constant level 0, periodic square wave (2 times the frequency of d3, period is 1/2), Fig. 4-1, Fig. 4-2, Fig. 4-3 ) These are the waveforms generated by verilog_xl corresponding to the above three cases. The SEG port is implemented using digital circuits and there is no requirement for transistor size.

Figure 4)

From the simulation waveforms of the common electrode and the segment code electrode, it can be seen that the designed circuit meets the requirements of the liquid crystal display principle, the common electrode changes 4 times per cycle and 3 different levels, and the period of the common electrode and the segment electrode must be consistent To make a stroke bright or off, the SEG and (COM) ports must satisfy a certain relationship. The relationship is as shown in the following table: When the SEG port and COM1 port are inverted, the corresponding segment is extremely bright. When in phase, the corresponding segment is extinct.
Four, summary
   The LCD driving circuit introduced in this article is completely implemented by hardware, and it is constructed by very few transistors. The design is exquisite. It can be well integrated in the application-specific integrated circuit. As the driving circuit LCD of LCD, this reduces the cost and has a competitive advantage in the market. . This is different from other hardware and software implementations of the LCD drive in the market. We have integrated the LCD driver circuit module in an ASIC coffee maker chip. The chip has already completed FPAG verification and placement and routing, and performs MPW in Shanghai.