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). The two displays are low in cost, flexible in configuration, and convenient to interface with a single chip microcomputer, and the latter has low driving current, low power consumption, long life, beautiful font shape, clear display, large viewing angle, flexible driving mode, and wide application . However, the LCD is more complicated in control, because the relative voltage DC average value between the LCD electrodes must be 0 , otherwise the LCD may be oxidized, so the LCD cannot simply be controlled by the level signal, but a certain waveform is used. Wave sequence to control. LCD display has static and time splitting
The former is simple, but requires more lines; the latter is complicated, but fewer lines are required. These two methods are determined by the choice of electrode leads. Taking the liquid crystal display of the electronic watch as an example, the display panel is as shown in (1), and the hour high is off or on at the same time. When the minute high is displayed in the digital 1 to 5, the top and bottom are also off or on at the same time. The two dot points are also turned on or off at the same time, and the driving method is a time division driving with a bias ratio of 1/2, and there are 11 segment electrodes and two common electrodes.
Second, the display principle of LCD
General substances can be divided into gases, liquids and solids. However, the properties of some substances do not belong to these three types. Liquid crystal is one of them. It is not a complete liquid or a complete solid. It flows like a liquid and has solid crystals. In the natural state, the liquid crystal molecules are placed in a very thin concave Cao, and the liquid crystal molecules are arranged in the direction of the groove . LCD displays work with these properties of liquid crystals. A liquid crystal material is added between the upper and lower electrodes of the LCD display, and the liquid crystal molecules are arranged in parallel, and have optical rotation, and are usually in a transparent state. When a certain voltage is applied between the upper and lower electrodes, the liquid crystal molecules are turned into vertical alignment, and the optical rotation is lost. Black . In order to prevent liquid crystal oxidation, it is required that the relative voltage DC average value between the LCD electrodes must be zero , so the LCD cannot be driven simply by the level signal, but is driven by a certain square wave sequence. The driving waveform is very particular, taking the time division method with a bias ratio of 1/2 as an example. Figure (2) shows the waveform that should be produced on the segment and common electrodes for a certain stroke to be bright or not. As can be seen from the figure (2), the waveform direction of B1 and COM2, so B1 is bright; B3 is in the same direction as the waveform of COM1, so B3 is off . (where B1 and B3 share a single SEG port)
Generally, the waveform of the COM port is always fixed. For the dynamic 1/2 time division mode, the waveforms of the COM1 and COM2 terminals are inverted. To control the display and extinction of each stroke, a suitable waveform must be generated on the corresponding electrode. The realization of the waveform has the following characteristics: 1), as seen on the two common electrodes, the two common electrodes have three levels, which are 0V, 1.5V and 3V respectively; 2), two common electrodes COM1 and The COM2 waveform is directional; 3), the period of the driving waveform of the common electrode and the segment code is the same, wherein the common electrode changes four times per cycle, and the segment code changes twice per cycle, which is a square wave signal. Because of the characteristics of the driving waveform of the common electrode, in the industry, most of the microcontroller and the corresponding software are used to generate the driving waveform of the common electrode, and for the design of the ASIC, if the above method is adopted, it takes up a large chip area and increases The cost. Therefore, this article will introduce a practical digital and analog circuit as a segment-type LCD driver.
Third, LCD display driver circuit design
1. COM1 and COM2 waveform generation circuits
Design points: As described in the Display Principles section, the waveforms of the two common electrodes are fixed. There are three levels, 0V, 1.5V, 3V, and each cycle changes 4 times. The waveforms of COM1 and COM2 are Direction. Figure (3) shows the solution. The circuit consists of an NMOS transistor and a 3-state control gate. The frequency of the DA is twice that of d3, where the NMOS transistor is connected to 1.5V and the 3-state gate is used with 3V. Each cycle changes 4 times and there are 3 levels of fixed common electrode waveforms. In order to be recognized by the human eye, the frequency of d3 is 10Hz, and the HSPICE waveform generated by this circuit is shown in Figure (3-1) (powered by 1.5V power supply, 3V voltage is generated by peripheral voltage doubler circuit). To achieve such design requirements, in Figure (3), W/L of the N-tube is 28uM/4uM, W/L of the two P-tubes of the 3-state gate is 8uM/3uM, and W/L of the two N-tubes is 4uM. /3uM.
Figure (3 -1)
2. SEG port circuit and waveform
Technical points: 11 segment poles and 2 common electrodes drive the display of the electronic watch together, and the segment pole and common electrode periods must remain the same. The solution is shown in Figure (4). Figure (4) is the segment pole drive circuit. It is composed of an XOR gate and a NOT gate circuit. In order to keep the common electrode and segment period consistent, the input signal d3 And d3 in the COM circuit is the same signal, which is a periodic square wave with a frequency of 10Hz; the signal of D1 is generated by the decoding circuit, which determines the digital decoding of the electronic watch, and the generated result is of three types, High level 1, constant level 0, periodic square wave (the frequency is twice the d3, the period is 1/2), Figure (4-1), Figure (4-2), Figure (4-3) ) are the waveforms generated by verilog_xl corresponding to the above three cases. The SEG port is implemented with a digital circuit, and there is no requirement for the size of the transistor.
It can be seen from the simulation waveforms of the common electrode and the segment electrode that the designed circuit conforms to the requirements of the liquid crystal display principle, the common electrode changes four times per cycle and three different levels, and the period of the common electrode and the segment electrode are kept consistent. To make a stroke bright or not, the segment (SEG) and (COM) ports must satisfy a certain relationship. The relationship is as shown in the following table: When the SEG port and the COM1 port are inverted, the corresponding segment is extremely bright. When in phase, the corresponding segment is extremely extinguished.
The LCD driver circuit introduced in this article is completely realized by hardware, and is built by few transistors. It is compact in design and can be well integrated in the ASIC as the driving circuit LCD of the LCD, which reduces the cost and has the competitive advantage in the market. . This is different from other LCD drivers that are implemented by hardware and software on the market. We have integrated the LCD driver circuit module into the ASIC coffee maker chip, which has completed FPAG verification, layout and routing, and MPW in China.