Dynamic driving method and application of LCD

    Abstract: This article takes a dot matrix liquid crystal display as an example to introduce its dynamic driving method, and provides a method to overcome the crossover effect. Finally, an example of using the dynamic driving method to drive a segmented LCD is given.

    Keywords: Liquid crystal display equipment, dynamic driving method, cross-effect

    liquid crystal display is due to the electric field applied to the display pixel. This electric field is the synthesis of the potential signals on the two electrodes before and after the display pixel. Since DC electric fields tend to reduce the life of liquid crystals, generally only AC electric fields with very small DC components are established. The DC component is usually less than 50mV. The driving of the liquid crystal display establishes a driving electric field to achieve display by adjusting the phase, peak value, frequency, etc. of the potential signal applied to the electrodes of the liquid crystal display.

    The driving methods of liquid crystal displays include: static driving method, dynamic driving method, dual-frequency driving method, etc. This article only explains the dynamic driving method that is currently the most widely used.

Dynamic driving method

    When the liquid crystal display displays many pixels, such as dot matrix type, in order to save the huge hardware driving circuit, the production and arrangement of the liquid crystal display electrodes are processed, and a matrix structure is implemented: that is, a horizontal group of display pixels are placed on the back The electrodes are all connected together and are called row electrodes; the segment electrodes of the vertical display pixels are all connected together and are called column electrodes. Each pixel on the display is uniquely identified by its column and column. The dynamic driving method of the liquid crystal display is to cyclically apply selection pulses to the row electrodes, and at the same time apply corresponding selection or non-selection driving pulses to all column electrodes, thereby realizing the display function of all pixels in a certain row. This scanning is performed line by line and the cycle period is very short, so that a stable image appears on the LCD screen.

    The selection time of each row in a frame is equal. Assume that the number of scanning lines in one frame is N and the scanning time is 1, then the selection time occupied by one line is 1/N of one frame time. This is the duty cycle coefficient of the LCD driver, also known as the duty cycle.

To overcome the crossover effect,

    in the dynamic driving mode, in order to display a certain position such as point (i, j), it is necessary to apply a selection voltage to the i-th column and j-th row at the same time to maximize the variable electric field strength at this point, but At this time, in addition to the (i, j) point, the other points in the i-th column and j-th row also bear a certain voltage. These points are called semi-selected points. If the effective voltage at the half-selected point is greater than the threshold voltage, an inappropriate display will appear on the screen, causing the contrast to decrease. This is the crossover effect. The way to solve the cross-over effect is the average voltage method, which means to average the voltages at the half-selected point and the non-selected point, and moderately increase the voltage at the non-selected point to offset part of the voltage at the half-selected point, so that the voltage at the half-selected point drops. Improve display contrast. Now illustrate it with Figure 1:

    The selection point in Figure 1 is (SEG1, COM2) [hereinafter referred to as (1, 2)]. Now the V1 voltage is applied to the second row, and the voltage of the other rows is 0V; the -V2 voltage is applied to the first column, and the rest are non-selected voltages 1/a'V1. Next analyze the potential difference at each point, which is the row voltage minus the column voltage.

    Selection points: (1, 2): V 1 + V 2

    Half selection points: (1, 1), (1, 3), (1, 4): V 2 (2, 2), (3, 2), (4, 2); V 1 -1/a'V 1

    non-selected point: -1/a'V 1

    To ensure the display effect of the selected point, keep V1+V2=VLCD at the required voltage value VLCD. At the same time, in order to improve the contrast of the display, let |V2|=|-1/a'V 1 |, that is:

    Solution:

    Let: a'+1=a, we get:

    Therefore, the voltage at each point in Figure 1 is:

    Select point : (1, 2): VLCD

    half selection points: (1, 1), (1, 3), (1, 4): (1/a) V LCD (2, 2), (3, 2), ( 4, 2): [(a-2)/a]V LCD

    non-selected point: - (1/a) V LCD

    can be seen, the voltages at the row half-selected point and the non-selected point are both 1/a of the display voltage VLCD . This 1/a is called the bias coefficient, also called bias. This method is called the average voltage method of 1/a bias, or simply the 1/a bias method. In this method, MAX{[(A-2)/a]VLCD, (1/a)VLCD} will become the scale for adjusting the display contrast.

    When the number of scanning lines N=1, the dynamic driving method is equal to the static driving method.

Application Example of Dynamic Drive Method

    In the process of selecting the display mode of the alcohol concentration detector, the author finally selected the VM807-2 8-bit code segment (7-segment) LCD produced by Hong Kong Jingdian Company through various investigations. This is not only because the LCD display consumes very little, but also because the MCU (PIC16C924) we use already has LCD driving capabilities. Therefore, whether from the perspective of cost reduction or circuit simplification, this choice is undoubtedly more reasonable. of. Below we will explain the specific application of the LCD module in the MCU.

LCD module configuration

    1. Configuration of the LCDCON register

    The LCDCON register is shown in Figure 2:

    among them: LCDEN: LCD module enable; SLPEN: sleep mode enable; VGEN: internal voltage generation enable; CS1: CS0: LCD clock selection bit, "00" = Fosc/256, Fosc = 4MHz; LMUX1: LUMX2: number of common terminals and offset selection, "10" = 3 common terminals, 1/3 bias.

    The configuration code I used is: 10000010.

The configuration of the LCDPS register

    is shown in Figure 3:

    Among them: LP3: LP0 frame clock frequency division selection bit.

    The configuration code I used is: ***0011, where "*" represents "0" or "1".

    According to the frame frequency calculation formula, the frame frequency is:

    Clock source/96(LP3:LP1+1)

    =4×106/96×(3+1)×256

    =40.69Hz

    2. Pixel control

    A pixel is uniquely identified by the first status bit of the pixel register. The LCD module has a total of 16 pixel registers and can control up to 4×29=116 pixels. The LCD we use has a total of 8×8=64 pixels. Therefore, the pixel data register is sufficient. The pixel data register of the LCD is shown in Figure 4:

    Bit 7: Bit 0: SEGCOMC represents the segment code and common terminal number that control the pixel data. Among them, the subscript "S" represents "0 to 32" segment codes, and the subscript "C" represents "1 to 4" common terminals.

    A register bit of "1" means the pixel is turned on (black); a register bit of "0" means the pixel is turned off (bright).

    3. Segment code enablement

    Segment code enablement is achieved through the LCDSE register. Because VIM807-2 is an 8-bit 7-segment display, and we choose 1/3 duty cycle (i.e. 3 common terminals), therefore, 3×8=24 segments must be selected through the LCDSE register to meet the needs, that is, the complete expression All code segment pixels. Of course there will be redundancy, 8×9=72, because every 3 common terminals and 3 segments can express 9 pixels. The public end and segment selection methods are shown in Figure 5.

    The LCDSE register is shown in Figure 6:

    the meanings represented by each bit are the number of bits, pin function, number of pin control segments in 3COM, and selected control segment.

    According to the above, the control code of LCDSE is: 00111011.

    4. Generation of LCD driving voltage

    There are two ways to generate LCD driving voltage, the internal charge pump method or the external ladder resistance network method. Since the LCD charge pump is under development, in order to reduce design risks, we use the more mature external ladder resistance network method. When using an external resistor ladder network, VGEN (LCDCON<4>) should be cleared.

    5. LCD module configuration program

    ...

    MAIN BCF STATUS, PRO

    BSF STATUS, RP1

    MOVLW 0X82

    MOVWF LCDCON

    }

    MOVLW 0X03

    MOVMF LCDPS

    MOVLW 0X3B

    MOVWF LCDSE

    ...