Interface Design of Control Driving Device and Liquid Crystal Display in Embedded Human-Machine Interface
“In this paper, the characteristics of LCD control driver and its MCU interface design and the specific designs of various common types are described in detail, and its basic circuit design is explained. Applying these rules to the design of embedded human-machine interface will surely be able to produce LCD interface with more compact structure, more stable and reliable performance, and lower cost.
Author: Qi Zhaoqian
Liquid crystal Display, stable and reliable, low cost, low power consumption, easy to control and drive, easy to use interface, compact modular structure, has been widely used as a human-machine interface in embedded systems. In recent years, many domestic manufacturers, such as Amethyst, Jiya, Jinghua, Truly, Pengyuan, etc., have been able to meet the needs of various customized liquid crystal displays; many famous semiconductor manufacturers, such as Hitachi, Seiko Epson, Toshiba, Holtek, Solomon, Samsung and others have successively launched many control drive devices. This paper comprehensively expounds how the existing control driving devices and liquid crystal displays constitute various embedded human-machine interfaces with compact structure, low cost, easy use and excellent performance.
1. Overview of liquid crystal display and its control drive and interface
Liquid crystal display LCD (Liquid Crystal Display) is an important flat panel display device made by utilizing the performance of liquid crystal material to block/transmit light due to positional change under the action of an electric field. Commonly used LCD devices are TN type (Twist Nematic, twisted nematic liquid crystal), STN type (Super TN, super twisted nematic liquid crystal) and TFT type (Thin Film Transistor, thin film transistor type liquid crystal). The performance of TN, STN, and TFT liquid crystals increases in turn, and the production cost also increases. TN and STN types are often used as monochrome LCDs. The STN type can be designed as a monochrome multi-level grayscale LCD and a pseudo-color LCD, and the TFT type is often used as a true color LCD. TN and STN type LCDs cannot be made into large-area LCDs, and the number of colors is less than 218. Those with less than 218 colors are called false colors, and those with 218 or more colors are called true colors. TFT type can realize large-area LCD true color display, and its pixels can be made about 0.3mm. TFT-LCD technology is becoming more and more mature, and the long-standing problems have been solved: the viewing angle is up to 170°, the brightness is up to 500cd/m2 (500 nits), the display size is up to 101.6cm (40in), and the change speed is up to 60 frames/s.
The LCD design is mainly about the control/driving of the LCD and the interface design with the outside world. The control is mainly to communicate with the outside world through the interface, manage the internal/external display RAM, control the driver, and distribute the display LCD for display. The controller also often contains an internal ASCII character library, or an externally expandable large-capacity Chinese character library. For small-scale LCD design, integrated control/driver is often used; for medium-scale LCD design, several controllers and drivers are often used, and appropriate display RAM, self-made character RAM or ROM font library are expanded externally. Control and driver mostly use low-voltage micro-power devices. The interface with the outside world is mainly used for LCD control, usually the 8/16-bit PPI parallel port that can be connected to the microcontroller MCU or the SPI serial port of several control lines. Except for some Samsung devices that need to use self-refresh dynamic SDRAM, most of the company’s devices use static SRAM. LCD types commonly used in embedded human-machine interfaces and their typical control/driving devices and interfaces are as follows:
Segment LCD, such as HT1621 (control/drive), 128-point display, 4-wire SPI interface;
Character LCD, such as HD44780U (control/drive), 2 lines × 8 character display, 4/8 bit PPI interface;
Monochrome dot-matrix LCD, such as SED1520 (control/drive), 61-segment × 16-line dot-matrix display, 8-bit PPI interface, such as T6963C (control) + T6A39 (column drive) + T6A40 (row drive), 640×64 Point dual-screen display, 8-bit PPI interface;
Gray-scale dot-matrix LCD, such as HD66421 (control/drive), 160×100-dot monochrome 4-level grayscale display, 8-bit PPI interface;
Pseudo-color dot matrix LCD, such as SSD1780 (control/drive), 104RGB×80 dots display, 8-bit PPI or 3/4-wire SPI interface;
True color dot matrix LCD, such as HD66772 (control/source drive) + HD66774 (gate drive), 176RGB×240 dots display, 8/9/16/18 bit PPI interface, 6/16/18 animation interface, synchronous serial interface ;
Video conversion LCD, such as HD66840 (CRT-RGB→CD-RGB), 720×512 dots display, monochrome/8-level grayscale/8-level color, 4-bit PPI interface.
The power supply circuit, the driving bias circuit, the backlight circuit, and the oscillation circuit that control the driving device constitute the basic circuit of LCD control and driving. It is the basis of LCD display.
The LCD together with its control driver, interface and basic circuit constitute LCM (Liquid Crystal Module, LCD module). In conventional embedded system design, most of the off-the-shelf LCMs are used as man-machine interface; in modern embedded system design, LCD, its control and driving devices, and basic circuits are often directly incorporated into the system. The overall consideration is not only compact in structure, but also reduces cost, and is conducive to reducing power consumption and realizing product miniaturization.
To control the LCD display, a single-chip MCU is often used, and through the PPI or SPI interface of the LCD part, it is executed according to several protocol instructions of the LCD controller. The LCD program of MCU generally includes initialization program, management program and data transmission program. Most LCD control driver manufacturers provide assembly or C language routine data with the device, which is very convenient for programming.
2. Common LCD control driver and interface design
2.1 Control driver and interface design of segment LCD
Segment LCD is used to display segment-shaped numbers or fixed-shaped symbols, and is widely used for counting, timing, status indication, etc. The commonly used control and drive device is Holtek’s HT1621, which contains video memory and oscillator circuits corresponding to LCD display points one-to-one, low-voltage and low-power consumption, 4-wire serial MCU connection, 8 control/transmission commands, and can perform 32 segments. ×4 lines = 128 points of control display, the display contrast can be adjusted externally, and the drive performance such as bias voltage and duty cycle can be selected programmable. The HT1621 controls and drives the LCD and its MCU interface as shown in Figure 1.
2.2 Control driver and interface design of character LCD
2.3 Control driver and interface design of monochrome dot matrix LCD
Monochromatic dot matrix LCD is used for graphics or graphic text mixed display, widely used in mobile communication, industrial monitoring, PDA products. Small-area LCDs often use monolithic integrated control and driving devices, such as Seiko Epson’s SED1520, which can achieve 61-column × 16-row dot matrix display; medium-area LCDs often use monolithic control/column driving devices and monolithic row driving devices, such as Hitachi The HD61202U (controller/column drive) and HD61203 (row drive) can realize 64×64 dot matrix display; larger area LCD often adopts the form of “controller + video memory + column driver + row driver”, such as Toshiba’s T6963C (controller ), T5565 (video memory), T6A39 (column drive), T6A40 (line drive), which can realize 640×128 dot matrix display. These drivers often require 12~18V negative power supply for bias and contrast adjustment. Most of the control devices can be connected to an external resistor-capacitor RC to form an oscillator or an external oscillator or an external clock. Each bit in the video memory corresponds to the LCD display point one-to-one. When text display is required, the ASCII font library integrated in the controller can be directly used for simple characters, and the large-capacity font library CGROM or the self-made font library CGRAM can be expanded outside the controller for displaying Chinese characters or self-made characters. The control interface is usually an 8-bit PPI 68XX or 80XX MCU interface (the connection with the MCU also has two forms of direct connection and indirect connection), 7~13 control/transmission instructions, which can realize drawing functions such as dots, lines and circles. Controllers T6963C, HD61830, SED1335, etc. can realize single and double screen LCD control. This is the result of adapting to the display of mobile communication, which essentially divides the video memory equally and corresponds to two LCD screens respectively. When compiling the data transmission program, pay attention to properly transforming the data form according to the characteristics of the video memory. For example, the 8-bit data in the SED1520 video memory is vertically arranged, and the data in the HD61202 video memory is vertically arranged. Figure 3 is a block diagram of Seiko Epson’s SED1335 controller, external expansion memory SRAM, self-made font library SGRAM, large-capacity Chinese character library CGROM, LCD and 80XX MCU interface composed of column driver SED1606 and row driver SED1635, which can realize 640×56 Monochrome dot matrix LCD display.
2.4 Control driver and interface design of gray-scale dot-matrix LCD
Gray-scale dot-matrix LCDs are widely used in small measurement and control systems and low-cost handheld devices. In the video memory of the controller used by this kind of LCD, every n bits correspond to one LCD display point, and the gray level realized by the entire LCD is 2n. Hitachi’s HD66421 is a commonly used economical grayscale dot matrix LCD control driver. A single piece of HD66421 can realize LCD grayscale display of 22 levels, 160 columns and 100 rows of dots with a few resistive components. Using HD66421 in parallel can realize a larger area of LCD display. HD66421 is embedded with 160 × 100 × 2-bit video memory, has 8-bit PPI interface, can be directly connected to 80XX MCU, 8 control/transmission instructions, programmable change drive characteristics and adjust grayscale types. HD66421 needs an external resistor R to form a system oscillation circuit, and a negative power supply is required to achieve bias. HD66421 is a highly integrated device with a 322-pin package. It is difficult to design the circuit board and should be paid enough attention. HD66421 controls and drives grayscale dot matrix LCD and its interface with 80XX MCU is shown in Figure 4.
2.5 Control driver and interface design of pseudo-color dot-matrix LCD
Color LCD display is based on the superposition principle of red R, green G, and blue B three primary colors. Each LCD pixel is composed of three RGB sub-pixels, which are respectively driven by three RGB color segments. Color LCD display requires larger video memory, each color segment has 2n colors, it needs to occupy n bits of video memory. Color LCD display is the inevitable result of LCD upgrading. Pseudo-color display often uses cheap STN type LCD, which is mostly used in mobile communication, PDA and other products. Solomon Systech’s SSD1780 is a typical monolithic highly integrated pseudo-color dot matrix LCD control driver device. It contains 312 × 81 × 4-bit graphics data memory GDDRAM, 477kHz oscillator circuit, integrated bias circuit and DC-DC circuit; with 8-bit PPI interface (can be directly connected to 80/68XX MCU) and 3/4-wire SPI Serial interface, 36 control/transfer commands. With the addition of a few capacitors, the SSD1780 can control and drive a 104RGB×81-dot color STN-type LCD, displaying 23n=4096 colors. The SSD1780 is a 627-pin package, and the PCB design of the circuit board is more difficult and must be taken seriously. SSD1780 controls and drives pseudo-color STN dot matrix LCD and its interface with 80XX MCU is shown in Figure 5.
2.6 Control driver and interface design of true color dot matrix LCD
Modern high-end PDAs, home appliances, display walls, etc. increasingly apply true-color dot-matrix LCD display technology. The number of colors displayed in LCD true color is more than 218. Compared with false color display, it requires larger video memory and higher control and driving technology, and needs to achieve high-speed animation. LCD true color display uses TFT type LCD, active dot matrix display, need to use source driver (source driver) and gate driver (gate driver) to control the source and gate of LCD field effect transistor FET. The source driver receives the display data to drive the LCD column display, also known as the data driver (data driver), and the gate driver controls the progressive scanning. Hitachi’s HD66772 series of true-color LCD control and drive devices are ideal for expressing a colorful world in embedded human-machine interface design. This family of devices includes the HD66772, HD66774, HD66775 and HD667P01. HD66772 is a controller with embedded 95KB video memory and a source driver of 176RGB segments, HD66774 is a 240-row gate driver with drive power supply, HD77665 is only a 120-row gate driver, HD667P01 is a drive power supply device, and HD66772 has a 80XX MCU Direct connect 8/16-bit PPI interface, 6/16/18-bit animation interface and synchronous serial interface. Use HD66772 series devices to control and drive 176RGB×240-dot TFT LCD true color display, there are two solutions: ①1 HD66772 + 1 HD66774; ②1 HD66772 + 2 HD66775 + 1 HD667P01. The former is compact and the latter is more economical. Figure 6 shows the block diagram of the LCD control driver connection and the 16-bit MCU interface of the previous scheme.
2.7 Control driver and interface design of video conversion LCD
In industrial control and embedded control systems, there are many LCD video driver designs. In this design, it is often necessary to select special devices, convert video signals, control and drive LCD, and perform animation display to achieve product compatibility and expand product performance. Hitachi’s HD66480F is such a typical device. It can easily take out the CRT signal from the video interface of the computer and directly drive the black and white or color LCD through video conversion, so that the display content on the CRT type display appears on the LCD screen at the same time. HD66840F can control and drive the LCD with a maximum of 720×512 dots to achieve monochrome, 8-level grayscale or 8-level color display. The HD66840F has a 4-bit controlled interface, which can be directly connected to an 8-bit MCU to achieve video display environment settings. When using HD66840F, 8-bit RGB display cache SRAM needs to be expanded. Figure 7 illustrates the design block diagram of using HD66840F to expand the display buffer HM6264, under the control of 8-bit 80XX MCU, to convert CRT signal and control and drive HD66772 color dot matrix LCD animation display.
3. Basic circuit design of LCD control driver
3.1 Design of the basic power supply circuit
The basic power supply voltage of LCD control and driving devices is generally 1.8~5.5V. Modern embedded system design emphasizes low-voltage and micro-consumption, and 1.8V, 2.5V, 3.0V or 3.3V devices are mostly used. The power consumption of all the devices mentioned above in the working state is below several to tens of mW, and can work in the voltage range of 1.8~3.6V. It is very important to select and design a power supply circuit with appropriate power and stable voltage. Many semiconductor manufacturers produce various types of series of micro-power high-performance power devices, such as Torex’s XC6203 series, Richtek’s RT9168/A series voltage regulators, AME’s AME8800 series, AME8811 series step-down devices, On Somlconductor’s NCP1400A series, Maxim’s MAX1795 family of boosters, and more. For these devices, the output voltage can be any value between 1.5~5V, the accuracy is ±1.2%~±2.5%, and the maximum output current is 100~500mA. By selecting these devices and adding a few resistance-capacitance-inductance devices or Schottky diode devices, a basic power supply circuit suitable for LCD control and driving devices can be designed. Figure 8 is the power supply circuit designed for HD66421, which is very simple.
3.2 Design of driver bias circuit
Graphical dot matrix LCD drivers often require a drive bias network and a negative power supply to achieve bias. The bias network can be configured according to the resistance-capacitance value recommended by the driver manufacturer, and the negative power supply can be realized by selecting an appropriate negative voltage device. Commonly used methods to generate negative power supply are: using 79 series three-terminal integrated voltage regulator, such as using LM7918, you can get -18V negative voltage source; using DC-DC IC production, such as Maxim’s MAX749, MAX680, MAX1860/1861, Motorola’s MC34063A, etc. Figure 9 is the -12V negative power supply circuit designed with MC34063A.
3.3 Backlight circuit design
LCD backlight, usually in the form of LED, EL (Electro Luminescence) and CCFL (Cold Cathode Lamp). Character type or small and medium dot matrix LCD, mostly use LED or EL backlight, LED is mainly yellow (red-green tone), generally driven by 4.2V; EL is mainly yellow-green (red, green and white tone), generally 1W, 400 ~ 800Hz, 70~120V AC drive. Medium and large dot-matrix STN type and TFT type LCDs are mostly white (red, green and blue) CCFL backlights, generally driven by 25kHz~100kHz, AC above 300V. EL and CCFL backlight circuits can be built with IC devices or finished modules. IC devices build a backlight circuit, such as IMP’s IMP525/562/803, with a few resistance-capacitance inductance devices, to form an EL backlight circuit, as shown in Figure 10; Maxim’s MAX1635 with a transformer to form an EL backlight circuit; Maxim’s MAX1610, Linear’s 1182 Or TI’s Vcc3972 and transformer components to build CCEL backlight circuit. Finished backlight modules, such as Senbao’s VET-N1210-01 CCEL module and Jingdian Fengyuan’s PYE series EL/CCEL modules. Building a backlight circuit with IC devices can compact the structure and reduce costs, and is often used in embedded system design.
3.4 Oscillation circuit design
Most LCD control drivers have internal oscillators and external oscillators or external clocks, which is very convenient for design. In order to simplify the peripheral circuit design, the internal oscillator of the control driver is often selected as the clock source. In this case, many control and drive devices often require external RC devices, which can be configured according to the instructions in the device guide.
The above has elaborated the characteristics of LCD control driver and its MCU interface design and the specific designs of various common types, and explained its basic circuit design. Applying these rules to the design of embedded human-machine interface will surely be able to produce LCD interface with more compact structure, more stable and reliable performance, and lower cost.