Introduction Liquid crystal displays (LCDs), due to their stability, reliability, low cost, low power consumption, convenient control and driving, simple and easy-to-use interfaces, and compact modular structure, have been widely used as human-machine interfaces in embedded systems. In recent years, many domestic manufacturers, such as Zijing, Jiya, Jinghua, Xinli, and Pengyuan, have been able to meet the needs of various customized LCDs; many well-known semiconductor manufacturers, such as Hitachi, Seiko Epson, Toshiba, Holtek, Solomon, and Samsung, have successively launched many control and driving devices. This paper comprehensively elaborates on how existing control and driving devices and LCDs can be combined to design various compact, low-cost, easy-to-use, and high-performance embedded human-machine interfaces. 1 Overview of Liquid Crystal Displays and Their Control and Interfaces Liquid crystal displays (LCDs) are important flat panel display devices that utilize the property of liquid crystal materials to block/transmit light by changing their position under the influence of an electric field. Commonly used LCD devices include TN (Twist Nematic) type, STN (Super TN) type, and TFT (Thin Film Transistor) type. The performance of TN, STN, and TFT LCDs increases sequentially, but so does their manufacturing cost. TN and STN types are commonly used as monochrome LCDs. STN types can be designed as monochrome multi-level grayscale LCDs and pseudo-color LCDs, while TFT types are commonly used as true-color LCDs. TN and STN LCDs cannot be made into large-area LCDs, and their color count is limited to 218 or less. Colors below 218 are called pseudo-color, while those with 218 or more are called true-color. TFT types can achieve large-area true-color LCD displays, with pixel sizes as small as 0.3mm. TFT-LCD technology is maturing, and long-standing problems have been solved: viewing angles reach 170°, brightness reaches 500 cd/m² (500 nits), display size reaches 101.6 cm (40 in), and refresh rates reach 60 frames per second. LCD design primarily involves the design of the LCD's control/drive and its interface with external systems. Control mainly involves communicating with the outside world through the interface, managing the internal/external display RAM, controlling the driver, and allocating display data. Driving mainly involves driving the LCD to display according to the controller's requirements. The controller often contains an internal ASCII character library or a large-capacity externally expandable Chinese character library. Small-scale LCD designs often use an integrated controller/driver; medium-to-large-scale LCD designs often use several controllers and drivers, and expand with appropriate display RAM, custom-made character RAM, or ROM font libraries. The control and driver components mostly use low-voltage, low-power devices. The interface with the outside world is mainly used for LCD control, typically connecting to an 8/16-bit PPI parallel port of a microcontroller (MCU) or an SPI serial port with several control lines. For display RAM, except for some Samsung devices which require self-refreshing dynamic SDRAM, most companies use static SRAM. The commonly used LCD types in embedded HMIs, along with their typical control/drive devices and interfaces, are as follows: Segment LCDs, such as HT1621 (controller/driver), 128-dot display, 4-wire SPI interface; Character LCDs, such as HD44780U (controller/driver), 2-line × 8-character display, 4/8-bit PPI interface; Monochrome dot matrix LCDs, such as SED1520 (controller/driver), 61-segment × 16-line dot matrix display, 8-bit PPI interface, or T6963C (controller) + T6A39 (column driver) + T6A40 (row driver), 640×64-dot dual-screen display, 8-bit PPI interface; Grayscale dot matrix LCDs, such as HD66421 (controller/driver), 160×100-dot monochrome 4-level grayscale display, 8-bit PPI interface; Pseudo-color dot matrix LCDs, such as SSD1780 (controller/driver), 104RGB × 80-dot display, 8-bit PPI or 3/4-wire SPI interface; True-color dot-matrix LCDs, such as HD66772 (controller/source driver) + HD66774 (gate driver), feature 176RGB×240 dots, 8/9/16/18-bit PPI interfaces, 6/16/18 animation interfaces, and synchronous serial interfaces. Video-converting LCDs, such as HD66840 (CRT-RGB→CD-RGB), offer 720×512 dots, monochrome/8-level grayscale/8-level color, and a 4-bit PPI interface. The power supply circuit, bias circuit, backlight circuit, and oscillation circuit of the control and driving devices constitute the basic control and driving circuit of the LCD. This is the foundation of the LCD display. The LCD, its control and driving components, interfaces, and basic circuits together form an LCM (Liquid Crystal Module). Conventional embedded system designs often use readily available LCMs as human-machine interfaces; modern embedded system designs often integrate the LCD, its control and driving devices, and basic circuits directly into the system. Overall, this approach results in a compact structure, reduced costs, and facilitates power consumption reduction and product miniaturization. Controlling LCD displays typically involves using a microcontroller (MCU), which executes several protocol instructions from the LCD controller via the LCD's PPI or SPI interface. The MCU's LCD program generally includes initialization, management, and data transmission programs. Most LCD driver manufacturers provide assembly or C language example code with their devices, making programming very convenient. 2. Common LCD Control Drivers and Interface Designs 2.1 Segment LCD Control Drivers and Interface Design Segment LCDs are used to display segmented numbers or fixed-shape symbols, widely used for counting, timing, and status indication. A commonly used control driver is Holtek's HT1621, which contains memory and oscillation circuits corresponding to each LCD display point. It features low voltage and low power consumption, a 4-wire serial MCU connection, 8 control/transmission instructions, and can control 32 segments × 4 lines = 128 points. The display contrast can be externally adjusted, and the bias voltage, duty cycle, and other driving performance can be programmably selected. The HT1621 control driver for the LCD and its MCU interface are shown in Figure 1. 2.2 Control Driver and Interface Design for Character LCDs Character LCDs are used to display 5×8 dot matrix characters and are widely used in industrial measuring instruments. Commonly used control driver devices include: Hitachi HD44780U, Novatek NT3881D, Samsung KS0066, and Sunplus SPLC78A01. The HD44780U is the most widely used. It has embedded SRAM (segmented RAM) corresponding to each LCD display point, a CGROM (character library for ASCII codes), and a self-made CGRAM character memory. It can display 1-2 lines of 8 5×8 dot matrix characters per line or 5×10 dot matrix characters of a corresponding size. Its internal oscillation circuit, with the addition of an external resistor and capacitor RC, can directly form an oscillator. The HD44780U has a 4/8-bit PPI interface that can be directly connected to a 68XX MCU, 9 control/transmission commands, and externally adjustable display contrast. There are two ways to connect the HD44780U to an 80XX MCU: direct connection and indirect connection. Direct connection requires external logic to transform the interface control signals, but no special operation program is needed. Indirect connection connects the control signals to the MCU's I/O port, requiring a specially written access program. The HD44780U controls and drives the LCD and its interface with the 80XX MCU is shown in Figure 2. 2.3 Control Driver and Interface Design of Monochrome Dot Matrix LCD Monochrome dot matrix LCDs are used for graphic or graphic-text mixed display and are widely used in mobile communications, industrial monitoring, and PDA products. Small-area LCDs often use single-chip integrated control and driver devices, such as Seiko Epson's SED1520, which can achieve a 61-column × 16-row dot matrix display. Medium-area LCDs often use single-chip control/column driver devices and single-chip row driver devices, such as Hitachi's HD61202U (controller/column driver) and HD61203 (row driver), which can achieve a 64×64 dot matrix display. Large-area LCDs often use a "controller + video memory + column driver + row driver" configuration, such as Toshiba's T6963C (controller), T5565 (video memory), T6A39 (column driver), and T6A40 (row driver), which can achieve a 640×128 dot matrix display. These drivers often require a 12-18V negative power supply for biasing and contrast adjustment. Most control devices can be externally connected to an RC oscillator to form an oscillator, or an external clock can be used. Each bit in the video memory corresponds one-to-one with a dot on the LCD display. When text display is required, simple characters can be directly displayed using the ASCII character library integrated into the controller. Chinese characters or custom-made characters can be displayed using a large-capacity CGROM or a custom-made CGRAM character library externally to the controller. The control interface is typically an 8-bit PPI 68XX or 80XX MCU interface (connection to the MCU can be direct or indirect), with 7-13 control/transmission instructions, enabling drawing functions such as dots, lines, and circles. Controllers such as the T6963C, HD61830, and SED1335 can control single or dual-screen LCDs. This is an adaptation to mobile communication displays, essentially dividing the video memory equally and assigning each memory to one of the two LCD screens. When programming the data transmission, it's important to consider the characteristics of the video memory and appropriately change the data format. For example, the 8-bit data in the SED1520 video memory is reversed vertically, while the data in the HD61202 video memory is vertically arranged. Figure 3 shows the block diagram of the LCD and its 80XX MCU interface, consisting of the Seiko Epson SED1335 controller, external SRAM, self-made SGRAM character library, large-capacity CGROM Chinese character library, column driver SED1606, and row driver SED1635. This configuration enables a 640×56 monochrome dot matrix LCD display. 2.4 Control Driver and Interface Design for Grayscale Dot Matrix LCDs Grayscale dot matrix LCDs are widely used in small-scale measurement and control systems and low-cost handheld devices. In this type of LCD, each n bits of the controller's video memory corresponds to one LCD display point, resulting in 2^n grayscale levels for the entire LCD. Hitachi's HD66421 is a commonly used and economical grayscale dot matrix LCD control driver. A single HD66421, along with a few resistors and capacitors, can achieve a 22-level, 160-column × 100-row LCD grayscale display. Using HD66421s in parallel allows for larger LCD displays. The HD66421 has 160×100×2 bits of video memory, an 8-bit PPI interface, and can directly connect to an 80XX MCU. It provides 8 control/transmission instructions, programmable drive characteristics, and adjustable grayscale levels. The HD66421 requires an external resistor R to form a system oscillation circuit and a negative power supply for bias. As a highly integrated device in a 322-pin package, the HD66421 presents challenges in PCB design and should be given sufficient attention. Figure 4 shows the HD66421's control and drive of the grayscale dot matrix LCD and its interface with the 80XX MCU. 2.5 Control Drive and Interface Design of Pseudo-Color Dot Matrix LCD Color LCD displays are based on the principle of superimposing the three primary colors: red (R), green (G), and blue (B). Each LCD pixel consists of three RGB sub-pixels, each driven by one of the three RGB color segments. Color LCD displays require more video memory; each color segment has 2^n colors, requiring n bits of video memory. Color LCD displays are an inevitable result of LCD upgrades. Pseudo-color displays often use inexpensive STN LCDs, primarily found in mobile communication and PDA products. Solomon Systech's SSD1780 is a typical monolithic, highly integrated pseudo-color dot-matrix LCD controller and driver. It contains 312×81×4-bit GDDRAM, a 477kHz oscillator circuit, integrated bias circuitry, and DC-DC circuitry; it features an 8-bit PPI interface (for direct connection to 80/68XX MCUs) and a 3/4-wire SPI serial interface, providing 36 control/transmission commands. With a few additional capacitors, the SSD1780 can control and drive a 104RGB×81-dot color STN LCD, displaying 2³n=4096 colors. The SSD1780 uses a 627-pin package, making PCB design more complex and requiring careful consideration. Figure 5 shows the SSD1780 controlling and driving the pseudo-color STN dot-matrix LCD and its interface with the 80XX MCU. 2.6 Control Driver and Interface Design of True-Color Dot-Matrix LCD Modern high-end PDAs, home appliances, and display walls increasingly utilize true-color dot-matrix LCD display technology. True-color LCD displays display more than 218 colors, requiring larger video memory and more advanced control driver technology compared to pseudo-color displays, and necessitating high-speed animation. True-color LCD displays use TFT-type LCDs with active dot-matrix display, requiring source drivers and gate drivers to control the source and gate of the LCD field-effect transistors (FETs). The source driver receives display data and drives the LCD column display, also known as the data driver, while the gate driver controls the line-by-line scanning. Hitachi's HD66772 series of true-color LCD control driver devices are ideal for representing a rich and colorful world in embedded human-machine interface design, enabling 176RGB×240 dots, 218 colors, high-speed animation TFT dot-matrix display. This series includes HD66772, HD66774, HD66775, and HD667P01. The HD66772 is a controller with embedded 95KB of video memory and a source driver for 176 RGB segments. The HD66774 is a 240-line gate driver with an integrated power supply. The HD77665 is only a 120-line gate driver. The HD667P01 is a power supply device. The HD66772 has an 8/16-bit PPI interface, a 6/16/18-bit animation interface, and a synchronous serial interface for direct connection to 80XX MCUs. Using the HD66772 series devices to control and drive a 176RGB×240-dot TFT LCD true-color display, there are two schemes: ① 1 HD66772 + 1 HD66774; ② 1 HD66772 + 2 HD66775 + 1 HD667P01. The former is more compact, while the latter is more economical. Figure 6 shows the block diagram of the LCD control and drive connection and the 16-bit MCU interface for the former scheme. 2.7 Control Driver and Interface Design for Video-Converted LCDs In industrial control and embedded control systems, many LCD video driver designs are used. These designs often require specialized components to convert video signals and control the LCD for animation display, achieving product compatibility and expanding product performance. Hitachi's HD66480F is a typical example. It can easily extract CRT signals from a computer's video interface and directly drive monochrome or color LCDs through video conversion, allowing the content displayed on the CRT monitor to simultaneously appear on the LCD screen. The HD66480F can control and drive LCDs with a maximum resolution of 720×512 dots, supporting monochrome, 8-level grayscale, or 8-level color display. The HD664840F has a 4-bit controlled interface, which can be directly connected to an 8-bit MCU to set up the video display environment. Using the HD664840F requires an external 8-bit RGB display buffer SRAM. Figure 7 illustrates the design block diagram of using the HD66840F external display buffer HM6264, under the control of an 8-bit 80XX MCU, to transform CRT signals and control the driving of the HD66772 color dot matrix LCD animation display. 3. Basic Circuit Design for LCD Control Driver 3.1 Design of Basic Power Supply Circuit The basic power supply voltage of LCD control driver devices is generally between 1.8V and 5.5V. Modern embedded system design emphasizes low voltage and low power consumption, and often uses 1.8V, 2.5V, 3.0V, or 3.3V devices. All the devices mentioned above have power consumption of several to tens of mW or less in their operating state and can operate within the voltage range of 1.8V to 3.6V. Therefore, selecting and designing a power supply circuit with appropriate power and stable voltage is very important. Many semiconductor manufacturers produce various types of low-power, high-performance power supply devices, such as Torex's XC6203 series, Richtek's RT9168/A series voltage regulators, AME's AME8800 series and AME8811 series buck converters, On Somlconductor's NCP1400A series, Maxim's MAX1795 series boost converters, and so on. These devices can provide output voltages of any value between 1.5 and 5V, with an accuracy of ±1.2% to ±2.5%, and a maximum output current of 100 to 500mA. By selecting these devices and adding a few external resistors, capacitors, inductors, or Schottky diodes, a basic power supply circuit suitable for LCD control drivers can be designed. Figure 8 shows a very simple power supply circuit designed for the HD66421. 3.2 Driver Bias Circuit Design Graphic dot matrix LCD drivers often require a bias network and a negative power supply to achieve bias. The bias network can be configured according to the resistor and capacitor values ​​recommended by the driver manufacturer, and the negative power supply can be implemented by selecting appropriate negative voltage devices. Common methods for generating negative power supplies include: using 79 series three-terminal integrated voltage regulators, such as the LM7918, which can provide a -18V negative voltage source; and using DC-DC ICs, such as Maxim's MAX749, MAX680, MAX1860/1861, and Motorola's MC34063A. Figure 9 shows a -12V negative power supply circuit designed using the MC34063A. 3.3 Backlight Circuit Design LCD backlights typically use LED, EL (electric luminescent) and CCFL (cold cathode lamp) backlights. Character-type or small-to-medium dot matrix LCDs mostly use LED or EL backlights. LEDs are mainly yellow (red-green tone) and are generally driven by 4.2V; ELs are mainly yellow-green (red-green-white tone) and are generally driven by 1W, 400-800Hz, and 70-120V AC. Large-format STN and TFT LCDs typically use white (red, green, and blue) CCFL backlights, generally driven by AC voltages above 300V and frequencies ranging from 25kHz to 100kHz. EL and CCFL backlight circuits can be built using IC devices or pre-built modules. IC devices can be used to build backlight circuits, such as IMP's IMP525/562/803, combined with a few resistors, capacitors, and inductors to form an EL backlight circuit, as shown in Figure 10; Maxim's MAX1635 combined with a transformer forms an EL backlight circuit; Maxim's MAX1610, Linear's 1182, or TI's Vcc3972 combined with a transformer form a CCEL backlight circuit. Pre-built backlight modules include Senbao's VET-N1210-01 CCEL module and Jingdian Fengyuan's PYE series EL/CCEL modules. Using IC devices to build backlight circuits allows for compact design and reduced costs, and is often used in embedded system design. 3.4 Oscillator Circuit Design Most LCD control drivers have an internal oscillator or can be connected to an external oscillator or an external clock; only one can be chosen for application, making design very convenient. To simplify peripheral circuit design, the internal oscillator of the control driver is often chosen as the clock source. In this case, many control driver devices often require external RC devices, which can be configured according to the device manual. Conclusion This article has detailed the characteristics of LCD control drivers and their MCU interface designs, as well as the specific designs of various common types, and explained their basic circuit design. Applying these principles to embedded human-machine interface design will certainly enable the creation of LCD interfaces with more compact structures, more stable and reliable performance, and lower costs.