Application of ST7920 controller in small and medium size LCD

Crystal oscillator

Crystal resonator 2.5*2.0mm 2520 16MHZ (16.000MH

Photocoupler

1. Introduction

LCD liquid crystal display devices have become a common choice for human-computer interaction interfaces in instrumentation due to their low voltage, low power consumption, large information display capacity, and long lifespan. With the rapid advancement of integrated circuit manufacturing technology and electronic engineering, LCDs have become one of the most important methods for information processing in the information industry. Additionally, an increasing number of portable digital devices across various applications use microcontrollers as their control core and LCDs as the display interface. The 12864 dot matrix LCD is a typical example of small to medium-sized displays, widely used in many applications. This article uses the ST7920 controller-driven 12864 LCD as a case study to demonstrate its practical implementation and application.

2. Introduction to the ST7920 Controller for 12864 LCD

2.1 LCD Display Control Instructions

To display data on an LCD, encoded information must be written into the display buffer. This is achieved through a set of predefined LCD control commands. The ST7920 controller uses two main signals—RS (Register Select) and R/W (Read/Write)—to manage these commands. The data bus DB0–DB7 facilitates communication between the external CPU and the internal processor of the LCD. These signals carry both control commands and display data. Table 1 lists the primary instructions used for controlling the 12864 LCD with the ST7920 controller.

2.2 LCD Display Control Timing

Like other controllable devices, an LCD requires precise timing to function correctly. Timing refers to the sequence of high and low levels of control signals. For LCD driver development, the host controller must communicate with the ST7920 according to the proper timing sequence. Figure 1 shows the timing diagram for an 8-bit parallel port write operation on the 12864 LCD controlled by the ST7920. As shown, during a write operation, three control signals—RS, R/W, and E—must be properly configured. First, RS determines whether the data being written is a command or display data. Then, R/W should be set to 0 for writing, followed by setting E to 1. Once these signals are stable, the 8-bit data from DB0–DB7 can be written to the LCD. After the operation, E should be set back to 0. It’s crucial to avoid unstable transitions during signal level changes to prevent errors. The read timing for the ST7920-controlled 12864 LCD is similar but differs in the R/W signal configuration.

3. Hardware Interface Circuit for LCD Display

This section provides an example of how the STC89C52 microcontroller connects to an LCD. Figure 2 illustrates the 8-bit parallel port interface for the STC89C52. The MCU's P1 port serves as the data bus (DB0–DB7), transmitting both commands and display data. Meanwhile, P3.0, P3.1, and P3.2 are connected to the RS, R/W, and E pins of the LCD, forming the control signal channel for the display.

4. Software Design for LCD Display

Based on the hardware connection diagram in Figure 2, this section demonstrates a sample program that writes characters such as "ST7920 Display Control" onto the LCD screen. The code includes initialization routines, command sending, and data writing functions. Several code snippets are provided to illustrate different aspects of the software implementation.

5. Conclusion

Small and medium-sized LCDs have become increasingly popular as user interfaces in electronic devices due to their advantages in size, power efficiency, and display capabilities. This paper explores the hardware and software implementation of the ST7920-controlled 12864 LCD, detailing the steps involved in developing and using such displays. The methods discussed here offer a general framework that can be adapted to other types of LCDs, providing a useful reference for engineers and developers working on similar projects.