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 are widely used in human-computer interaction interfaces of instrumentation due to their low voltage, low power consumption, large display capacity, and long service life. With the rapid development of integrated circuit manufacturing technology and electronic technology, LCDs have become one of the most important information processing methods in the information industry. Additionally, more and more portable digital devices across various applications use microcontrollers as control cores and LCDs as display terminals. The 12864 dot matrix liquid crystal is a typical representative of small to medium-sized displays and is widely adopted. This article introduces the practical application of the 12864 LCD using the ST7920 controller as an example.

2. Introduction to the ST7920 Controller for 12864 LCD

2.1 LCD Display Control Instructions

To display information on an LCD, encoded data must be written into the display buffer. This is achieved through a series of LCD control commands. The ST7920 controller uses signals RS and R/W to manage these commands. DB0–DB7 serve as the data bus, facilitating communication between the external CPU and the internal processor of the LCD. The data exchanged includes both control commands and display content. Table 1 lists the main instructions for the 12864 Chinese character library dot matrix screen controlled by the ST7920.

2.2 LCD Display Control Timing

Like other controllable devices, an LCD requires specific 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 follow the appropriate timing when communicating with the ST7920. 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 via the 8-bit parallel port (DB0–DB7), three control signals—RS, R/W, and E—are needed. RS determines whether the data being sent is a command or display data. R/W is set to 0 for writing, and E is set to 1 after the signal stabilizes. Once all three signals are stable, the data can be written to the LCD. After writing, E is set back to 0. It's crucial to avoid unstable transitions during signal level changes and ensure that read/write operations occur only when the signals are stable. The read timing for the ST7920-controlled 12864 LCD is similar but involves different read/write control signals.

3. Hardware Interface Circuit for LCD Display

This section explains how to connect the MCU (Microcontroller Unit) to the LCD using the STC89C52 as an example. Figure 2 shows the 8-bit parallel port interface diagram for the STC89C52 controlling the LCD. The P1 port of the MCU connects to the DB0–DB7 of the LCD, serving as a data bus for both commands and display data. 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 the program code for displaying characters like "ST7920 display control" on the LCD screen. The software design includes initializing the display, sending commands, and writing data to the display buffer. Several code examples are provided to illustrate the process clearly.

5. Conclusion

Small and medium-sized LCDs have become increasingly popular as user interfaces for electronic devices due to their advantages such as low power consumption and ease of integration. This paper discusses the hardware and software implementation of the 12864 LCD using the ST7920 controller. It also outlines the development and usage steps for LCD applications. The method presented here is versatile and can serve as a reference for designing other types of LCD displays.