Method of designing universal keyboard for single chip microcomputer system using old computer

Calculators are of various types, exquisitely made, sensitive to trigger, and cheap. Using an old calculator to transform it into a universal keyboard for a single-chip microcomputer system can be easily connected to any single-chip microcomputer application system. Whether in experimental production or practical application, it is a simple and convenient method.

I made a universal keyboard with a calculator and used it in several application systems. The effect is better than using ordinary keys and it is very convenient to use.

The circuit is shown in Figure 1 and Figure 2. First, choose a calculator with a suitable and beautiful appearance (this article takes the RS-3890 calculator as an example), open the back cover, find the row and column lines of the keyboard matrix, and mark them. Draw a corresponding diagram of the matrix keyboard and symbols, as shown in Figure 2.

Cut off the printed circuits of the original keyboard's row and column lines leading to the mask chip, and then use thin enameled wires to connect the row and column lines to the corresponding L/O ports of the AT89C2051 microcontroller as the detection lines of the new keyboard.

This article uses P1.0~P1.5 as row lines and P3.2~P3.7 as column lines. If more buttons are needed, the microcontroller can be AT89C51. Use an I/O port of the microcontroller as the sound control port (P1.7 is used in this article) to control the sound of the original calculator beeper.

The keyboard code is outputted by the serial port of the single-chip microcomputer to provide key control information to the single-chip microcomputer application system. This keyboard has only three connections with the application system, and the power supply is taken from the application system, which is very convenient to use.

The flowcharts of the keyboard sending program and the application system key value receiving program are shown in Figure 3 and Figure 4. Write the keyboard code sending source program into 89C2051, and the keyboard code receiving source program into the microcontroller in the application system. The two programs set the same baud rate, use KEYNUM as the keyboard information export, and then use KEYREC and KEYNUM values ​​as control program codes in the application system to complete the operation of the application system.

The attached table gives the corresponding relationship between the key code and the key value converted after receiving. The key value KEYNUM in the table is defined by the author in a certain application system. Readers can also change the key value according to the requirements of their own application. The KEYREC value is the actual code sent by the keyboard corresponding to the key symbol shown in Figure 2.