A Practical Design of Single Chip Microcomputer Matrix Keyboard

The matrix keyboard is a keyboard group with a matrix-like layout used in the external device of the single-chip microcomputer. The keyboard with a matrix structure is obviously more complicated than the direct method, and the recognition is also more complicated. The column line is connected to the positive power supply through a resistor, and the I/O port of the single-chip microcomputer connected to the row line is used as the output end, and the I/O port connected to the column line is used as the input. The advantage of the matrix keyboard is that it saves the IO port of the single-chip microcomputer. For example, the 8 IO ports of an ordinary keyboard can only be used as 8 keys, while the matrix keyboard can be used as 16 keys.

This article uses 51 single-chip microcomputer as the carrier to introduce the principle, circuit and software design points of a 4*4 matrix keyboard to realize 16 key operations.

1. General circuit of matrix keyboard

Figure 1 Matrix keyboard circuit

As shown in the matrix keyboard circuit of Figure 1

, the 4*4 matrix keyboard has 4 rows and 4 columns of keys. The 4 I/O ports of the microcontroller are connected to the row lines of the matrix keyboard, and the other 4 I/O ports are connected to the column lines of the matrix keyboard. The key recognition and operation are completed by operating the row and column lines.

2. The principle of matrix keyboard

The process of matrix keyboard key recognition is generally as follows:

(1) Set the first row to low level (0) and the other rows to high level. Read the column line data. A low level on the column line indicates that a key is pressed in this row.

(2) Set the second row to a low level (0) and the remaining rows to a high level. Read the column line data. A low level on the column line indicates that a key is pressed in this row.

......

(N-1) According to the different levels of the row and column lines, it is possible to identify whether a key is pressed, which key is pressed, and obtain the key number.

(N) Jump to the corresponding key processing procedure according to the key number.

3. An example of a matrix keyboard program

Figure

2 shows the main flow of key processing. The idea is to pull down each row of keys in turn, and then read the data of the column line. If the column line has a low level, it is considered that a key is pressed in this row, and the key is marked as pressed in this row and the row value is stored.

The idea of ​​reading column line data is to read columns 1-4 in sequence. If the column is low, mark and store the column value.

Here is some code:

//(1) Button recognition program

void key() //Key scan

{

unsigned char key_value_temp; //temporary key value, default 1111 1111 (binary)

    key_value=0xff;

 //key value

key_value_temp=0xff; //Key value temporary variable

//Matrix keyboard program flow

// Pull down the 1234th row in turn and read the column line data

Pin_r_1=0; Pin_r_2=1;Pin_r_3=1;Pin_r_4=1; //Pull row 1 low.

//Pin_r_1 is the first row of wire, Pin_r_2 is the second row of wire, and the rest are similar

key_value_temp=read_column(); //Read column data

if (key_value_temp != 0xff)

 // indicates that a key is pressed in the first row

{key_value=key_value_temp & 0x1f;
//Get the key number, for example 0001

1101 means the key at row 1, column 2 is pressed

}

Pin_r_1=1; Pin_r_2=0;Pin_r_3=1;Pin_r_4=1; //Pull down the 2nd line

key_value_temp=read_column(); //Read column data

if (key_value_temp != 0xff)

 // indicates that a key is pressed in row 2

{key_value=key_value_temp &
0x2f; 

}

Pin_r_1=1; Pin_r_2=1;Pin_r_3=0;Pin_r_4=1; //Pull down line 3

key_value_temp=read_column(); //Read column data

if (key_value_temp != 0xff)

 // indicates that a key is pressed in row 3

{key_value=key_value_temp &
0x3f; 

}

Pin_r_1=1; Pin_r_2=1;Pin_r_3=1;Pin_r_4=0; //Pull down the 4th line

key_value_temp=read_column(); //Read column data

if (key_value_temp != 0xff)

 // indicates that a key is pressed on line 4

{key_value=key_value_temp &
0x4f; 

}

}

//(2) Read column line data

unsigned char read_column() //Read the column of the matrix keyboard

{unsigned char key_column;

key_column=0xff;

if(Pin_c_1==0 ) key_column=key_column & 0xfe;
//1110

means the key in column 1 is pressed. The lower 4 bits of key_column represent the key number, and the upper 4 bits are usually 1111

if(Pin_c_2==0 ) key_column=key_column & 0xfd;
//1101

means the key in the second column is pressed.

if(Pin_c_3==0 ) key_column=key_column & 0xfb;
//1011

means the key in the third column is pressed.

if(Pin_c_4==0 ) key_column=key_column & 0xf7;
//0111

means the key in the 4th column is pressed.

return key_column;

}

The key value table corresponding to the above program is as follows:

Finally, process the key value in the program and jump to the corresponding operation.

There is a Proteus simulation example of a 4*4 matrix keyboard that implements the following functions:

1. Use the row scanning method to read the key values ​​of the 4x4 matrix keyboard;

2. The value of the pressed key is displayed on LCD1602.