For example, if we have to show a 6-Digit value on a segment display, then instead of using a delay to eliminate bouncing effect of the key, the display can be refreshed 10 to 50 times. If there are some other outputs in the system like, multiplexed segment displays or matrix displays etc., which should be updated continuously, then instead of using delay function, it is better to execute one cycle of such updating. If this is not desired, the delay function can be included in each case of the switch label. This delay function is executed even if any of the keys are not pressed. The Instruction Pointer executes the Switch label and after identifying the key, the delay function is executed. This delay can be included in the program flow before or after the execution of the key function. To avoid this wrong interpretation, a delay time can be included in the program after detecting the pressed key such that, the transient state is settled. They produce a transient output which might be understood by the microcontroller as if, the key is operated repeatedly. The Mechanical parts of the push buttons do not change their states in a smooth fashion. The switches that are used in the matrix keypad can be of various types. For this purpose, a switch label with 16 cases for 16 keys is used to show the key value in Hexadecimal format. In this example program, let’s show the number of the pressed key. The identified key can be used according to the required application. Thus, our logic is converted into C-Program. When a switch from the second row is pressed, as there are 4 switches before row 2, it is valued as ’4+column’. As every row contains 4 switches, the switches of row 1 can be valued as columns. ‘For loop’ is restarted with temp=1 and column=1. This temporary variable is initially loaded with value ‘1’ and is left shifted by multiplying it with ‘2’ each time when the loop is executed. The same data is loaded into PORTC using a temporary variable. The variable named as ‘column’ contains the information regarding the enabled column. For 4X4 Matrix, in a microcontroller, we need 4-Pins as outputs for enabling the columns and 4-Pins as inputs for reading the rows. Now, let’s put the logic into a C-Program. The same procedure can be followed by interchanging the rows and columns. If there is no key in pressed state, then all the rows are read as LOW. For this purpose, we will enable the columns one by one (by issuing logic HIGH) and then read the row which is enabled due to the pressed key for each of the columns. The pressed key is identified in terms of rows and columns. Programming a Microcontroller to Read the Pressed Key Similarly, switches in each column have one of their terminals connected to the remaining switches in the same column, thus giving out 4 pins representing four rows and 4 pins representing four columns.
As every button switch has two terminals, Terminal 1 of each switch in a row is connected to other switches. These are electrically connected as shown below. For easy understanding, let’s arrange these 16 push buttons as the elements of a 4X4 Matrix i.e, in four rows and four columns. For a 4X4 keypad there are 4*4=16 push buttons. Let’s take 4X4 Matrix Keypad as a reference and understand the connections. Connecting the Push Buttons in a Matrix Keypad Format But, by using the Matrix type of configuration just 8-Pins are sufficient.
We need 16-Pins of the microcontroller in order to read the status of these 16 switches/push buttons individually. For example, consider a 4X4 Matrix keypad, which contains 4*4=16 Keys.
The advantage of these matrix keypads is that all the switches of the matrix can be read using lesser number of I/O pins of a processor/microcontroller. Here arrangement of keys doesn’t mean the layout of the keys or physical placement on the circuit board, but the keys are identified in terms of elements of a matrix like M11, M12, and so on… up to M44 for a 4X4 Matrix Keypad. The name itself infers that it has some keys arranged in rows and columns. The Computer keyboard is also a matrix keypad. The Matrix keypads are used in systems/control panels where Human Machine Interface (HMI) is required in order to change the operating parameters of a system/machine. In this article let’s discuss designing of matrix keypads and the programming part to get the information regarding the pressed key.
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