Matrix Keyboard Interfacing with Microprocessor

4 x 4 Matrix Keyboard Interfacing – The keyboard is the most widely used input device of any microprocessor. For an interfacing keyboard to the microprocessor-based system, usually push buttons are used. These push-button keys, when pressed, bounce a few times, closing and opening the contacts before providing a steady reading. The microprocessor must wait until the key reaches a steady state. This is known as key debounce. The problem of key bounce can be eliminated using the key debounce technique, either hardware or software.

At the lowest level, keyboards are organized in a matrix of rows and columns. The microprocessor accesses both rows and columns through the ports. To connect an 8×8 matrix of keys, two 8-bit ports are required. Therefore, the PPI (8255) can be used to interface an 8×8 matrix keyboard to the microprocessor. When a key is pressed, a row and a column make contact; otherwise, there is no connection between rows and columns.

Figure 1 shows an example to interface a 4×4 matrix keyboard with the microprocessor using ports A and B of the 8255.

A 4×4 keyboard matrix is connected to ports A and B of the 8255 (PPI). Port A is considered as output and Port B as input port. The rows are connected to port A (output), and the columns are connected to port B (input). All the rows are made low through port A. If no key has been pressed, reading port B will yield 1s for all columns since they are all connected to high (V_cc). If a key is pressed, one of the columns will have a low since the key pressed provides the path to ground. It is the function of the microprocessor to scan the keyboard continuously to detect and identify the key pressed.