Using 4x5 Keypad with PIC16F877

A PIC16877 microcontroller is used to connect in particular to a 4X5 keypad which can also be connected with logic and analogue circuits.

The PIC16F877 was designed to consider a supply of 5V, I/O pins leakage current, the voltage level recognized as a high or low state while in TTL or ST mode, and others. With four pins, the keys are normally open. Only pins on the same side are connected together when keys are not pressed but when the key is pressed, the four pins are connected all together.

The circuit is connected only to the positive supply via JP1 connector where each row of the matrix keyboard is connected by its own pull-up resistors and through a protection resistor, going to the microcontroller I/O pin used to read the state of that row. The PIC drives the columns via JP3 connector that still goes through protection resistors.

The ports of microcontroller connected to the rows must always be set as inputs or must be kept in high impedance and this also applies to the columns. The state of the keys of a column can be read by driving the column low.

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