Keyboard Matrix Interface
Posted on Apr 7, 2012 12615
Keyboards can be slotted into two categories, at least as far as the manner that the switches are connected is co
ncerned: those with a common connection and those with the switches arranged in a matrix. The matrix type has the important advantage that the number of connections is an absolute minimum. Such an arrangement is ideal for ICs; many of these are designed for use with a matrix keyboard. However, many keyboards are available in job lots, for instance, that apart from a common connection also have a connector for each key. Such keyboards can be connected to ICs that require a matrix type with the aid of a number of electronic switches. The principle is straightforward: each key of the keyboard controls an electronic switch that is included in a matrix. As an example, the diagram shows a hexadecimal keyboard that is arranged in a 4- -4 matrix. Each of the electronic switches is held in the open position by a pull-down resistor. The current drawn by the circuit is very small and is determined mainly by the value of the pull-down resistors and the number of keys being pressed. The CMOS switches draw virtually no current.