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Input devices Keyboard Matrix Interface Circuit

2017-08-15 21:14  
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Matrix keypads are very common input devices in embedded systems. They have simple architecture and are easy to interface. One good thing about them is that they allow you to interface a large number of input keys to a microcontroller with minimum usage of  I/O resources.Keyboards can be slotted into two categories, at least as far as the manner that the switches are connected is concerned: 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. TheCMOSswitches draw virtually no current.A 4x4 matrix keypad requiring eight Input/Output ports for interfacing is used as an example.Rows are connected to Peripheral Input/Output (PIO) pins configured as output. Columns are connected to PIO pins configured as input with interrupts. In this configuration, four pull-up resistors must be added in order to apply a high level on the corresponding input pins as shown in Figure 1. The corresponding hexadecimal value of the pressed key is sent on fourLEDs .
Keyboard Matrix Interface