Keypad

A keypad is a set of buttons arranged in a block which usually bear digits and other symbols but not a complete set of alphabetical letters. If it mostly contains numbers then it can also be called a numeric keypad. Keypads are found on many alphanumeric keyboards and on other devices such as calculators, combination locks and telephones which require largely numeric input.

Figure 1 A 4X3 matrix keypad

A computer keyboard usually contains a small numeric keypad with a calculator-style arrangement of buttons duplicating the numeric and arithmetic keys on the main keyboard to allow efficient entry of numerical data. This number pad (commonly abbreviated to "numpad") is usually positioned on the right side of the keyboard because most people are right-handed. Many laptop computers have special function keys which turn part of the alphabetical keyboard into a numerical keypad as there is insufficient space to allow a separate keypad to be built into the laptop's chassis. Separate plug-in keypads can be purchased.
By convention, the keys on calculator-style keypads are arranged such that 123 is on the bottom row. In contrast, a telephone keypad has the 123 keys at the top. It also has buttons labelled * (star) and # (octothorpe, number sign, "pound" or "hash") either side of the zero. Most of the keys also bear letters which have had several auxiliary uses, such as remembering area codes or whole telephone numbers. The keypad of a calculator contains the digits 0 through 9, together with the four arithmetic operations, the decimal point and other more advanced functions. Keypads are a part of mobile phones that are replaceable and sit on a sensor board. Some multimedia mobile phones have a small joystick which has a cap to match the keypad. Keypads are also a feature of some combination locks. This type of lock is often used on doors, such as that found at the main entrance to some offices.
There are some type(s) of keypad: 4X3, 4X4, etc. Each type of those sould be accessed with different manner. Microcontroller must use I/O port to source and sink current to achieve data in which keypad pressed.

Constructing a Matrix Keypad
Martix keypads are well known for their simple architecture and ease of interfacing with any microcontroller. In this part of tutorial we will learn how to interface a 4x4 matrix keypad with AVR and 8051 microcontroller. Also we will see how to program then in Assembly and C.

Figure 3 Keypad Switch

Constuction of a keypad is really simple. As per the outline shown in the figure below we have four rows and four columns. In between each overlapping row and column line there is a key. Now our keypad is ready, all we have to do is connect the rows and columns to a port of microcontroller and program the controller to read the input.

Scanning a Matrix Keypad
There are many methods depending on how you connect your keypad with your controller, but the basic logic is same. We make the coloums as i/p and we drive the rows making them o/p, this whole procedure of reading the keyboard is called scanning.
In order to detect which key is pressed from the matrix, we make row lines low one by one and read the coloums. Lets say we first make Row1 low, then read the columns. If any of the key in row1 is pressed will make the corrosponding column as low i.e if second key is pressed in Row1, then column2 will give low. So we come to know that key 2 of Row1 is pressed. This is how scanning is done.
So to scan the keypad completely, we need to make rows low one by one and read the columns. If any of the button is pressed in a row, it will take the corrosponding column to a low state which tells us that a key is pressed in that row. If button 1 of a row is pressed then Column 1 will become low, if button 2 then column2 and so on...