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2.8. K E Y P A D 4 x 4


Keypads are often used as a primary input device for embedded microcontrollers. The keypads actually consist of a number of switches, connected in a row/column arrangement as shown in Fig 2.8.1.

In order for the microcontroller to scan the keypad, it outputs a nibble to force one (only one) of the columns low and then reads the rows to see if any buttons in that column have been pressed. The rows are pulled up by the internal weak pull-ups in the 8051 ports. Consequently, as long as no buttons are pressed, the microcontroller sees a logic high on each of the pins attached to the keypad rows. The nibble driven onto the columns always contains only a single 0. The only way the microcontroller can find a 0 on any row pin is for the keypad button to be pressed that connects the column set to 0 to a row. The controller knows which column is at a 0-level and which row reads 0, allowing it to determine which key is pressed. For the keypad, the pins from left to right are: R1, R2, R3, R4, C1, C2, C3, C4.

Keypad Diagram

Figure 2.8.1. Keypad 4 X 4 Connection

The Algorithm

Matrix-type keypads consist of a rectangular array of momentary push button. Each row and each column of push buttons is connected to a common rail. Suppose a four by four array of push button are used. A four by four array is often used to input hexadecimal numbers. There are four comun rails and four row rails. Each pushbutton has two terminals, one connected to its column rail and the other to its row rail. The row and column rails are connected to the microcontroller ports. The columns are driven low by output port. The rows are then read into the input ports. If no key is pressed, the rows read 1. When a row is detected to be 0, it indicates that a key in that row is pressed. the task now is to detect which key of the row is actually pressed. The microcontroller loops through each column, driving only one column low at a time as it inspects the row.The microcontroller needs to poll the rows to see if a key is pressed. Only when the column in which the pressed key resides is driven low is the row rail grounded, and thus the voltage is low. The rows and columns are interchangeable, that is, the rows may be driven low as the columns are read by the input ports.

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