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Monitor 15 contacts with a single PIC input

13 Nov 2014  | Benabadji Noureddine

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There are already a number of articles [1-4] that show how to use many pushbuttons with a minimum number of inputs. They require an RC circuit where the timing can be measured to identify which pushbutton has been pressed, or an ADC input, with resistors forming a divider for each pushbutton pressed.

The following design idea shows another simple way to use up to 15 pushbuttons with only one I/O. The microcontroller chosen must contain an internal comparator with selectable values for the internal voltage reference VREF. The majority of 8bit PIC microcontrollers in the mid-range family feature one or two internal comparators with just this.


Figure: Sense 15 buttons with one input.


As a simple and practical example, the circuit in the figure uses an 8bit PIC controlling a 4bit LED display to show which button has been pressed. A comparator interrupt routine reacts immediately to any pushbutton pressed. The routine cycles through VREF values until the comparator output COUT returns high, which will indicate the pushbutton pressed.

The fully commented assembler source code takes less than 100 words. It is not overly optimised, so as to simplify understanding and make translation to other controllers easy.


References
[1] Bernhard Linke: Sense multiple pushbuttons using only two wires

[2] Petre Petrov: Measure two resistive sensors or multiple switches with a single Schmitt

[3] AN234: Hardware techniques for PICmicro microcontrollers (MICROCHIP | 2003)

[4] Tips'n tricks for 8-pin PICmicro microcontrollers (MICROCHIP | 2003)


To download the PDF version of this article, click here.




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