Monitor 15 contacts with one PIC input

Several previously published Design Ideas and appnotes [1-4] 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 V_REF. The majority of 8-bit PIC microcontrollers in the mid-range family feature one or two internal comparators with just this.

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