Jumpers are often used to configure microcontroller circuits. That's easy if there are only two possible states to be configured - the desired state can be determined by fitting or not fitting a jumper on a port pin. However, if you have to distinguish between three possible states, you normally need an extra pin. Many years ago a Microchip application engineer gave me a tip that in any case works with PIC microcontrollers: the three settings are defined by the High, Low and Open states of a single pin (see the figure).
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