In digital logic, there's a state called "1" which is a defined high voltage, for example 2.7 V – 5.0 V. To achieve it we must connect it to VCC.
And "0" which is a low voltage, for example 0 V until 0.8 V. To achieve it we must connect it to GND.
And there's another state called "don't care", or "high impedance" or "invalid state" in which those wires are not connected neither to GND nor VCC. It's called floating wire.
The problem is, how do I check of the current state of the wire is floating?
It's easy to check the logic state of a wire using voltmeter. If it displays 2.7 – 5 volt, we can say 1. It depends on the logic level.
But what if the voltmeter displays 0 volt? There are two possibilities: that the wire's logic is 0, or it's a floating wire.
So what tool/method should I use to check if the wire is floating?
Best Answer
Measure the voltage when the pin is pulled to Vcc by a high value resistor, like 100K.
Make the same measurement when the pin is pulled to GND by a high value resistor.
If the pin has an asserted logic level it should read that level in both cases.
If the pin is floating the voltage at the pin should measure HIGH for measurement #1 and LOW for measurement #2.
If it's configured as an input pin with an internal pull-up or pull-down resistor you will get an intermediate voltage determined by the relative value of your resistor and the pull-up/down.