How do I know if a wire is floating or not?

Question

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 V_CC.

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 V_CC. 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?

Answer 1

So what tool/method should i use to check if wire is floatinf?

1. Measure the voltage when the pin is pulled to Vcc by a high value resistor, like 100K.

2. 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.

Answer 2

You can measure this with a voltmeter only, without adding any extra resistors. The voltmeter itself typically has an impedance of about 10 megaohm.

1. Measure between GND and pin
2. Measure between V_CC and pin

If both readings are 0, the pin is floating. If the pin is low, the second reading would be equal to V_CC.

Answer 3

The method described by @ErikR works fine but requires two steps. These can be combined into a single check by connecting the input to a 50%-50% voltage divider between Vcc and ground. Again, something like 100K and 100 K:

^{simulate this circuit – Schematic created using CircuitLab}

If the input is driven high, the voltage will be near Vcc. If the input is driven low, the voltage will be zero. If floating, the voltage will be near Vcc/2.

The disadvantage of this method is that it requires an analog input where as the 2-step method can be done with a digital input. In fact, this kind of voltage divider must never be connected to a digital input; depending on the logic family, sustained input at an intermediate voltage can damage the chip.