Understanding a floating input pin on an IC

[tiofilo75]

Hi I am trying to use the IC HT9200B and need some clarification on what it is meant by "floating pin". I am trying to figure out what this statement means:

Data synchronous clock input for the serial mode:
When the IC is operating in the parallel mode, the input terminal (CLK) is included
with a pull-high resistor. When the IC is operating in the serial mode, this pin becomes floating

Now I want to use the serial mode so I am confused by the floating portion of the statement. I thought that having a floating input pin was a not a good thing so can someone shed some light please.

[digsys]

Firstly, a floating Input is really only "dangerous" if the environment is noisy AND it is tied to an IRQ or similar (in most cases).
If it isn't tied to internal hardware / IRQs, then it can't do much "damage". It's just NOT good "practice" to leave Inputs floating.
In this case, they rely on the External Clock being dynamic, and therefore the Input is really never floating. Same as if you drive
it from logic ICs etc No need for pull-up / down resistors. IF you check the specs of all I/O, you'll usually find details of how they
are set up, and what you MAY need to be careful of.

[jmole]

It's basically saying that in parallel mode, the pin has an internal pullup resistor. In serial mode, the internal pullup is disconnected, leaving the pin "floating", or in a High-Z state, disconnected from any other internal circuitry (aside from the gate(s) of whatever transistor(s) it's connected to).

[JuKu]

Firstly, a floating Input is really only "dangerous" if the environment is noisy AND it is tied to an IRQ or similar (in most cases).
If it isn't tied to internal hardware / IRQs, then it can't do much "damage". It's just NOT good "practice" to leave Inputs floating.

It is not that simple. Floating input is an ESD hazard. Also, the noise can cause rapid switching of input states, wasting power (bad, if you running on battieries). This will also make the power rails inside the chip very noisy, causing all kinds of random behavior. Also, it can easily float to in-between voltages, which causes both input transistors to conduct. I haven't seen a chip actually burn because of that, but would you think a chip works as it should if i/o section power is shorting to ground every now and then?

Open input is worse than just bad practice, it is bad engineering.

[CarlG]

I fully agree with JuKu 

In your particular case,  the CLK pin will obviously be driven by whatever is running the serial interface to the HT9200B, so the node will not be floating in your circuit.

The manufacturer have provided each pin with a PU where needed, and enabling/disabling them by the serial/parallel selection pin, so that you don't have to add discrete PU's to the unused pins.