Floating Node Help........

I have one basic doubt......regarding floating node.....I am not able
to find sufficient information on internet so my doubt is still a
doubt............

Q. Why a node Floats ?

consider a scenario.....suppose we have a standard inverter using pmos
and nmos.........now suppose supply is not present at all and input is
0 then what will be the output ?? will it float or just remain like a
simple wire ??

and for the other case suppose supply was present and the output is
driven by it and after some time suppy is removed........so what will
be the output in that case???? will it float or just remain like a
simple wire ??

Q. also i want to know with the above scenario that why we say that
floating node can attain any value between 0 to Vdd ?????


Thanxs,
Puneet

 

Hi Puneet,

From a physical stand point, a floating node is a node that is not
driven. The floating situation might be permanent if this node is not
driven by any signal at all or may occur at a certain condition in the
circuit simulation.
An obvious example of the latter a gate driven by a signal that goes
to Z state (high impedance) at a certain point. From a Spectre point
of view, a net is considered floating if the number of instTerms
+terminals attached to it is < 2 (Not 100% sure though, may be
confirmed by Andrew). But anyway, a net could be seen as alright by
Spectre but could be floating if the signal on this net goes high
state. Spectre would not catch this I'm afraid. Spectre keeps the last
state if nothing is driving the net.
What you need to do to ensure you are having the right outputs when
your inputs are no longer driven is to add pullup/pulldown resistors.

Does this make any sens ?

Cheers,
Riad.

 

thanxs Raid......