unknown value x in digital design

i understand that unknown value x occurs when output cannot be determined to be 1 or 0 (sorry if i'm wrong)..... but how is this possible... i mean when we get the output it has to be a particular value (either 1 or 0 right) how can it not be determined???

or is it like if we expect the value 0 in the output and we get 1.. does that mean 1 is an unknown value??? but then it should be termed as an error rather than unknow right???

or is it like the output value keeps changing constantly between 1 and 0 in which case we can't then determine if the ouput settles to 0 or 1.... so is toggling between 0 or 1 at the output result in unknown x value???

and what do we do with this value?? it is just ignored or wat is done??? what if this output value has to be given as an input to another circuit. then wat happens... sorry if i am wrong but i am confused.. can someone pls help...

preethi19 said:

is it like the output value keeps changing constantly between 1 and 0 in which case we can't then determine if the ouput settles to 0 or 1.... so is toggling between 0 or 1 at the output result in unknown x value???

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That's correct. Value x means output toggles between 1 and 0 continuously. It's like different values for the same clock pulse.
If we give this as an input to other circuit, then it's obvious that it won't work properly, unless the output of the circuit is independent of input values.

It is the same as in algebra, if you have a variable, but not enough information to solve for that variable, it is unknown. For example, when you turn the power on to a sequential device, you don't know if it will come up as the value 1 or a 0. So we say that the value 'x' represents the potential to be either 1 or a 0. The we can let the Boolean logic propagate that value out to an observable output, or not. In verification, we want to make sure the design will eventually turn the unknown to a known value. For example (x & 0) = 0, (x | 1) = 1. This can be done in simulation or using formal tools.

An 'x' is also used to specify an unspecified input. Meaning that we want the synthesis tool to generate logic assuming that an input could be either 1 or 0.