RE: [sv-dc] 4-state vs 3-D

I think discussing a particular encoding is not a good solution. If the capability to represent X and Z for real numbers is built into the language then the particular encoding becomes an implementation detail. Note that for the digital domain, the encoding of X/Z used by the simulation kernel is not visible to users (even when some API's make it seem so).

What you seem not be discussing here is an encoding that can be adopted by convention, and, as long as the same value can be deposited on the floating-point variable by some other mean then there is ambiguity as to whether that particular value is indeed a Z, an X, or some other malformed number due to some other reason. This does bring up some real use issues, for example, how should a waveform tool display a floating-point variable that contains NaN: should it show NaN, Z, X.

I contend that the X / Z should be handled internally by the implementation. Attempting to formalize the encoding using an existing floating-point value seems like a memory optimization to avoid storing the information elsewhere (say 2 more bits), which is probably not the best use of the committee. This something that the implementation can better address.

        Arturo

-----Original Message-----
From: owner-sv-dc@eda.org [mailto:owner-sv-dc@eda.org] On Behalf Of Achim Bauer
Sent: Wednesday, December 08, 2010 12:42 PM
To: Kevin Cameron
Cc: sv-dc@eda.org
Subject: Re: [sv-dc] 4-state vs 3-D

Hi Kev,

looks good, just my analog five cents:

In the analog or real domain "x" or undefined might be interpreted as
undriven (strength=0!) or floating. A floating or high-impedance
(strength=0) node can not really maintain a stable signal, it is prone
to any kind of (unknown) disturbances, the (voltage) value is kind of
undefined. For real values z and x are kind of the same.

So one might even code it in a 2-dimensional way:

Value ( real: -inf -> +inf )
Strength ( real: undriven/undefined = 0, driven = ]0:+inf) )

Which would comply nicely with a sparse struct notation.

Achim

On Wed, 2010-12-08 at 09:13 -0800, Kevin Cameron wrote:
> 4-state refers to the received values "0,1,Z,X", since 0,1 are the range of known good logic values (for a single bit), the same nomenclature doesn't really apply for real values (since there are a lot of valid discrete values).
>
> I'd prefer to call it "3-Dimensional", the dimensions being:
>
> Value (logic: 1,0, real: -inf -> +inf)
> Strength (undriven = z, regardless of type)
> Certainty (uncertain logic = x, uncertain real = NaN)
>
>
> Comments?
>
> Kev.
>
>

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