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

I agree in terms of the internal sim encoding -- the additional question
that Kevin and you both mention is the externally visible encoding.
There are various tradeoffs there; personally I'd prefer to have a
non-NaN external encoding and users can re-code to a NaN if the
external use really prefers that.

Gord.

On 12/8/2010 1:20 PM, Arturo Salz wrote:
> 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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Gordon Vreugdenhil                                503-685-0808
Model Technology (Mentor Graphics)                gordonv@model.com
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