RE: EXTERNAL: Re: [vhdl-200x] Sizing of variables and signals from initial values

David,

I agree whole-heartedly with the "lossless" theory of operations (except for division). I sometimes use ufixed instead of unsigned just because of the lossless arithmetic* when integers won't handle the range.

While in vhdl-2008 I can declare constants using the initialization expression to set the range, I cannot do that with variable/signal declarations.  You can however declare "sizer" constants this way, and then use their ranges to declare the variables/signals. Just a little more verbose...

I don't really think there is a better way to do it with functions than what you did, and the packages as a whole are fundamental game-changers for VHDL synthesis. Not even SV can do that!

*I only wish there was a seamless way to make ufixed subtraction expand to sfixed, and handle mixed sfixed/ufixed arithmetic in a clean yet reasonable way. I often use the (natural - 1 < 0) trick to catch the carry/borrow on down-counters.

Andy D Jones
Electrical Engineering
Lockheed Martin Missiles and Fire Control
Dallas TX





-----Original Message-----
From: owner-vhdl-200x@eda.org [mailto:owner-vhdl-200x@eda.org] On Behalf Of David Bishop
Sent: Wednesday, April 24, 2013 4:00 PM
To: vhdl-200x@eda.org
Subject: Re: EXTERNAL: Re: [vhdl-200x] Sizing of variables and signals from initial values

On 04/24/2013 03:02 PM, Jones, Andy D wrote:
> Formatting error... example should be:
>
> Variable a, b : ufixed(arg'range) := (others => '0'); -- 0.0
>
> Variable sum : ufixed := a + b; -- also 0.0, but "bigger"
>
> Variable product : ufixed := a * b; -- still 0.0, but even "bigger"
>
Cleaver.  This will work to bound the types.

I admit the helper scripts here are a bit verbose, but I could not think 
of a better way to do it.   The idea here is that none of the operations 
do any rounding, they just let the number grow.   The way I fix this 
issue is like this:

variable a, b : ufixed (arg'range);
variable result : ufixed (arg'range);

result := resize ((a + b) * a, result'high, result'low);


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