Hello Tamas,


At 23.06 2008.07.14, you wrote:
>I think we have to find out what OP wanted to say from the equations, which
>was:
>
>> out=(adc-min)*65536/(max-min);
>
>So I guess max-min could be less or greater than 65536, but even then the
>number have to be evenly scaled over the full 16bit range, right?

I already knew how to get max precision from just 32bit division, what I
asked was if anybody had multiprecision / extendedprecision routines to
share (here I am rephrasing), which is the only way to compute without
approximations out=(adc-min)*65536/(max-min)


>Multiply by 2^16 is equal to shift by 16 bits, so the storage needed from
>the 24 bits is 24+16 bit, which is 40. You have 32 and 64 bit sizes
>supported by C30 so I guess you have no other choice than use 64 bit. But I
>think you would need to cast at least the constant to 64bit number to make
>sure it will use that:
>
>out=(adc-min)*(unsigned long long)65536/(max-min);
>
>or
>
>out=(adc-min)*65536ULL/(max-min);

yes, in fact I have specified that was pseudo C, and anyway my target was
assembly (DIV and MUL were mentioned).

Anyhow, it doesn't matter, I got acceptable precison, but a library with
signed and unsigned 64bit/32bit division and 32bit*32bit=64bit multiplication
would still be handy. I'm very new to the dsPIC.

With kind regards,
Mario


>
>whichever way you like.
>
>Tamas
>
>
>
>
>On Mon, Jul 14, 2008 at 8:18 PM, Olin Lathrop <olin_piclist@embedinc.com>
>wrote:
>
>> Electron wrote:
>> > I received many answers (for which I'm grateful), but none addressed
>> > my request, although I think was quite clear.
>>
>> If everyone mistunderstood the question, just maybe you should consider the
>> question was poorly worded.
>>
>> I still don't understand how this isn't just a scaling problem after a
>> subtraction.
>>
>> > However, I've done my math and by maximixing precision (shifting left
>> > the data
>> > before division to get maximum precision even from 32bit), I've been
>> > able to make
>> > it work satisfactorily with 32bit math.
>>
>> Sounds exactly like one of the suggestions I made.  I guess my answer
>> wasn't
>> clear enough.
>>
>>
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