My first, simple question on LUT... 
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 My first, simple question on LUT...

I'm a student and so I apologize for this silly question ;-P...
I have to implement a logarithm-function on a FPGA ...how can I do this in
VHDL using a LUT?

( in particular the function is quite complex:
Y=max*(A,B)=max(A,B)+log(1+exp(-|A-B|))   --- where A,B,Y is a 8bit
"integer" values )

thank you for the help!



Thu, 30 Jan 2003 03:00:00 GMT  
 My first, simple question on LUT...
what is the required precision?  If 0.5
dB or less then a combination of a
barrel shifter and a 4 LUT will get you
a pretty compact solution.  Look at my
"quick and dirty logarithms" post
http://www.dspguru.com/comp.dsp/tricks/alg/quicklog.htm.
That describes a log circuit I've used
many times in FPGAs.

Doing it in VHDL depends on the
capability of your synthesis tools.  If
you are using Synplicity, you can encode
the LUT portion as a ROM, and the tools
will correctly infer the ROM.  If you
are using a less capable tool, such as
the foundation tools then you might have
to structurally instantiate the LUTs and
load up the look up values.

Quote:

> I'm a student and so I apologize for this silly question ;-P...
> I have to implement a logarithm-function on a FPGA ...how can I do this in
> VHDL using a LUT?

> ( in particular the function is quite complex:
> Y=max*(A,B)=max(A,B)+log(1+exp(-|A-B|))   --- where A,B,Y is a 8bit
> "integer" values )

> thank you for the help!

--
-Ray Andraka, P.E.
President, the Andraka Consulting Group,
Inc.
401/884-7930     Fax 401/884-7950

http://www.andraka.com  or
http://www.fpga-guru.com


Thu, 30 Jan 2003 03:00:00 GMT  
 
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