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Old 03-29-2005, 09:59 PM
John_H
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Default Re: Dividing a 24 bit std_logic_vector by a decimal number

I'm a Verilog guy and your question is for VHDL - it would be great if a
VHDL guy gould give the proper code snippets. The question is addressed in
general:

"genlock" <[email protected]> wrote in message
news:[email protected] ups.com...
> Yes the decimal number is a constant value.


For the example of division by 1.36, multiplying by (2^24/1.36) and taking
the 24 MSbits of the result, you get an "effective" division. If you
generate the "reciprical integer multiplier" from real literals and do a
type conversion to std_logic_vector, the multiply would follow as a simple
multiply. Then just shift or select the upper bits and you have your
result.

> What do you mean by embedded multipliers?


The modern FPGAs tend to have multipliers as part of the logic fabric.
You're using ISE so my expectation is you're using a
Virtex(-E)Virtex-II(Pro), Virtex-4, Spartan-II(E), or Spartan-3/3L/3E.
These should all have multipliers if memory serves me right. Check the data
sheets.

> Is there a VHDL code available for that or how do we go about coding
> one.


I'd like to see someone on this newsgroup provide you a snippet to do
(roughly) what I suggest. If you wanted Verilog, it'd be something like
result[23:0] <= In[23:0] * ((1<<24)/1.36 + 0.5) >> 24;

but I haven't used real variables in my code much if at all. I think this
would synthesize.

> I dont need a clock for this one.
>
> Thankyou



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