Exponential function in vhdl

[Altera_Forum]

hiii 

I want to implement exp(x.^2) in VHDL for synthesis. Can any one suggest how to do it. Here x is in signed format and i want result in fixed point arithmatic i.e. without rondoff operation. Is there any instruction like in MATLAB 'exp' is there . Thanks and regards!

[Altera_Forum]

For exponential, you will need to create a look up table filled with all the results. 

The address input of the LUT is x and the output is your function.

[Altera_Forum]

--- Quote Start ---  For exponential, you will need to create a look up table filled with all the results. The address input of the LUT is x and the output is your function.  --- Quote End ---    Thank you Tricky for your reply. May be I can write this lookup table in a package and call it for places where I need exponential function. If you have any sample program then please share.

[Altera_Forum]

I suggest NOT writing a function to do it - vhdl is a hardware description language, NOT a proramming language. You need to understand what hardware will be produced before you write any code.

[Altera_Forum]

You can use matlab/python/tcl or whatever language you prefer to write a program that generates a source file for your lookup table.

[Altera_Forum]

How can I generate sourse file for my lookup table by using MATLAB. Are you taking about HDL coder. I tried it but it doesnot accept "exp" function.

[Altera_Forum]

If not function then should I write any package for exp. Sorry I havent written any lookup table before so I dont know. And actually I want to design a Gaussian filter with variable window size & sigma value. For that I am using two 1D gaussian.  

So the part I want to impliment is  Gauss= exp^(-(x^2)/2*(sigma)^2); here sigma will vary from 0.1 to 5. and x is a vector =[-2 -1 0 1 2] for length equal to 5. I want to vary length also. How to do it.please help me with this. If you know any book for these type of operation then please suggest.

[Altera_Forum]

I highly suggest you learn about digital design. What you are going to create is a ROM - a read only memory. This can be done in VHDL using a constant which can be initialised from a function, a specified constant or a .mif file. you can have matlab generate the values to go into the constant. 

There are plenty of examples out on the web.

[Altera_Forum]

--- Quote Start ---  If not function then should I write any package for exp. Sorry I havent written any lookup table before so I dont know. And actually I want to design a Gaussian filter with variable window size & sigma value. For that I am using two 1D gaussian.   So the part I want to impliment is  Gauss= exp^(-(x^2)/2*(sigma)^2); here sigma will vary from 0.1 to 5. and x is a vector =[-2 -1 0 1 2] for length equal to 5. I want to vary length also. How to do it.please help me with this. If you know any book for these type of operation then please suggest.  --- Quote End ---    Any equation can be implemented in fpga as LUT either fully or partially. In the partial case you do in fpga what is doable and then use LUT. In your case I suggest: get x^2 in fpga (x * x), one multiplier get 2* sigma^2 in fpga, one multiplier and add one zero lsb truncate above results to suitable bitwidth divide -x2/(2*sigma^2) in fpga if you can afford a divider, else include this section within LUT finally get LUT for your exp in software e.g. matlab and put it in a rom  To get LUT of exp: you need to know range of your result. then just calculate exp of that range in suitable steps and rounded to your rom bitwidth Then in fpga address LUT with actual fpga result.