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denis2gif
Visitor
Visitor
5,652 Views
Registered: ‎06-04-2011

opening the vhdl code of a predefined ise symbol.

Hello !

 

Question from a beginner.

I need a 24-bit barrel shifter in a schematics.

First, I find only 4- and 8-bit ones in the predefined symbols of my ISE11.

So second, I try to find shifters in the IP Core and don't find them.

So third, I look in the langage template for shifters and don't find them.

Finally, I put an 8-bit barrel shifter BRLSHFT8 and find the name of its VHDL model: BRLSHFT8_MXILINX.

Is there a way to open this VHDL model in order to use it as a template for creating my own VHDL BRLSHT24_denis2gif?

 

Thanks for your help.

 

denis2gif

 

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drjohnsmith
Teacher
Teacher
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Registered: ‎07-09-2009

if you want to stay with the symbols

 

can you think of a way of cascading barrel shifters to make the width you want ?

 

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denis2gif
Visitor
Visitor
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Registered: ‎06-04-2011

Not really...

There might be a tricky solution but a variable circular permutation on 24 items does not seem to be a simple cascade of 3 circular permutations on 8 items...

But may be I don't see the trick.

 

Denis

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drjohnsmith
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Teacher
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Registered: ‎07-09-2009

keep thinking

 

you wil get it,

 

clue , how could you cascade a 4 bit shifter ?

 

out with pen and paper , 

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denis2gif
Visitor
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Registered: ‎06-04-2011

Dear John,

 

Thank you for the clue:smileywink:, but I'd like to be sure we are talking of the same thing.

 

A 4-bit barrel shifter rotates four inputs (I3 : I0) up to four places. The control inputs (S1 and S0) determine the number of positions, from one to four, that the data is rotated. The four outputs (O3 : O0) reflect the shifted data inputs. Note that you d'ont know in advance the amount of shift, which is encoded in S1S0 

You tell me that it is possible to make an 8-bit barrel shifter with  inputs (I7 : I0) controlled by S2S1S0 using only 4-bit barrel shifters with inputs (Ia3 : Ia0) , (Ib3 : Ib0),... and controls Sa1Sa0,   Sb1Sb0,...( and probably also many multiplexers).

Do I understand well?

Note also that I am looking for a minimum lantency solution (1 clk cycle - barrel shifters and not shift registers).

 

denis2gif

 

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drjohnsmith
Teacher
Teacher
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Registered: ‎07-09-2009

you have it

 

If you want to use schematics, thats the way to do it.

 

it works, you just have to work out the wirring and any inverting of the control codes.

 

NOTE:

  I'm not saying that schematics is the way I'd code up a barrel shifter,

      I'd probably use a HDL like Verilog or VHDL.

 

e.g. 

  http://www.cs.umbc.edu/portal/help/VHDL/samples/bshift.vhdl

 

 

don't worry this is an assignment that comes up evry year or two on the forum,

    good luck.

 

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denis2gif
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Registered: ‎06-04-2011

Thanks John !!!

 

Denis

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