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binary sorter circuit

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cyboman

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I want to get back to circuit design and decided to practice some of the problems. I'm using VHDL for coding. The problem looked simple to me, but to my surprise I was not able to solve it. Please bear in mind that it is not a homework problem. I work full time and simply want to do more of circuit design and want to pick my brain. Here is the problem.

Binary sorter.

Using concurrent code, design a circuit capable of ordering the bits of a bit vector. The ordering should be from left to right, with all '1's coming first (for example, "00011001" would become "11100000").

Does anybody have any idea?
Any help is appreciated.
 

One solution:

Design a sorter with 2 inputs and 2 outputs:

In Out
00 00
01 10
10 10
11 11

It is easy to solve your problem with a bunch of these 2-bit sorters.
As an excercise, try to do it with as few source code lines as possible.
You can use a VHDL "for generate" loop.
 

What std_match said.

If you do the 2-bit swap method, for a 4-bit input it would look something like this in ascii art:

Code: 
  3 2 1 0
  o o o o
   X   X
  o o o o
  |  X  |
  o o o o
   X   X
  o o o o

Where o represents a bit, X is a 2-bit sorter, and | is just a straight copy. Data flow is from top to bottom.
 

One more stage is needed, otherwise input "0011" will fail:

Code: 
  3 2 1 0
  o o o o
   X   X
  o o o o
  |  X  |
  o o o o
   X   X
  o o o o 
  |  X  |
  o o o o
 

Well, darnit. I knew I should have checked it more carefully. :p In any event 3 stages should by definition be enough. The trick is to do the first stage as swap(D3,D0) ; swap(D2,D1) and then you can avoid my silly mistake. I knew there was going to be a price to be payed for laziness and not making optimal use of the opportunity of swapping more bits at 2nd stage. :p
 

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