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How to avoid 'unable to synthesize' errors

 
 
Anon Anon
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      03-21-2007
Can somebody recommend a book or on-line document that explains how to
avoid generating un-synthesizable code?

I recently wrote some VHDL code that looked fine to me and was close to
one of the Xilinx examples I'd seen, but it would not synthesize. I've
fixed that problem now (by a complete re-write) but I'd like to avoid
such problems in future and also get a clearer idea of what it is that
causes such issues.

Note that I am working in a Xilinx environment, using ISE 9.1.

Many thanks for any help!

AA
 
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Mike Treseler
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      03-21-2007
Anon Anon wrote:
> Can somebody recommend a book or on-line document that explains how to
> avoid generating un-synthesizable code?


Google this group.

Use a synchronous template.

No wizards/generators for simple stuff.

Verify functionality with vhdl simulation first.


-- Mike Treseler
 
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Ralf Hildebrandt
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      03-21-2007
Anon Anon schrieb:

> Can somebody recommend a book or on-line document that explains how to
> avoid generating un-synthesizable code?


You have 3 things that you can model with a HDL: combinational logic,
latches and flipflops:

process(a,b)
begin
if (a='1') then
logic<='0';
else logic<=b;
end if;
end process;

process(enable,data)
begin
if (enable='1') then
latch<=data;
end if;
end process;

process(async_reset,clock)
begin
if (async_reset='1') then
flipflop<='0';
elsif rising_edge(clock) then
flipflop<=data;
end if;
end process;

Three-state logic is something similar to combinational logic.

There are a lot of ways how to model these things. You can even put
everything into one process, as Mike Treseler suggests. I think for
beginners it is often more clear what will be the result if one uses
these templates.


Ralf
 
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Ahmed Samieh Ahmed Samieh is offline
Junior Member
Join Date: Mar 2007
Posts: 5
 
      03-22-2007
Quote:
Can somebody recommend a book or on-line document that explains how to
avoid generating un-synthesizable code?
read this paper
http://www.alse-fr.com/archive/VHDL_Coding_eng.pdf

--------
asm4pic
 

Last edited by Ahmed Samieh; 03-22-2007 at 11:10 AM..
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