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VHDL Wait-Statement after Synthese

 
 
Roman
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      10-28-2004
Hy all,

I have a little problem to write a kind of WAIT-Procedure in VHDL. (
like in C, "loop until i = 0,...")
This is necessary to pass some data to an external chip (Cypress SL811)
to the right time.

In the end it should look like this:
send_data(xyz);
wait(t);
send_data(xyz);
wait(t);
send_data(xyz);
wait(t);
.....

I know that I can do it in an process with a counter, but I would prefer
a procedure, which I will put into the package body, and then call it
when I needed.
normal wait statements and "a <= x after t;" is not available after
the synthese.( i hope I am wrong and there is a smart way to do this)

Has anyone of you gurus an idea how to manage this?

Thank you very much,
Roman
 
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Mike Treseler
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      10-28-2004
Roman wrote:

> I have a little problem to write a kind of WAIT-Procedure in VHDL. (
> like in C, "loop until i = 0,...")
> This is necessary to pass some data to an external chip (Cypress SL811)
> to the right time.
>
> In the end it should look like this:
> send_data(xyz);
> wait(t);
> send_data(xyz);
> wait(t);
> send_data(xyz);
> wait(t);


You can do this in a test bench, but not for synthesis.

Synthesis code is based on the synchronous process.
Writing a controller in vhdl is a little like writing an interrupt
routine for the event of a rising clock edge. You have to check the
relevant inputs and internal variables and update the appropriate
variables and outputs on *every* rising edge.

The notion of waiting can't be encapsulated in a procedure call.
In your example, you need to check for a handshake or maybe
a counter value on every tick and make a signal assignment only in
the appropriate case.

A procedure can be used in synthesis code to collect
frequently used sequences of sequential statements,
but it can't isolate you from the tick by tick nature
of a hardware controller.


-- Mike Treseler
 
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vipinlal vipinlal is offline
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Join Date: Feb 2010
Posts: 39
 
      03-04-2010
'wait for' statement is useless when it comes to synthesize.you can actually use a mod-n counter fo generating a delay.
vhdlguru.blogspot.com/2010/03/synthesizable-delay-generator-instead.html
 
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