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-   -   VHDL--usage of WAIT statement in PROCESS (http://www.velocityreviews.com/forums/t291843-vhdl-usage-of-wait-statement-in-process.html)

anil 05-13-2006 08:52 AM

VHDL--usage of WAIT statement in PROCESS
 
Hello,

iam confused with the flow of simulation when a WAIT statement
is used in the middle of statements within a PROCESS.
please ,help me out of this with some relevant description.



Reiner Huober 05-15-2006 08:28 AM

Re: VHDL--usage of WAIT statement in PROCESS
 

> iam confused with the flow of simulation when a WAIT statement
> is used in the middle of statements within a PROCESS.
> please ,help me out of this with some relevant description.


In a simulation cycle, a fraction of the processes become active,
either when there is an event on the sensitivity list (process has no
wait statement) or when the condition in the current wait statement
becomes true.

Active processes with no sensitivity list will then run code from the
current wait statement until the next wait statement is encountered:

process
begin
-- statements1
WAIT UNTIL CLK='1';
-- statements2
WAIT UNTIL CLK='1';
-- statements3
end;

This process will execute statements1 on initialization. Then it
becomes inactive until CLK becomes "1". Note that if CLK is already
"1", it will wait until CLK has some other value and then CLK becomes
"1".

After CLK becomes "1", the processes will execute statements2 and then
wait until CLK becomes "1" for a second time.

After that, it will execute statements3 and loop back to statements1,
waiting until CLK becomes "1" for a third time.

--
If you do not like the behaviour that the process waits if the CLK is
already one, you must use IF statements, see the LRM

http://www.microlab.ch/courses/vlsi/...1076_8.HTM#8.1


process
begin
-- statements1
if CLK /='1' then
wait until CLK='1'
end if;

Hubble.


vipinlal 03-04-2010 10:43 AM

'wait for' statement is useless when it comes to synthesize.you can actually use a mod-n counter for generating a delay.
vhdlguru.blogspot.com/2010/03/synthesizable-delay-generator-instead.html


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