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regarding coding using signal assignment..........

 
 
ekavirsrikanth@gmail.com
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      11-03-2006


i have a doubt regarding the coding style in vhdl
........................


if ( reset = '1' ) then


data_temp <= 0;


elsif (clock'event and clock = '1') then


data <= data_temp;
data_out <= data_temp and not data;


end if;
end if;


1. when i am giving clock data temp data is loaded into data at

present clock cycle or at next clock cycle.


2. when i am taking data_out output as given above it is
getting data_out at next clock cycle not at the present clock cycle
where data_temp is high and data is low. why this is happening


i need to get the data_out (high for one clock cycle at the
instant data_temp is high not at the next clock cycle .)


is this problem will be solved during synthesis since i am
facing the problem during simulation but does it behave the same way
during synthesis also. since it is a signal assignment signal " <= " a

minimum of delta delay will be there but will it take the entire clock
cycle delay.........


regards
kil

 
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Ralf Hildebrandt
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      11-03-2006
http://www.velocityreviews.com/forums/(E-Mail Removed) schrieb:

> if ( reset = '1' ) then
>
>
> data_temp <= 0;
>
>
> elsif (clock'event and clock = '1') then
>
>
> data <= data_temp;
> data_out <= data_temp and not data;
>
>
> end if;
> end if;
>
>
> 1. when i am giving clock data temp data is loaded into data at
>
> present clock cycle or at next clock cycle.


Simulate it!

All signals are updated after a delta delay. This means for "data", that
the new value ("data_temp") will be assigned to "data" in the next delta
delay. A delta delay is only a step of the simulator in reality it is
zero time.



> 2. when i am taking data_out output as given above it is
> getting data_out at next clock cycle not at the present clock cycle
> where data_temp is high and data is low. why this is happening


Again the delta delay is the reason for this: The new values of "data"
and "data_out" will be written in the next delta delay. At the moment
the simulator read the _old_ value of these signals.

This has an expression in reality: bot "data" and "data_out" are
flipflops. If you read the value of data_out at the same time when
"data" gets a new value, then you see the old value at the output of the
"data" flipflop. The signal has to propagate through the flipflop, which
takes some time.


> i need to get the data_out (high for one clock cycle at the
> instant data_temp is high not at the next clock cycle .)


I am not shure if I understand you right, but I guess that

data_out <= data_temp;

would lead to the solution you look for. Why do you need "data"?



> is this problem will be solved during synthesis since i am
> facing the problem during simulation but does it behave the same way
> during synthesis also. since it is a signal assignment signal " <= " a


You described 2 flipflops in a chain. Therefore you see the same
behavior after synthesis.


> minimum of delta delay will be there but will it take the entire clock
> cycle delay.........


A delta delay has zero time length.
A VHDL simulator works hardly as follows:
* If there is a an event of a signal, then evaluate, what will happen.
In our case let the event be the rising_edge(clock).
* During evaluation the new values of all signals will be computed. The
new values will be not stored at the signals immediately, because the
simulator does not know, which signal has to be updated before an
other. Therefore the new values are scheduled for being stored later -
in the next simulator step after one delta cycle.
* After all values of all signals have been computed, evaulation is done
and the simulator advances to the next step - a delta delay later.
Then the simulator assigns the previously computed values to the
signals.


Variables get their values immediately - not after a delta delay. Thats
one difference to signals.

Ralf
 
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krushi krushi is offline
Junior Member
Join Date: May 2006
Posts: 1
 
      11-06-2006
Thanks Ralf,

i need to generate the data_out for a clock cycle and then it will be disabled so i to that i have generated the data signal and doing the not operation so that i will get the data_out for one clock cycle thats what my reqirment is but insted i am getting the data_out at the clock event data is high (what i need is data_out to be high at present clock event where data_temp is high and data is low).

thanks for the reply............

regards

krushi
 
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