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Syntax help

 
 
atul.ee@gmail.com
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      11-15-2006
What is the VHDL syntax to specify rise and fall time of the signal

 
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Jonathan Bromley
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      11-15-2006
On 15 Nov 2006 05:08:35 -0800, http://www.velocityreviews.com/forums/(E-Mail Removed) wrote:

>What is the VHDL syntax to specify rise and fall time of the signal


There is none. Logic signals in VHDL have discrete values and
change instantaneously from one value to another. (Things are
a little more difficult for signals of type REAL, but let's skip
that issue for now.)

You can mimic some of the features of risetime by making
a signal go to 'X' for some time...

process
constant t_rise: time := 1 ns;
constant t_fall: time := 2 ns;
begin
sig <= '0';
wait for 10 ns;
sig <= 'X', '1' after t_rise;
wait for 10 ns;
sig <= 'X', '0' after t_fall;
...

But please NEVER do that with a clock signal!
--
Jonathan Bromley, Consultant

DOULOS - Developing Design Know-how
VHDL * Verilog * SystemC * e * Perl * Tcl/Tk * Project Services

Doulos Ltd., 22 Market Place, Ringwood, BH24 1AW, UK
(E-Mail Removed)
http://www.MYCOMPANY.com

The contents of this message may contain personal views which
are not the views of Doulos Ltd., unless specifically stated.
 
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atul.ee@gmail.com
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      11-16-2006

Jonathan Bromley wrote:
> On 15 Nov 2006 05:08:35 -0800, (E-Mail Removed) wrote:
>
> >What is the VHDL syntax to specify rise and fall time of the signal

>
> There is none. Logic signals in VHDL have discrete values and
> change instantaneously from one value to another. (Things are
> a little more difficult for signals of type REAL, but let's skip
> that issue for now.)
>
> You can mimic some of the features of risetime by making
> a signal go to 'X' for some time...
>
> process
> constant t_rise: time := 1 ns;
> constant t_fall: time := 2 ns;
> begin
> sig <= '0';
> wait for 10 ns;
> sig <= 'X', '1' after t_rise;
> wait for 10 ns;
> sig <= 'X', '0' after t_fall;
> ...
>
> But please NEVER do that with a clock signal!
> --
> Jonathan Bromley, Consultant
>
> DOULOS - Developing Design Know-how
> VHDL * Verilog * SystemC * e * Perl * Tcl/Tk * Project Services
>
> Doulos Ltd., 22 Market Place, Ringwood, BH24 1AW, UK
> (E-Mail Removed)
> http://www.MYCOMPANY.com
>
> The contents of this message may contain personal views which
> are not the views of Doulos Ltd., unless specifically stated.


Thanks, That was helpfull.
Atul

 
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