Simple problem, understanding the case sentence

[Geo]

Hi, I've just started a digital electronics course where we will be
mainly working with VHDL, I've started reading a book and trying some
of the exercises. I'm just starting VHDL and got some problems with an
exercise requiring to describe a 2-to-1 multiplexer using a CASE
statement (in the sequential statements chapter).

I've already implemented the multiplexer using a when statement in the
output assignent, but failed when trying with the case statement, see:

[code:1:f9eea22e47]
library ieee;
use ieee.std_logic_1164.all;

entity multiplexor is
port(
entrada0, entrada1: in std_logic;
selector: in std_logic;
salida: out std_logic
);
end multiplexor;

architecture comp_multiplexor of multiplexor is
begin
-- it works using this:
-- salida <= entrada0 when selector = '0' else entrada1;

-- but this one doesn't work:
case selector is
when '0' => entrada0;
when '1' => entrada1;
end case;


end data_flow;
[/code:1:f9eea22e47]
Am I doing things correctly? The compiler tells me about an unexpected
prefix for a array/slice/ (or something like that) on the when choices
lines.

I have problems with another exercise, however I'm sure it's about the
same thing (it's a 2-4 decoder using a CASE statement too), but I'd
like to ask if it's possible to do something like this:

entity dec24 is
port( ...
input: in std_logic_vector( 1 downto 0 );
output: out std_logic_vector( 3 downto 0)
);
end dec24

architecture bhv_dec24 of dec24 is
begin
case input is
when B"00" => ...
when B"01" => ...
...
end case;
end bhv_dec24;
[/code]
My question is about the when B"00" choices, is it correct to use such
a expression here?

Also, would the next expression be ok?

output <= ( 0 => '1', others => '0' );

I would use this one, for example, in the when B"00" choice for
assigning '1' to the appropiate output bit and '0' to all of the
others.

Last, I'd like to apologize for having some none English identifiers in
the first code, I'm not a native English speaker and just copied&pasted
that one. Thanks in advance for your help.

Regards,
José Jorge Enríquez.

PS: sorry for my poor English.

 

[ALuPin@web.de]

Hola José,

you have to use the case statement within a PROCESS.

ARCHITECTURE xy OF ts IS

BEGIN

PROCESS(Selector, a, b)
BEGIN
CASE Selector IS
WHEN '1' => Salida <= a;

WHEN '0' => Salida <= b;

END CASE;
END PROCESS;

END xy;

Rgds
André

 

[vishnu]

i think it will work if you like this
library ieee;
use ieee.std_logic_1164.all;


entity multiplexor is
port(
entrada0, entrada1: in std_logic;
selector: in std_logic;
salida: out std_logic
);
end multiplexor;


architecture comp_multiplexor of multiplexor is
begin
case selector is
when '0' => salida<=entrada0;
when '1' => salida<=entrada1;
end case;


end data_flow;

 

[Geo]

That's it! I just didn't notice any comment/advice about it, thank you.

Best wishes,
José Jorge Enríquez.

 

[Geo]

Thanks for your comments, I just didn't pasted the the correct code
here . (You can see the end data_flow at the end of the code that
does not match any previously declared identifier).

The solution is to place the case statement within a process, as André
pointed out in the previous post.

Thanks (and sorry for posting incorrect code),
José Jorge Enríquez.

 

[Isaac Bosompem]

vishnu wrote:

i think it will work if you like this
library ieee;
use ieee.std_logic_1164.all;


entity multiplexor is
port(
entrada0, entrada1: in std_logic;
selector: in std_logic;
salida: out std_logic
);
end multiplexor;


architecture comp_multiplexor of multiplexor is
begin
case selector is
when '0' => salida<=entrada0;
when '1' => salida<=entrada1;
end case;


end data_flow;

You could also do:

architectur comp_multiplexor of multiplexor is
begin

salida <= entrada0 when selector='0' ELSE entrada1;


end comp_multiplexor;

 

[Geo]

I've already done that, you can see the code commented out in my first
post. The problem was that the book exercise required the use of a case
statement and I couldn't make it work (but it's working now). Thank you
anyway.

Regards,
José Jorge Enríquez.