need help with 8 to 3 encoder in vhdl

[rg350dxlover]

Hi... I'm new to VHDL and I have to do the code for 8 to 3 encoder. There seem to be a problem with my code but I can't figure out what it is. Help! Thanks...


library ieee;
use ieee.std_logic_1164.all;

entity encoder is
port( y : in std_logic_vector(7 downto 0);
s : out std_logic_vector(2 downto 0));
end encoder;

architecture behavior of encoder is
begin
with y
s <= "000" when "0000 0001"
"001" when "0000 0010"
"010" when "0000 0100"
"011" when "0001 0000"
"100" when "0010 0000"
"101" when "0100 0000"
"110" when "1000 0000"
end behavior;

 

[TrickyDicky]

you missed select out:

with y select
s <= "000" when "0000 0001"
etc

 

[barry]

You're also missing the "when others" case.

 

[TrickyDicky]

You're also missing the "when others" case.

You dont actually need when others, but without it you will create a latch.

 

[barry]

You dont actually need when others, but without it you will create a latch.

Unless you expressly WANT a latch then you DO NEED the 'when others'. It's generally bad form to not explicitly cover every case.

 

[std_match]

The with-select construct must cover all combinations, so it is not possible to get a latch by skipping one or more cases.

 

[TrickyDicky]

The with-select construct must cover all combinations, so it is not possible to get a latch by skipping one or more cases.

I stand corrected. I just confirmed it in my VHDL reference (I dont use with select much).

 

[harerama]

whats the error u got ?

 

[barry]

I stand corrected. I just confirmed it in my VHDL reference (I dont use with select much).

I've NEVER used 'with-select' . I use 'case/when' (inside a process). I generally put EVERYTHING in a process. Maybe that's just style, but I don't like to have stuff just floating around free; seems like a good way to make a mistakes. (I also like to clock everything unless there's a good reason not to. At the very least, it will yield better performance.)

 

[Bustigo]

where is the comma ? , "011" when "0001 0000",
is that the error ? or u put it

 

[rg350dxlover]

Thank you all for your help. :smile: I've fixed my code. Added "select", "," and "when others" case and yet there's error. It says that "object with std_logic type cannot contain characters". So meaning I have to change the input and output type? or add library?

 

[TrickyDicky]

its probably complaining about the space in the middle of the output word. Either remove it or change it to _

 

[barry]

Thank you all for your help. :smile: I've fixed my code. Added "select", "," and "when others" case and yet there's error. It says that "object with std_logic type cannot contain characters". So meaning I have to change the input and output type? or add library?

Get rid of that space. "0000 0001"==> "00000001"

 

[rg350dxlover]

Thank you all for the help. It's working now! =)
Merry Christmas!

 

[Bustigo]

Merry Christmas!

 

[rg350dxlover]

A new problem now. How do I connect a module with another one? Like right now, I have decoder, counter, etc. in separate files. How do I put them together?

 

[Bustigo]

read about COMPONENT INSTANTIATION in vhdl

 

[rg350dxlover]

all right. will look into it. thanks Bustigo.