assigning output to input

[Simone Winkler]

Hello!

I tried to do a RS-flip-flop with nor-gates in VHDL. But i got the following
error message:

Parameter notq of mode out can not be associated with a f
ormal parameter of mode in.


the vhdl code was:

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity rs is
Port ( S : in std_logic;
R : in std_logic;
Q : out std_logic;
notQ : out std_logic);
end rs;

architecture Behavioral of rs is

begin
Q <= S nor notQ;
notQ <= R nor Q;
end Behavioral;


....it was just to try the functionality of vhdl......
what can i do to solve this problem?

thanx...

 

[Jonathan Bromley]

I tried to do a RS-flip-flop with nor-gates in VHDL. But i got the following
error message:

Parameter notq of mode out can not be associated with a f
ormal parameter of mode in.

Sounds like you may have two problems.

First, your code is trying to use the value of an "out" port,
which is illegal in VHDL. You must create an internal signal, use
that internal signal for all the useful work, and then copy it
to the out port(s).

Second, the error message: it sounds as if your testbench for
this module is connecting to the ports inappropriately - or
perhaps it's just a confusing error message.

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

You don't need arithmetic packages for this code.
Get rid of them.

entity rs is
Port ( S : in std_logic;
R : in std_logic;
Q : out std_logic;
notQ : out std_logic);
end rs;

OK so far.

architecture Behavioral of rs is

begin
Q <= S nor notQ; -- Can't do this (notQ is an out port)
notQ <= R nor Q;
end Behavioral;

architecture Corrected of rs is
signal Q_int: std_logic;
signal Qbar_int: std_logic;
begin
Q_int <= S nor Qbar_int; -- this is OK
Qbar_int <= R nor Q_int;
-- now get the signals out to the ports:
Q <= Q_int;
notQ <= Qbar_int;
end Corrected;

...it was just to try the functionality of vhdl......

OK, but be aware that combinational feedback is always a bad
idea for synthesisable logic.
--

Jonathan Bromley, Consultant

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