Question: inout signal assignment


J

John Potter

Hello, I have a inout signal assignment problem.
I can't assign a value to a inout signal which is a vector. I get
"UUUUUUU" everytime.

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

entity Testing is
Port ( CLK: in std_logic;
A : inout std_logic_vector(7 downto 0);
B : inout std_logic_vector(7 downto 0);
C : inout std_logic_vector(7 downto 0));
end Testing;

architecture Behavioral of Testing is

begin

bbbb: process is
begin

wait on CLK until CLK = '0';

B <= A;
B <= "11111111" after 50ps;
B <= C after 80ps;
wait;

end process bbbb;

aaaa: process is
begin

A <= "10101011" after 0ns;
C <= "00000000" after 10ps;
wait;

end process aaaa;

end Behavioral;
-----------------------------------------

A, B, C should behave like this:

A = "10101011" 0ps
B = "UUUUUUUU" 0ps, "11111111" 50ps, "0000000" 80ps
C = "00000000" 10ps

But I just get "UUUUUUUU" for B. My result:
http://www.angelfire.com/hi/Garrychang/VHDL.jpg.

Anybody know why I got this result? If I delete line - "B <= C after
80ps;" the final result is correct.

Thanks.
 
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R

Renaud Pacalet

John said:
Hello, I have a inout signal assignment problem.
I can't assign a value to a inout signal which is a vector. I get
"UUUUUUU" everytime.

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

entity Testing is
Port ( CLK: in std_logic;
A : inout std_logic_vector(7 downto 0);
B : inout std_logic_vector(7 downto 0);
C : inout std_logic_vector(7 downto 0));
end Testing;

architecture Behavioral of Testing is

begin

bbbb: process is
begin

wait on CLK until CLK = '0';

B <= A;
B <= "11111111" after 50ps;
B <= C after 80ps;
wait;

end process bbbb;

multiple inertial delay assignments to the same signal in the same
simulation cycle: the last one overrides the others (it's a bit more
complex but let's start with this). What you'll get is B = (others=>'U')
until the first falling edge of CLK and then B = the value of C at first
falling edge of CLK.
aaaa: process is
begin

A <= "10101011" after 0ns;
C <= "00000000" after 10ps;
wait;

end process aaaa;

end Behavioral;
-----------------------------------------

A, B, C should behave like this:

A = "10101011" 0ps
B = "UUUUUUUU" 0ps, "11111111" 50ps, "0000000" 80ps
C = "00000000" 10ps

But I just get "UUUUUUUU" for B. My result:
http://www.angelfire.com/hi/Garrychang/VHDL.jpg.

As the first (and only) falling edge of CLK occurs at t=0ps the 3
assignements of your bbbb process are all executed at t=0ps and B is
assigned the value of C at 0ps, that is, (others=>'U').
Anybody know why I got this result? If I delete line - "B <= C after
80ps;" the final result is correct.

Tell us what behaviour you need and we'll give you a solution or two.

Regards,
 

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