signals to connect ports

Young Guest 2011-01-05 16:57 #2002345

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Hello everyone,

I am new to VHDL and I like it !

I have a question about usage of signals in VHDL, I have been told that 
it is more conenient to use signals to connects ports, instead of 
connecting ports directly. Now I wonder why is that ?! and how to do it 
a sequential process ?

when should the value from the input port should be taken into the 
signals ? the signal passes the value to the output with the clock edge, 
so what about the input ?!!

Thanks in advance
Young

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Na sowas Guest 2011-01-05 17:36 #2002401

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> I have been told that it is more conenient to use signals to connects
> ports, instead of connecting ports directly.
In my opinion only output ports are affected, because you cannot read 
back an out port.
So given this short sample:

entity runningLED is
 Port   ( 
           Clk     : in    STD_LOGIC;
           LED     : out   STD_LOGIC_VECTOR(7 downto 0)
end runningLED ;
architecture Behavioral of runningLED  is
signal LEDs   :        STD_LOGIC_VECTOR (7 downto 0) := "00000001";
   process begin
      wait until rising_edge(clk);
      if (LEDs="10000000") then     --  this would not be possible on the output port "LED"
         LEDs <= "00000001";
      else
         LEDs <= LEDs(6 downto 0) & '0';
      end if;
   end process;
   LED <= LEDs;
end Behavioral;

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Young Guest 2011-01-07 10:09 #2004764

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thanks so much for your reply.

I don't understand why the  LED <= LEDs; is not in the end of the 
process! the output port should take the value each clock, right ?!

I believe it should be as a part of the process

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Jan M. (mueschel) 2011-01-07 10:53 #2004802

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If you put the assignment inside the process, you would have another 
flipflop generated.
Here, this assignment is just connecting the output of the internal 
register, namely LEDs, which updates with each clock to the output of 
the entity. This is not different to put a wire between the output of an 
IC to the actual connector on the edge of your board. So, a 
combinatorial assignment without involving the clocked process is what 
you need in here.

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