VHDL Code help

Tai T. (john9686) OP 2013-12-07 20:25 #3435989

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Ok so when I run Isim through xilink I get this:

Simulator is doing circuit initialization process.
at 0 ps, Instance /testbench/uut/ : Warning: NUMERIC_STD."<": metavalue 
detected, returning FALSE
ERROR: In process functionT.vhd:25
 Target Size 10 and source size 20 for array dimension 0 does not match.


Is it referring to "z <= y * 3;" ????


Code:

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;
entity functionT is
    port(
        clk: in std_logic;
        switches: in std_logic_vector(9 downto 0);
        output: out std_logic_vector(9 downto 0)
end functionT;
architecture ARCH of functionT is
--signals
constant A: unsigned(9 downto 0):=("0110101001"); -- A=425
signal B: unsigned(9 downto 0);
signal sig512: unsigned(9 downto 0):=("1000000000");
signal T: unsigned(9 downto 0);
signal x, y, z: unsigned(9 downto 0);
    B <= unsigned(switches); --something like that
    x <= A when A>sig512 else sig512;
    y <= x when x<B else B;
    z <= y * 3;
    T <= z(7 downto 0) & "00";
    output <= std_logic_vector(T);
end ARCH;

Attached files:

isim.png (130 KB)

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Olga Guest 2013-12-07 20:33 #3435998

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Tai Tai wrote:
> Is it referring to "z <= y * 3;" ????

IF it is line 25 then yes. But I'm not going to count it. It's your 
task to tell us, which line the error is referring to.

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Tai T. (john9686) OP 2013-12-07 20:45 #3436014

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That is line 25.

Why would I be receiving this error in isim?

ERROR: In process functionT.vhd:25
 Target Size 10 and source size 20 for array dimension 0 does not match.

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Gustl B. (-gb-) 2013-12-07 23:26 #3436205

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Y is 10 bits, 3 is at least 2 bits. So the result 1023*3=3069 is 
101111111101 is 12 bits but z is only 10 bits so z is too short. The 
error says z should be 20 bits long, not 12. I cant say why, but just do 
it and use the last 8 bits.

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Tai T. (john9686) OP 2013-12-08 02:38 #3436332

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Thanks I have it working. The only problem now is how to divide the "T" 
by 4 so I get the right "output". With this code T=672 and ouput=672, 
168 is the correct output meaning I need to divide by 4.


CODE:

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;
entity functionT is
    port(
        clk: in std_logic;
        output: out std_logic_vector(19 downto 0)
end functionT;
architecture ARCH of functionT is
--signals
signal A: unsigned(9 downto 0):=("0110101001"); -- A=425
signal B: unsigned(9 downto 0):=("0011100000"); -- B=224
signal sig512: unsigned(9 downto 0):=("1000000000"); -- sig512=512
signal T: unsigned(19 downto 0);
signal x, y: unsigned(9 downto 0);
signal z: unsigned(19 downto 0);
    x <= A when A>sig512 else sig512; --Maximum
    y <= x when x<B else B; --Minimum
    z <= y * 3;
   T <= z(17 downto 0) & "00";
    output <= std_logic_vector(T);
end ARCH;

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Tai T. (john9686) OP 2013-12-08 04:21 #3436349

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Sorry with out the whole picture it wouldn't make sense. Attached 
expression I was working on
I need to mutilate by 0.75
so multiply by 3 then divide by 4

Attached files:

VHDL_Algorithm.jpg (7.5 KB)

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Lothar M. (lkmiller) Moderator (Company: Titel) 2013-12-08 08:27 #3436375

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Tai Tai wrote:
> The only problem now is how to divide the "T" by 4
Why don't you simply try it this way:  output <= std_logic_vector(T/4);
That will work, because an unsigned division by 4 is just "truncating 
the last two bits"...


BTW: why not using integer datatypes?

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;
entity functionT is
  clk: in std_logic;
  inA: in std_logic_vector(19 downto 0),
  inB: in std_logic_vector(19 downto 0),
  output: out std_logic_vector(9 downto 0) -- can never be longer than A or B
end functionT;
architecture ARCH of functionT is
--signals
signal A: integer := 425; -- initialazation makes no sense
signal B: integer := 224; -- because A nd B are not in registers!
signal T, x, y: integer;
  -- convert inputs to integers
  A <= to_integer(unsigned(inA));
  B <= to_integer(unsigned(inB));
  -- do the calculation
  x <= A when A>512 else 512; -- Maximum
  y <= x when x<B   else B;   -- Minimum
  T <= (y*3)/4;               -- *0.75
  -- convert back for output
  output <= std_logic_vector(to_unsinged(T,10));
end ARCH;


For the magic conversions between vectors and integers see the picture 
there:
http://www.lothar-miller.de/s9y/categories/16-Numeric_Std

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Gustl B. (-gb-) 2013-12-08 10:32 #3436437

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Why don't you just write

T<=z(9 downto 0);

Because attaching "00" is a left shift with 2 digits means you multiply 
with 4.

I also see no need for T to be 20 bits long when your right output is 
168 you only need 8 bits.

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Lothar M. (lkmiller) Moderator (Company: Titel) 2013-12-08 16:41 #3436873

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Gustl Buheitel wrote:
> Why don't you just write
> T<=z(9 downto 0);
If at all such a trick, then it should be

   T <= z(11 downto 2)

Because then the last two bits are truncated (whats ok for an unsigned 
division by 4). But all in all I prefer the z/4 style...

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Gustl B. (-gb-) 2013-12-08 16:54 #3436883

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Hello,
he has:

z <= y * 3;
T <= z(17 downto 0) & "00";

so T = z*4. And the output is factor 4 too high (so he says), so he 
could just leave away the &"00" and gets it perfectly? I think he 
accidentially multiplies z by 4.

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Lothar M. (lkmiller) Moderator (Company: Titel) 2013-12-08 17:00 #3436895

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Gustl Buheitel wrote:
> (so he says)
Maybe he's wrong...

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Tai T. (john9686) OP 2013-12-09 01:28 #3437617

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Yeah thanks for all your input. This the code I have working perfectly.
Thanks :)

Code:

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.numeric_std.all;
entity functionT is
    port(
        clk: in std_logic;
        output: out std_logic_vector(19 downto 0)
end functionT;
architecture ARCH of functionT is
--signals
signal A: unsigned(9 downto 0):=("0010111001"); -- A=185
signal B: unsigned(9 downto 0):=("0100100100"); -- B=292
signal sig512: unsigned(9 downto 0):=("1000000000"); -- sig512=512
signal x, y: unsigned(9 downto 0);
signal z: unsigned(19 downto 0);
    x <= A when A>sig512 else sig512; --Maximum
    y <= x when x<B else B; --Minimum
    z <= y * 3; --Multiple by 3
    output <= std_logic_vector(z/4); --Divide by 4
end ARCH;

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