Forum Discussion

Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
17 years ago

Alu

Hey all,

This is my first experience with Quartus and VHDL and I am trying to make an ALU. Below is the code I have got so far.

LIBRARY ieee;

USE ieee.std_logic_1164.ALL;

USE ieee.std_logic_signed.ALL;

USE ieee.std_logic_arith.ALL;

ENTITY alu IS

PORT(

a : IN STD_LOGIC_VECTOR(11 downto 0);

opcode : IN STD_LOGIC_VECTOR(4 downto 0);

b : IN STD_LOGIC_VECTOR(11 downto 0);

alu_out : OUT STD_LOGIC_VECTOR(11 downto 0);

y : BUFFER STD_LOGIC_VECTOR(11 downto 0));

END alu;

ARCHITECTURE start OF alu IS

BEGIN

PROCESS (a, b, opcode,y)

BEGIN

CASE opcode IS

WHEN "000--" =>

alu_out <= a ; -- Pass a

WHEN "001--" =>

alu_out <= a + b; -- Add (a + b)

WHEN "010--" =>

alu_out <= a - b; -- Subtract (a - b)

WHEN "011--" =>

alu_out <= a and b; -- Logical AND (a AND b)

WHEN "100--" =>

alu_out <= a or b; -- Logical OR (a or b)

WHEN "101--" =>

alu_out <= a + 1; -- Increment A (a+1)

WHEN "110--" =>

alu_out <= a - 1; -- Decrement A (a-1)

WHEN "111--" =>

alu_out <= b; -- Pass b

WHEN "---00" =>

y <= alu_out; -- Pass alu_out to y

--WHEN "XXX01" =>

-- y <= ; --

--WHEN "XXX10" =>

-- y <= ; --

WHEN "XXX11" =>

y <= ("000000000000"); -- Pass 0's to y

WHEN others =>

y <= (others => '0');

END CASE;

END PROCESS;

END start;

I get warnings including:

Warning (10325): VHDL Choice warning at alu.vhd(21): ignored choice containing meta-value ""000--""

Warning (10034): Output port "alu_out[11]" at alu.vhd(12) has no driver

Warning: Output pins are stuck at VCC or GND

Warning (13410): Pin "alu_out[0]" is stuck at GND

Warning (13410): Pin "alu_out[1]" is stuck at GND

Warning (13410): Pin "alu_out[2]" is stuck at GND

Warning (13410): Pin "alu_out[3]" is stuck at GND

Warning (13410): Pin "alu_out[4]" is stuck at GND

Warning (13410): Pin "alu_out[5]" is stuck at GND

Warning (13410): Pin "alu_out[6]" is stuck at GND

Warning (13410): Pin "alu_out[7]" is stuck at GND

Warning (13410): Pin "alu_out[8]" is stuck at GND

Warning (13410): Pin "alu_out[9]" is stuck at GND

Warning (13410): Pin "alu_out[10]" is stuck at GND

Warning (13410): Pin "alu_out[11]" is stuck at GND

Warning (13410): Pin "y[0]" is stuck at GND

Warning (13410): Pin "y[1]" is stuck at GND

Warning (13410): Pin "y[2]" is stuck at GND

Warning (13410): Pin "y[3]" is stuck at GND

Warning (13410): Pin "y[4]" is stuck at GND

Warning (13410): Pin "y[5]" is stuck at GND

Warning (13410): Pin "y[6]" is stuck at GND

Warning (13410): Pin "y[7]" is stuck at GND

Warning (13410): Pin "y[8]" is stuck at GND

Warning (13410): Pin "y[9]" is stuck at GND

Warning (13410): Pin "y[10]" is stuck at GND

Warning (13410): Pin "y[11]" is stuck at GND

Warning: Design contains 29 input pin(s) that do not drive logic

Warning (15610): No output dependent on input pin "a[0]"

Warning (15610): No output dependent on input pin "a[1]"

Warning (15610): No output dependent on input pin "a[2]"

Warning (15610): No output dependent on input pin "a[3]"

Warning (15610): No output dependent on input pin "a[4]"

Warning (15610): No output dependent on input pin "a[5]"

Warning (15610): No output dependent on input pin "a[6]"

Warning (15610): No output dependent on input pin "a[7]"

Warning (15610): No output dependent on input pin "a[8]"

Warning (15610): No output dependent on input pin "a[9]"

Warning (15610): No output dependent on input pin "a[10]"

Warning (15610): No output dependent on input pin "a[11]"

Warning (15610): No output dependent on input pin "opcode[0]"

Warning (15610): No output dependent on input pin "opcode[1]"

Warning (15610): No output dependent on input pin "opcode[2]"

Warning (15610): No output dependent on input pin "opcode[3]"

Warning (15610): No output dependent on input pin "opcode[4]"

Warning (15610): No output dependent on input pin "b[0]"

Warning (15610): No output dependent on input pin "b[1]"

Warning (15610): No output dependent on input pin "b[2]"

Warning (15610): No output dependent on input pin "b[3]"

Warning (15610): No output dependent on input pin "b[4]"

Warning (15610): No output dependent on input pin "b[5]"

Warning (15610): No output dependent on input pin "b[6]"

Warning (15610): No output dependent on input pin "b[7]"

Warning (15610): No output dependent on input pin "b[8]"

Warning (15610): No output dependent on input pin "b[9]"

Warning (15610): No output dependent on input pin "b[10]"

Warning (15610): No output dependent on input pin "b[11]"

I think it is to do with using the "-" for dont care in the when statement. Can anybody please help??

2 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    17 years ago

    Am I all alone on this then? :-(

    Any help would be greatly appreciated!! Please!
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    17 years ago

    No you are not alone.

    1) don't use buffer port at all. I am not sure what y output is for.

    2) use clock in your design(clock your process). This is not essential right now but will be soon in your project

    3) use one case statement for each output; one for alu_out and one for the mystery y output. You can't read alu_out(as you are assigning alu_out to y), I don't know how your compiler didn't issue an error.

    4) split up your opcode into 3 bits and 2 bits(assuming you need them separately) one case for opcode(4 downto 2):

    case opcode(4 downto 2) is

    when "000" etc.

    do not use (- or x)

    for y output use a separate case:

    case opcode(1 dwnto 0) is

    when "00" .etc

    Hope this helps