Alu

[Altera_Forum]

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??

[Altera_Forum]

Am I all alone on this then? :-( 

 Any help would be greatly appreciated!! Please!

[Altera_Forum]

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