Simulation works, FPGA displays random value of 3...wheres it coming from??

Zeppelin1007 · Dec 9, 2012

Hey guys...first post..be gentile...

Im pulling my hair out over this one..

I was assigned to create a simple repeat adder. InputA is added to itself a number of InputB times. Works fantastically in simulation. WHen it hits the board however, the outputs i get have a "3" added to the output on the left side.

For example:
if i input InputA= 0101 and inputB = 0100, my output should be 01000101. Ok.
Load+clock = Display Inputs A and B on the LEDs. This doesnt exactly happen. Instead i get 00110101....in fact, no matter what i put in, the "B" side, ALWAYS has 0011...

So if i begin addition, enable = high + Clock, I should get 00000101, then 000001010, 00001111...NOPE, i get 00111010...00111111

Can somebody point out where this magical number is coming from? Im using XIlinx ISE 14.1. I do get the warning that InputB has no load, PAR will not attempt to route this signal. Is it because im routing my inputs to variables?

Code:

library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_signed.all;


entity muladder is 
port(
	klok: in std_logic;
	load: in std_logic:='0';
	InputA: in std_logic_vector(7 downto 0):="00000000"; --Preset states to nothing is "undefined"
	InputB: in std_logic_vector(3 downto 0):="0000";
	enable: in std_logic:='0';
	Output: out std_logic_Vector(7 downto 0):="00000000"
	);

end muladder;

architecture multiplier of muladder is	
Begin
	process (klok, load, enable)
		Variable Temp, Adda: std_logic_vector(7 downto 0):="00000000"; --Used variables to update them 
											
		Variable InputBSub: std_logic_vector(3 downto 0):="0000";
	

begin	

if (rising_edge(klok)) then
	if load = '1' and enable = '0' then
		Temp:= InputA; 	--loads Input A 
		Adda:=InputA;										--Loads input A to be added to itself
		InputBSub:= InputB;  							--THe counter variable
		
		Output(3 downto 0)<= Temp(3 downto 0);   --Supposed to output InputA showing its been loaded
		Output(7 downto 4)<= InputBSub;             -- Supposed to output InputB showing its been loaded
		
	elsif (load = '0' and enable = '1') then
		Temp:= Temp+AddA;
		InputBSub:= InputBSub - 1;
		
	
	end if;
	
	Output<= Temp;
		
end if;

end process;
	
end multiplier;

I know im not the best VHDL coder. As you can probably tell im more of a C++ guy. But im giving it a shot. I just cant figure out where this extra 2 bits are coming from. I should mention that if A= 0, and B = 0, i still get 00110000. if i enable and pulse the clock, it begins to count by 3, going 00110000, 01100000, so on and so fourth. My output is supposed to be 8 bit, with leftmost being MSB. yet it looks like the output is still split? despite giving it an 8 bit output.

Im kicking around the idea my board is screwed up? its the xilinx spartan 6, but some other projects run fine on it...ALU, Shift register...i say this because i have another version of the same program, although done long hang addition/subtraction using bitwise XOR/OR/AND...and guess what? SAME RESULT! So im guessing something in my if/then handling?

Thanks in advance guys. Im really puzzled on this one and i'd really like to understand why im getting what im getting.

FvM · Dec 9, 2012

if i input InputA= 0101 and inputB = 0100, my output should be 01000101. Ok.

Not according to your code.
The below line is never evaluated:
Output(7 downto 4)<= InputBSub;

Output is finally assigned to Temp, which is InputA in load state. Apparently, you didn't even check InputA upper four bits.

Zeppelin1007 · Dec 10, 2012

Ah, thats interesting thank you! i get it, it outputs InputA only, because it initially outputs the bottom first 4 bits of A, then the 4 bits of B, then it all immediately gets over written by Ouput<=Temp, which is 00000101...ok that makes sense. I guess i've been staring at this so long its beginning to lose meaning.

So i solved that issue, but im still chasing these phantom bits around....

Code:

library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_signed.all;


entity muladder is 
port(
	klok: in std_logic;
	load: in std_logic;
	InputA: in std_logic_vector(7 downto 0); --Preset states to nothing is "undefined"
	InputB: in std_logic_vector(3 downto 0);
	enable: in std_logic:='0';
	Output: out std_logic_Vector(7 downto 0)
	);

end muladder;

architecture multiplier of muladder is	
Begin
	process (klok, load, enable)
		Variable Temp, Adda: std_logic_vector(7 downto 0); --Used variables to update them 
			Variable Active: std_logic:='0';																	-- instantly, rather after the process
		Variable InputBSub: std_logic_vector(3 downto 0);
	

begin	

if (rising_edge(klok)) then
	if load = '1' and enable = '0' and active = '0' then
		Active := '1';
		Temp:= InputA; 	--loads Input A 
		Adda:=InputA;										--Loads input A to be added to itself
		InputBSub:= InputB;  							--THe counter variable
		
	[B]	Output(3 downto 0)<= Temp(3 downto 0); [/B]
	[B]	Output(7 downto 4)<= InputBSub;            [/B]
		
	elsif (load = '0' and enable = '1' and active = '1') then
		Output<="00000000";  --Attempt to "clean out" the output....
		Temp:= Temp+AddA;
		InputBSub:= InputBSub - 1;
		if(InputBSub = "0001") then
				Active  := '0';
		end if;
	[B]	OutPut<=Temp; [/B]
	end if;
			
end if;


end process;
	

end multiplier;

Still, addition on the right side works....not the left side...im not getting 3's anymore, im 1's...

Simulation works, FPGA displays random value of 3...wheres it coming from??

Zeppelin1007

Newbie level 2

FvM

Super Moderator

Zeppelin1007

Newbie level 2

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Simulation works, FPGA displays random value of 3...wheres it coming from??

Zeppelin1007

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FvM

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Zeppelin1007

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