A trouble With Code

[Altera_Forum]

Hi all, 

well am a new VHDL programmer, and I was trying to write a code for a generic multiplier using a 4 bit ripple adder, the results are correct, but my design should include two modes, test mode and normal mode, when I was trying to compile the following code, an error occured as follows. Error (10028): Can't resolve multiple constant drivers for net "output[10]" at fast_multiplier.vhd(19)  here is the code LIBRARY IEEE; USE IEEE.STD_LOGIC_1164.ALL;  ENTITY fast_multiplier IS  GENERIC (n: POSITIVE:=7);  PORT(mode,TDI,clk_normal,clk_test:IN STD_LOGIC;  multiplier: IN STD_LOGIC_VECTOR(n-1 DOWNTO 0);  multiplicand: IN STD_LOGIC_VECTOR(3 DOWNTO 0);  result: OUT STD_LOGIC_VECTOR(n+3 DOWNTO 0);  TDO: OUT STD_LOGIC); END ENTITY;  ARCHITECTURE simple OF fast_multiplier IS SIGNAL input1: STD_LOGIC_VECTOR(n-1 DOWNTO 0); SIGNAL input2: STD_LOGIC_VECTOR(3 DOWNTO 0); SIGNAL output: STD_LOGIC_VECTOR(n+3 DOWNTO 0); SIGNAL clk: STD_LOGIC; BEGIN  g0: ENTITY WORK.multiplier_ripple(simple) GENERIC MAP(7) PORT MAP(input1,input2,output);  g1: ENTITY WORK.my_mux(simple) PORT MAP(clk_normal,clk_test,mode,clk);  PROCESS(mode,clk)  BEGIN  IF RISING_EDGE(clk) THEN  IF mode='0' THEN  input1<=multiplier;  input2<=multiplicand;  result<=output;   ELSIF Mode='1' THEN  input2(3 DOWNTO 0)<= (TDI & input2(3 DOWNTO 1)) ;  input1(n-1 DOWNTO 0)<= input2(0) & input1(n-1 DOWNTO 1);  output(n+3 DOWNTO 0)<= input1(0) & output(n+2 DOWNTO 0);  TDO<=output(n+3);  END IF;   END IF;  END PROCESS; END ARCHITECTURE;  when mode=0 this is the normal mode and when mode=1 this is the test mode any help plz as soon as possible

[Altera_Forum]

output is unconditionally driven by the multiplier, so it can't be driven in the process. I guess, you would want to drive result instead.

[Altera_Forum]

To avoid double drive on "output" connect "temp1" to multiplier output at G0 line and assign temp2 instead of output in the line: 

output(n+3 DOWNTO 0)<= input1(0) & output(n+2 DOWNTO 0);  Then create a switch: if ... output <= temp1;  else  output <= temp2; end if;

[Altera_Forum]

Kaz is right i think.Because simple meaning of the this common error "multiple constant driver" is that compiler is unable to assign more than one value to that specified stuff that u wanted. 

Because your code is simultaneously assigning more than one values to output[10],so compiler always confuses out of two values,which one i should use.So it will generate an error of multiple constant driver To resolve this,always use intermediate variable,like "temp" for storing values and use value whenever u need it to your required output.