How to Synthesize While Loop in Verilog?

Question

I have tried to design a Booth multiplier and it runs well in all compilers including:

Modelsim,Verilogger Extreame,Aldec Active Hdl & Xilinx's Isim......

I know Simulation and Synthesis are two Different Process and only few Verilog constructs with various restrictions are there for synthesis. But I don't know what happen While loop in my program not work in Synopsys Synplify 9.6 as well as in Xilinx ise 14.2.

When I try to synthesize Synopsys says "loop iteration limit 2000 exceeded" while Xilinx' XST says " This Xilinx application has run out of memory or has encountered a memory conflict"

I have attached my code below. I Also write this <-------"Error Generated Here due to this while loop" where Synthesizer generates error due to while loop......

/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////Author/Coder-Shrikant Vaishnav///////////////////////////////////////////////
/////////////////////////////////////////Design-Booth Algorithm Demonstration////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////


module booth_synt(input wire [4:0]a,input wire [4:0]b,output reg signed[9:0] g);
reg signed[10:0]c;// One extra bit for sign bit.....I mean 11th bit.
//We used Signed Reg bcoz ASR fill vacant bit with MSB and then shift other for unsigned reg they fill it with zeros and then shift.. 
reg[4:0]d;
reg [4:0]e;
reg [2:0]count1; 
reg [2:0]count2; 
reg [2:0]count3; 
reg [2:0]count4; 

//Always start whenever any changes happens

always@(a,b)
 begin :close



//If's for sign bit check...
//Then 2's Complement...

count1=3'b000; //Initialize Counter
count2=3'b000;
count3=3'b000;
count4=3'b000;

//For negative
if(a[4]==1'b1) //Internal checking
  begin
  if(a==5'b10000)
    begin
     g[9:0]=10'b0000000000;
    end
  else
  begin
  d=~{1'b0,a[3],a[2],a[1],a[0]};  //we place 1'b0 because its inversion is 1   
  d=d+5'b00001;  //2's Complement we use additional register d for data holding.....bcoz wire not hold data
     if(d[4]==1'b0)//This "if" is used bcoz if due to calculation if accidently d[5]==1'b0 then this changs sign bit and thus ans
    begin
    d[4]=1'b1;
    end
    c[5:1]=d;
  end
  end

 if(b[4]==1'b1)
  begin 

   if(b==5'b10000)
    begin
     g[9:0]=10'b0000000000;
     disable close;
    end
  else
   begin
   e=~{1'b0,b[3],b[2],b[1],b[0]}; //we place 1'b0 because its inversion is 1
   e=e+5'b00001;
   if(e[4]==1'b0)//This "if" is used bcoz if due to calculation if accidently e[4]==1'b0 then this changs sign bit and thus ans
    begin
   e[4]=1'b1;
    end
   end
  end


//For positive 
if(b[4]==1'b0)
begin
e[4:0]=b[4:0];

end

if(a[4]==1'b0)
begin

c[1]=a[0];
c[2]=a[1]; //"a" is multiplier while "b" is multiplicand...
c[3]=a[2];
c[4]=a[3];
c[5]=a[4];

end



//Initialization of Output ........
c[0]=1'b0;

//All MSB's are Initially set to Zeros
c[6]=1'b0;
c[7]=1'b0;
c[8]=1'b0;
c[9]=1'b0;
c[10]=1'b0;



//Four Different Conditions Checking......
case({c[1],c[0]})

2'b00:begin   //Case 1

       while(count1<3'b101)  **<-------"Error Generated Here due to this while loop"**
      begin


         if({c[1],c[0]}==2'b10) //cond1 for 10
            begin
             c[10:6]=(c[10:6]-e[4:0]);
              c=c>>>1;
              count1=count1+1'b1;


            if(count1==3'b101)// Counter value check
             begin
               if(c[10]==1)
                 begin
                 c=~{1'b0,c[9],c[8],c[7],c[6],c[5],c[4],c[3],c[2],c[1],c[0]};//we place 1'b0 because its inversion is 1
                 c=c+10'b0000000010;
                 c[10]=1'b1; //Again giving 1 for surity
                 g[9:0]=c[10:1];

                 end

                 if(c[10]==0)
                 begin
                 c[10]=1'b0;
                 g[9:0]=c[10:1];

                 end
            end //end if==4
          end
          if(({c[1],c[0]}==2'b00) || ({c[1],c[0]}==2'b11))      //cond 2 in it we describe both 00 and11.........Arithemetic Right Shift operation
             begin
              c=c>>>1;     
              count1=count1+1'b1;                    


             if(count1==3'b101) // Counter value check
               begin
                if(c[10]==1)
                 begin
                 c=~{1'b0,c[9],c[8],c[7],c[6],c[5],c[4],c[3],c[2],c[1],c[0]};//we place 1'b0 because its inversion is 1
                 c=c+10'b0000000010;
                 c[10]=1'b1; //Again giving 1 for surity
                 g[9:0]=c[10:1];

                 end

                 if(c[10]==0)
                 begin
                 c[10]=1'b0;
                 g[9:0]=c[10:1];

                 end

              end
              end   

           if({c[1],c[0]}==2'b01) //cond3 for 01
            begin
             c[10:6]=(c[10:6]+e[4:0]);
              c=c>>>1;
              count1=count1+1'b1;


             if(count1==3'b101) // Counter value check
               begin
                if(c[10]==1)
                 begin
                 c=~{1'b0,c[9],c[8],c[7],c[6],c[5],c[4],c[3],c[2],c[1],c[0]};//we place 1'b0 because its inversion is 1
                 c=c+10'b0000000010;
                 c[10]=1'b1; //Again giving 1 for surity
                 g[9:0]=c[10:1];

                 end

                 if(c[10]==0)
                 begin
                 c[10]=1'b0;
                 g[9:0]=c[10:1];

             end
              end
              end
     end  

   end//while's end


 //Case2
 2'b11:begin
      while(count2<3'b101) **<-------"Error Generated Here due to this while loop"**
      begin



         if({c[1],c[0]}==2'b10) //cond1 for 10
            begin
             c[10:6]=(c[10:6]-e[4:0]);
              c=c>>>1;
              count2=count2+1'b1;


             if(count2==3'b101) // Counter value check
               begin
                if(c[10]==1)
                 begin
                 c=~{1'b0,c[9],c[8],c[7],c[6],c[5],c[4],c[3],c[2],c[1],c[0]};//we place 1'b0 because its inversion is 1
                 c=c+10'b0000000010;
                 c[10]=1'b1; //Again giving 1 for surity
                 g[9:0]=c[10:1];

                 end

                 if(c[10]==0)
                 begin
                 c[10]=1'b0;
                 g[9:0]=c[10:1];

              end
              end
              end

          if(({c[1],c[0]}==2'b00)||({c[1],c[0]}==2'b11))//cond 2 in it we describe both 00 and11.........Arithemetic Right Shift operation
             begin
              c=c>>>1;     
              count2=count2+1'b1;                    


             if(count2==3'b101)// Counter value check
               begin
                if(c[10]==1)
                 begin
                 c=~{1'b0,c[9],c[8],c[7],c[6],c[5],c[4],c[3],c[2],c[1],c[0]};//we place 1'b0 because its inversion is 1
                 c=c+10'b0000000010;
                 c[10]=1'b1; //Again giving 1 for surity  
                 g[9:0]=c[10:1];

                 end

                 if(c[10]==0)
                 begin
                 c[10]=1'b0;
                g[9:0]=c[10:1];

             end
              end
              end   

           if({c[1],c[0]}==2'b01) //cond3 for 01
            begin
            c[10:6]=(c[10:6]+e[4:0]);
              c=c>>>1;
              count2=count2+1'b1;


             if(count2==3'b101)// Counter value check
               begin
                 if(c[10]==1)
                 begin
                 c=~{1'b0,c[9],c[8],c[7],c[6],c[5],c[4],c[3],c[2],c[1],c[0]};//we place 1'b0 because its inversion is 1
                 c=c+10'b0000000010;
                 c[10]=1'b1; //Again giving 1 for surity 
                 g[9:0]=c[10:1];

                 end

                 if(c[10]==0)
                 begin
                 c[10]=1'b0;
                 g[9:0]=c[10:1];

              end
              end
             end
           end 
          end //while's end



  //Case 3
  2'b10:begin
     while(count3<3'b101) **<-------"Error Generated Here due to this while loop"**
      begin

     if({c[1],c[0]}==2'b10) //Cond1 for 10
            begin
             c[10:6]=(c[10:6]-e[4:0]);

              c=c>>>1;
              count3=count3+1'b1;


             if(count3==3'b101)// Counter value check
               begin
                if(c[10]==1)
                 begin
                 c=~{1'b0,c[9],c[8],c[7],c[6],c[5],c[4],c[3],c[2],c[1],c[0]};//we place 1'b0 because its inversion is 1
                 c=c+10'b0000000010;
                 c[10]=1'b1; //Again giving 1 for surity 
                 g[9:0]=c[10:1];

                 end

                 if(c[10]==0)
                 begin
                 c[10]=1'b0;
                 g[9:0]=c[10:1];

              end
              end
              end

          if(({c[1],c[0]}==2'b00)||({c[1],c[0]}==2'b11))//cond 2 in it we describe both 00 and11.........Arithemetic Right Shift operation
             begin
              c=c>>>1;     

              count3=count3+1'b1;                    


             if(count3==3'b101)// Counter value check
               begin
                if(c[10]==1)
                 begin
                 c=~{1'b0,c[9],c[8],c[7],c[6],c[5],c[4],c[3],c[2],c[1],c[0]};//we place 1'b0 because its inversion is 1
                 c=c+10'b0000000010;
                 c[10]=1'b1; //Again giving 1 for surity 
                 g[9:0]=c[10:1];

                 end

                 if(c[10]==0)
                 begin
                 c[10]=1'b0;
                 g[9:0]=c[10:1];

               end
              end
              end   

           if({c[1],c[0]}==2'b01) //cond3 for 01
            begin
             c[10:6]=(c[10:6]+e[4:0]); 

              c=c>>>1;
              count3=count3+1'd1;

             if(count3==3'b101)// Counter value check
               begin
                if(c[10]==1)
                 begin
                 c=~{1'b0,c[9],c[8],c[7],c[6],c[5],c[4],c[3],c[2],c[1],c[0]};//we place 1'b0 because its inversion is 1
                 c=c+10'b0000000010;
                 c[10]=1'b1; //Again giving 1 for surity 
                 g[9:0]=c[10:1];

                 end

                 if(c[10]==0)
                 begin
                 c[10]=1'b0;
                 g[9:0]=c[10:1];

                 end

               end
             end
           end 
          end //while's end



  //Case 4
  2'b01:begin
         while(count4<3'b101) **<-------"Error Generated Here due to this while loop"**
          begin

         if({c[1],c[0]}==2'b10) //cond1 for 10
            begin
             c[10:6]=(c[10:6]-e[4:0]);
              c=c>>>1;
              count4=count4+1'b1;


             if(count4==3'b101)// Counter value check
               begin
                if(c[10]==1)
                 begin
                 c=~{1'b0,c[9],c[8],c[7],c[6],c[5],c[4],c[3],c[2],c[1],c[0]};//we place 1'b0 because its inversion is 1
                 c=c+10'b0000000010;
                 c[10]=1'b1; //Again giving 1 for surity 
                 g[9:0]=c[10:1];

                 end

                 if(c[10]==0)
                 begin
                 c[10]=1'b0;
                 g[9:0]=c[10:1];

              end
              end
              end

          if(({c[1],c[0]}==2'b00)||({c[1],c[0]}==2'b11))//cond 2 in it we describe both 00 and11.........Arithemetic Right Shift operation
             begin
              c=c>>>1;     
              count4=count4+1'b1;                    


             if(count4==3'b101)// Counter value check
               begin
                if(c[10]==1)
                 begin
                 c=~{1'b0,c[9],c[8],c[7],c[6],c[5],c[4],c[3],c[2],c[1],c[0]};//we place 1'b0 because its inversion is 1
                 c=c+10'b0000000010;
                 c[10]=1'b1; //Again giving 1 for surity 
                 g[9:0]=c[10:1];

                 end

                 if(c[10]==0)
                 begin
                 c[10]=1'b0;
                 g[9:0]=c[10:1];

              end
              end
              end   

           if({c[1],c[0]}==2'b01) //cond3 for 01
            begin
             c[10:6]=(c[10:6]+e[4:0]);
              c=c>>>1;
              count4<=count4+1'b1;


             if(count4==3'b101)
               begin
                if(c[10]==1)
                 begin
                 c=~{1'b0,c[9],c[8],c[7],c[6],c[5],c[4],c[3],c[2],c[1],c[0]};//we place 1'b0 because its inversion is 1
                 c=c+10'b0000000010;
                 c[10]=1'b1; //Again giving 1 for surity 
                 g[9:0]=c[10:1];

                 end

                 if(c[10]==0)
                 begin
                 c[10]=1'b0;
                 g[9:0]=c[10:1];

                  end
                end
               end
              end  //while's end

          end//01's end  

   endcase //case end


end       //always end
endmodule

Answer 1

While loops tend to imply something dynamic, like checking a condition. This is not a good use of verilog intended for synthesis. For loops which can be statically unrolled are more commonly used to shorten the written code.

If you need some thing more dynamic a dedicated state machine should be written.

To answer some of the questions raised in the comments:

Combinatorial logic uses assign or is contained in always @* this is continuously evaluated and all runs in parallel think AND, OR, NOR gates.

Sequential logic will be contained in an always @( posedge clk ) this is executed on every positive edge of the clock. The registers or memory elements used inside of this typically represent Flip-Flops.

Answer 2

Here are some tips:

• Remove the latches:
  • c[5:1] is a latch because it is not initialized in the if(a==5'b10000) branch.
  • d is not defined in if(a==5'b10000) and could be a potential latch depending on the synthesizer optimization capabilities.
  • e is not defined in if(b==5'b10000) and could be a potential latch like d.
  • At the beginning of the always block, make sure all the combination logic have initial values.
• Try to avoid using disable for code to be synthesized. Use if-else statements instead.
  • Note: This is a guideline not a rule.
• There is lot of redundant and poorly organized code:
  • All branches with in the case statement are the same. Should be cleaned up in consolidated.
  • There is code inside the while loop that is better suited outside the loop (e.g. the if(count*==3'b101) block)
• If the while loop is having problems, try a for loop.

Answer 3

This is poorly written code.You have written it like a computer program. Verilog is a Hardware Description Language - not a programming language. In your case, synthesizer is trying to replicate logic inside the while loop in the case statement.

• Design the hardware on a piece of paper before translating it to HDL
• Identify the combinational and sequential logic in the design before coding.
• Think what logic will synthesizer use to realize the logic you have written.