How to fix inferring latch(es) error when I use a for loop?

Question

I'm implementing a module to count number of '1's in an input vector and cannot fix the inferring latches error.

Warning (10240): Verilog HDL Always Construct warning at top_module.v(15): inferring latch(es) for variable "count", which holds its previous value in one or more paths through the always construct File: /var/www/verilog/work/vlgaaQTZu_dir/top_module.v Line: 15 Warning (10240): Verilog HDL Always Construct warning at top_module.v(15): inferring latch(es) for variable "mask", which holds its previous value in one or more paths through the always construct File: /var/www/verilog/work/vlgaaQTZu_dir/top_module.v Line: 15

I've tried to add an else expression in the for loop after if statement, and also add an initial statement to initialize the value of count and mask.

module top_module( 
    input [254:0] in,
    output [7:0] out );

    wire [7:0]count;
    wire [254:0]mask;

    initial begin 
    count = 8'h0;
    mask = 255'h1;
    end 

    always @(*)
    begin
        for(int i = 0; i < 255; i = i + 1)
            begin
                if(in & mask)
                  begin
                    count = count + 1;
                    mask = mask << 1;
                  end
                else
                  begin
                    count = count;
                    mask = mask;
                  end
            end
      out = count;
    end

endmodule

I failed to pass the complying.

dave_59 dave_59 · Accepted Answer · 2019-01-05T06:59:50

Adding the initial statement only initializes the variables once at time 0. You need to initialize every time you enter the always block so you never reference the previous values.

 always @(*)
    begin
        count = 8'h0;
        mask = 255'h1;
        for(int i = 0; i < 255; i = i + 1)
            begin
                if(in & mask)
                  begin
                    count = count + 1;
                    mask = mask << 1;
                  end
            end
      out = count;
    end

How to fix inferring latch(es) error when I use a for loop?

1 Answers