Issue unexpected output from hdlcoder

Asked by satish morigiri on 31 Jan 2018

Latest activity Answered by Tim McBrayer on 31 Jan 2018

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test_hdl.zip

Dear All,

I'm trying to make a simple verilog code by using hdlcoder. so I used as the below script and function. but I came across unexpected result when I run the hdlcoder.

The expected data is all 0 in simulation.

Would you please help me how to resolve this problem?

-calc.m

%%Function 
function out=calc(u)
dx=1.71;
u = u / 1000;
out = u*dx;
out = floor(out);
end

-test.m

    clear
  clc
u=3000;
a=calc(u);
u=4000;
a=calc(u);
u=5000;
a=calc(u);

Especially, I got the snippet code from calc_fixpt.v But It does not make sense.

    assign out_2 = 4'b0000;
     always @(posedge clk or negedge reset_x)
      begin : out_reg_process
        if (reset_x == 1'b0) begin
          out_3 <= 4'b0000;
        end
        else begin
          if (enb) begin
            out_3 <= out_2;
          end
        end
      end

From this code, the output is always 0. Would you let me know how do resolve this problems?

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Answer by Tim McBrayer on 31 Jan 2018

This looks like an issue with your data types. Note that your output Verilog code has the output being a 4-bit value. You don't show your input data types, so I don't know if you went through float to fixed conversion, or if the data you are supplying is 4-bit data.

Whichever way you arrived at this data type, the problem is in the line u = u / 1000. Since u is defined as 4 bits, it can only store the values 0..15. When you divide any of these values by 1000, in a 4-bit data type the result is always 0.

Possible fixes:

Change the data type to a type that does not always round to 0 when dividing by 1000
Change your logic so that you are not redefining your input u, but compute u / 1000 into a separate variable

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