Why is this counter assignment wrong?

mouad 0
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I am learning Verilog using the HDLBits website, and I solved this problem (circuit counts the number of '1's in an input vector), but I want to understand why my previous tries were wrong.

correct answer

module top_module( 
    input [254:0] in,
    output [7:0] out );
    
    int i ;
    reg [7:0] counter;
    
    always @(*) begin 
        counter =0;
        for (i=0;i<255;i++)begin 
            counter = (in[i]==1)? counter+1:counter;
        end 
    out = counter ;
    end 
   

endmodule

1st wrong answer

module top_module( 
    input [254:0] in,
    output [7:0] out );
    
    int i ;
    reg [7:0] counter;
    
    always @(*) begin 
        counter =0;
        for (i=0;i<255;i++)begin 
            counter = (in[i]==1)? counter+1:counter;
        end 
    end 
    out = counter ;

endmodule

2nd wrong answer

module top_module( 
    input [254:0] in,
    output [7:0] out );
    
    int i ;
    
    
    always @(*) begin 
        out=0;
        for (i=0;i<255;i++)begin 
            out = (in[i]==1)? out+1:out;
        end 
    end 
    

endmodule
verilog system-verilog
1 Answers

All 3 code samples have syntax errors. If the HDLBits website did not report errors, try the EDA Playground website simulators.

In your "correct" answer you need to change

output [7:0] out );

to:

output reg [7:0] out );

When you make an assignment to a signal inside an always block (a procedural assignment), you need to declare the signal as a reg.

In your "1st wrong answer", change:

out = counter ;

to:

assign out = counter ;

Continuous assignments (outside of always blocks) require the assign keyword.

In your "2nd wrong answer", use reg for out.

toolic0
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