How does case statement and assignment of values work in system-verilog/verilog?

Question

I have a design module (a partially implemented seven segment display) with a case statement as shown below. However, it looks as if, if a case statement is not catered for a bcd value the previously assigned segment value is returned as the segment value for the bcd value which is not catered for in the switch statement.

Why is it behaving that way? Assuming I don't want to use a default statement.

I printed out the values of the bcd, segment and the expectedOutput and I observed what I just wrote above.

module seven_segment_display(output logic[6:0] segment, input logic[3:0] bcd);
    always@(*)
        begin
            case (bcd)
                4'b0011 : begin segment = 7'b1011011; end
                4'b1000 : begin segment = 7'b1111011; end
                4'b1010 : begin segment = 7'b0000000; end
                4'b0000 : begin segment = 7'b1111110; end
            endcase
        end
endmodule

bcd  segment expectedOutput
0000 1111110 1111110
0001 1111110 0110000
0010 1111110 1101101
0011 1011011 1111001
0100 1011011 0110011
0101 1011011 1011011
0110 1011011 1011111
0111 1011011 1110000
1000 1111011 1111111
1001 1111011 1111011
1010 0000000 0000000
1011 0000000 0000000
1100 0000000 0000000
1101 0000000 0000000
1110 0000000 0000000
1111 0000000 0000000

Matthew · Accepted Answer · 2019-05-21 20:31:42Z

1

segment is a variable. Like in any other (software) language, variables remember their value until you overwrite their value with some other value.

Your first input (bcd) is 4'b0000. There is a branch of the case statement that matches that value and so the value of 7'b1111110 is assigned to the variable segment. Then you change the value of bcd to 4'b0001. There is no branch that matches that value, so no new value is assigned to the variable segment. So, it retains it's old value.

answered May 21, 2019 at 20:31

Matthew

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3 Comments

Matthew Over a year ago

I can't understand what else you'd expect to happen. The expected value corresponding to an input of 4'b0001 is not assigned anywhere (because there is no branch of the case statement that corresponds to 4'b0001). I can't understand, therefore, why you would expect that value to be assigned to segment.

user2987773 Over a year ago

I get it now, I mistakenly assumed an unknown value should be assigned.

Greg Over a year ago

@user2987773 add a default condition to your case statement.

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