I am a beginner in verilog and came across this question-

Given the finite state machine circuit as shown, assume that the D flip-flops are initially reset to zero before the machine begins.

Build this circuit. enter image description here

My Code is -

module top_module (
input clk,
input x,
output z
reg q,q1,q2;
always @(posedge clk)

        q<= q^x;
        q1<= ~q1 && x;
        q2<= ~q2 || x;
        z=~(q | q1 | q2);

Suggest me where i am going wrong!

  • 1
    \$\begingroup\$ Welcome! You haven't said what's failed, but try moving z=... outside the always block and change it to assign z = ~(q | q1 | q2); \$\endgroup\$
    – awjlogan
    Commented Jan 24, 2020 at 9:54
  • \$\begingroup\$ It previously showed 43 mismatches but now its working fine. Could you please tell me the reason behind this? \$\endgroup\$ Commented Jan 24, 2020 at 9:57
  • \$\begingroup\$ Briefly, you were mixing combinatorial (continuous) assignment (z=...) and sequential logic (q<=...) in an edge sensitive always block. Continous assignments are either done using assign, or in an always @* (Verilog) or always_comb (SystemVerilog) construct. \$\endgroup\$
    – awjlogan
    Commented Jan 24, 2020 at 12:04

1 Answer 1


From the diagram, z is driven by a combinational logic. In the code, you are trying to drive z using sequential logic inside clockedge. You have to either use z as a wire and drive it using assign statement. Or use z as a reg and drive it inside always@* block.

General coding guideline is to not mix blocking and non-blocking assignments in the same always block.


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