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Trying to simulate JK-FF with gate level code

I've been trying to simulate a JK-FF with gate level code, but it's not working. Any help is appreciated.

Circuit code:

module circuit1_3_c(j,k,r,cp,q,q1);
input j,k,r,cp;
output q,q1;
wire t1,t2,t3,t4;

nand(t1,t2,q);
nand(t2,t1,j,cp,t3);
nand(t3,cp,k,t4);
nand(t4,t3,t1);
nand(q,q1,t2);
nand(q1,q,t3,r);

endmodule

Testbench code:

module circuit1_3_ctest;
parameter STEP=10;
parameter HALF_STEP=5;

reg j,k,r,cp;
wire q,q1;
circuit1_3_c circ(j,k,r,cp,q,q1);
initial begin
$dumpfile("circuit1_3_c.vcd");
$dumpvars(0,circuit1_3_ctest);
$monitor("\%t: J=%b, K=%b, R=%b, Cp=%b, Q=%b, Qbar=%b", $time, j,k,r,cp,q,q1);
r<=1'b1;
cp<=1'b0;
j<=1'b0; k<=1'b0; r<=1'b1;
#STEP; 
j<=1'b0; k<=1'b1; r<=1'b1;
#STEP;
j<=1'b1; k<=1'b0; r<=1'b1;
#STEP;
j<=1'b1; k<=1'b1; r<=1'b1;
#STEP;
j<=1'b0; k<=1'b0; r<=1'b1;
#STEP; 
j<=1'b0; k<=1'b1; r<=1'b1;
#STEP;
j<=1'b1; k<=1'b0; r<=1'b1;
#STEP;
j<=1'b1; k<=1'b1; r<=1'b1;
#HALF_STEP
$finish;
end
always #HALF_STEP cp=~cp;     
endmodule

Output. As you can see, q and q1 are always unknown (x):

[Output. As you can see, q and q1 are always unknown (x).1

Solution

  • You need to reset your logic properly. One way is to drive r as 0 at time 0. Change:

    r<=1'b1;
cp<=1'b0;
j<=1'b0; k<=1'b0; r<=1'b1;

    to:

    cp<=1'b0;
j<=1'b0; k<=1'b0; r<=1'b0;

    Output:

                   0: J=0, K=0, R=0, Cp=0, Q=0, Qbar=1
               5: J=0, K=0, R=0, Cp=1, Q=0, Qbar=1
              10: J=0, K=1, R=1, Cp=0, Q=0, Qbar=1
              15: J=0, K=1, R=1, Cp=1, Q=0, Qbar=1
              20: J=1, K=0, R=1, Cp=0, Q=0, Qbar=1
              25: J=1, K=0, R=1, Cp=1, Q=1, Qbar=0
              30: J=1, K=1, R=1, Cp=0, Q=1, Qbar=0
              35: J=1, K=1, R=1, Cp=1, Q=0, Qbar=1
              40: J=0, K=0, R=1, Cp=0, Q=0, Qbar=1
              45: J=0, K=0, R=1, Cp=1, Q=0, Qbar=1
              50: J=0, K=1, R=1, Cp=0, Q=0, Qbar=1
              55: J=0, K=1, R=1, Cp=1, Q=0, Qbar=1
              60: J=1, K=0, R=1, Cp=0, Q=0, Qbar=1
              65: J=1, K=0, R=1, Cp=1, Q=1, Qbar=0
              70: J=1, K=1, R=1, Cp=0, Q=1, Qbar=0