CODEWITHSUNDEEP
verilogcounterfpgahdl
Masood Salik
Masood Salik

Reputation: 139

Implementing a 4 bit counter using D flipflop.in Verilog

I am implementing a 4 bit counter using a D flip flop. For that, I have first written the code of D flip-flop then converted it to T flip-flop and then used it to make a counter.

The problem I am facing is that only first instance of T_flipflop "T0" is working while other bits are on unknown state.

The output of the code!!

#                    0 T = 1 , Clock=0 , q=  x , q= xxxx , Reset= 1 
#                   10 T = 1 , Clock=1 , q=  X , q= xxx0 , Reset= 1 
#                   15 T = 1 , Clock=1 , q=  X , q= xxx0 , Reset= 0 
#                   20 T = 1 , Clock=0 , q=  X , q= xxx0 , Reset= 0 
#                   30 T = 1 , Clock=1 , q=  X , q= xxx1 , Reset= 0 
#                   40 T = 1 , Clock=0 , q=  X , q= xxx1 , Reset= 0 
#                   50 T = 1 , Clock=1 , q=  X , q= xxx0 , Reset= 0 
#                   60 T = 1 , Clock=0 , q=  X , q= xxx0 , Reset= 0 
#                   70 T = 1 , Clock=1 , q=  X , q= xxx1 , Reset= 0 
#                   80 T = 1 , Clock=0 , q=  X , q= xxx1 , Reset= 0 
#                   90 T = 1 , Clock=1 , q=  X , q= xxx0 , Reset= 0 
#                  100 T = 1 , Clock=0 , q=  X , q= xxx0 , Reset= 0 
#                  110 T = 1 , Clock=1 , q=  X , q= xxx1 , Reset= 0 
#                  120 T = 1 , Clock=0 , q=  X , q= xxx1 , Reset= 0 
#                  130 T = 1 , Clock=1 , q=  X , q= xxx0 , Reset= 0 
#                  140 T = 1 , Clock=0 , q=  X , q= xxx0 , Reset= 0 
#                  150 T = 1 , Clock=1 , q=  X , q= xxx1 , Reset= 0 
#                  160 T = 1 , Clock=0 , q=  X , q= xxx1 , Reset= 0 
#                  170 T = 1 , Clock=1 , q=  X , q= xxx0 , Reset= 0 
#                  180 T = 1 , Clock=0 , q=  X , q= xxx0 , Reset= 0 
#                  190 T = 1 , Clock=1 , q=  X , q= xxx1 , Reset= 0 
#                  200 T = 1 , Clock=0 , q=  X , q= xxx1 , Reset= 0

I have figured out a problem that my T flip flop is not working as expected. If I remove the input T from the T_flipflop module and assign the Q_bar to D, it work properly. How can i correct the T flip-flop without removing the input T_flipflop module.

module D_flipflop (Q, Q_bar ,D, clk,reset);
output reg Q;
output  Q_bar;
input clk, reset ,D;

assign Q_bar=~Q;

always @(posedge clk) 
begin
if(reset) //Active high reset
    Q<=1'b0;
else 
    Q<=D;
end
endmodule   



module T_flipflop (output Q, Q_bar , input T, clk,reset);
wire w;

assign w=T^Q; 


D_flipflop D1(.Q(Q), .Q_bar(Q_bar) ,.D(w), .clk(clk),.reset(reset));

endmodule



module ripple_carry_counter(Q,T,Clk,reset);
output [3:0]Q ;
input Clk,reset,T;

T_flipflop  T0(.Q(Q[0]), .Q_bar() ,.T(T), .clk(Clk),.reset(reset));
T_flipflop  T1(.Q(Q[1]), .Q_bar() ,.T(T), .clk(Q[0]),.reset(reset));
T_flipflop  T2(.Q(Q[2]), .Q_bar() ,.T(T), .clk(Q[1]),.reset(reset));
T_flipflop  T3(.Q(Q[3]), .Q_bar() ,.T(T), .clk(Q[2]),.reset(reset));

endmodule

module exp_masood_bench_ripple_carry_counter;
reg clk,reset,T;
wire [3:0]q;
ripple_carry_counter MS(q,T,clk,reset);

initial
$monitor($time , " T = %b , Clock=%b , q= %d , q= %b , Reset= %b ",T,clk,q,q,reset);

initial
begin
clk=1'b0;
T=1'b1;
reset=1'b1;
#15 reset=1'b0;
#400 $finish;
end
always #10 clk=~clk;
endmodule

Upvotes: 1

Views: 13029

Answers (1)

Laleh
Laleh

Reputation: 475

That's because your reset is level sensitive and always high in simulation. So output of first TFF (clock of next TFF) is always zero. And because reset is synchronous next TFF wont reset and its output which is X, won't change.

Make reset asynchronous so that all FFs will reset at the same time.

Upvotes: 1

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