A D flip-flop is a sequential element that follows the input pin d at the given edge of a clock.
Design #1: With async active-low reset
module dff ( input d,
input rstn,
input clk,
output reg q);
always @ (posedge clk or negedge rstn)
if (!rstn)
q <= 0;
else
q <= d;
endmodule
Hardware Schematic
Testbench
module tb_dff;
reg clk;
reg d;
reg rstn;
reg [2:0] delay;
dff dff0 ( .d(d),
.rsnt (rstn),
.clk (clk),
.q (q));
// Generate clock
always #10 clk = ~clk;
// Testcase
initial begin
clk <= 0;
d <= 0;
rstn <= 0;
#15 d <= 1;
#10 rstn <= 1;
for (int i = 0; i < 5; i=i+1) begin
delay = $random;
#(delay) d <= i;
end
end
endmodule
Design #1: With sync active-low reset
module dff ( input d,
input rstn,
input clk,
output reg q);
always @ (posedge clk)
if (!rstn)
q <= 0;
else
q <= d;
endmodule
Hardware Schematic
Testbench
module tb_dff;
reg clk;
reg d;
reg rstn;
reg [2:0] delay;
dff dff0 ( .d(d),
.rsnt (rstn),
.clk (clk),
.q (q));
// Generate clock
always #10 clk = ~clk;
// Testcase
initial begin
clk <= 0;
d <= 0;
rstn <= 0;
#15 d <= 1;
#10 rstn <= 1;
for (int i = 0; i < 5; i=i+1) begin
delay = $random;
#(delay) d <= i;
end
end
endmodule