We want to create a timer that:

  (1) is started when a particular pattern (1101) is detected,
  (2) shifts in 4 more bits to determine the duration to delay,
  (3) waits for the counters to finish counting, and
  (4) notifies the user and waits for the user to acknowledge the timer.

In this problem, implement just the finite-state machine that controls
the timer. The data path (counters and some comparators) are not included
here.

The serial data is available on the data input pin. When the pattern 1101
is received, the state machine must then assert output shift_ena for
exactly 4 clock cycles. After that, the state machine asserts its
counting output to indicate it is waiting for the counters, and waits
until input done_counting is high.At that point, the state machine must
assert done to notify the user the timer has timed out, and waits until
input ack is 1 before being reset to look for the next occurrence of the
start sequence (1101). The state machine should have a active high
synchronous reset, setting the state to where it begins searching for the
input sequence 1101.

module TopModule (
  input clk,
  input reset,
  input data,
  output reg shift_ena,
  output reg counting,
  input done_counting,
  output reg done,
  input ack
);


Reference solution:

module RefModule (
  input clk,
  input reset,
  input data,
  output reg shift_ena,
  output reg counting,
  input done_counting,
  output reg done,
  input ack
);

  typedef enum logic[3:0] {
    S, S1, S11, S110, B0, B1, B2, B3, Count, Wait
  } States;

  States state, next;

  always_comb begin
    case (state)
      S: next = States'(data ? S1: S);
      S1: next = States'(data ? S11: S);
      S11: next = States'(data ? S11 : S110);
      S110: next = States'(data ? B0 : S);
      B0: next = B1;
      B1: next = B2;
      B2: next = B3;
      B3: next = Count;
      Count: next = States'(done_counting ? Wait : Count);
      Wait: next = States'(ack ? S : Wait);
      default: next = States'(4'bx);
    endcase
  end

  always @(posedge clk) begin
    if (reset) state <= S;
    else state <= next;
  end

  always_comb begin
    shift_ena = 0; counting = 0; done = 0;
    if (state == B0 || state == B1 || state == B2 || state == B3)
      shift_ena = 1;
    if (state == Count)
      counting = 1;
    if (state == Wait)
      done = 1;

    if (|state === 1'bx) begin
      {shift_ena, counting, done} = 'x;
    end

  end

endmodule


Testbench:
`timescale 1 ps/1 ps
`define OK 12
`define INCORRECT 13


module stimulus_gen (
	input clk,
	output reg reset,
	output reg data, done_counting, ack,
	input tb_match
);
	bit failed = 0;
	
	always @(posedge clk, negedge clk)
		if (!tb_match) 
			failed <= 1;
	
	initial begin

		@(posedge clk);
		failed <= 0;
		reset <= 1;
		data <= 0;
		done_counting <= 1'bx;
		ack <= 1'bx;
		@(posedge clk) 
			data <= 1;
			reset <= 0;
		@(posedge clk) data <= 0;
		@(posedge clk) data <= 0;
		@(posedge clk) data <= 1;
		@(posedge clk) data <= 1;
		@(posedge clk) data <= 0;
		@(posedge clk) data <= 1;
		@(posedge clk);
			data <= 1'bx;
		repeat(4) @(posedge clk);
			done_counting <= 1'b0;
		repeat(4) @(posedge clk);
			done_counting <= 1'b1;
		@(posedge clk);
			done_counting <= 1'bx;
			ack <= 1'b0;
		repeat(3) @(posedge clk);
			ack <= 1'b1;
		@(posedge clk);
			ack <= 1'b0;
			data <= 1'b1;
		@(posedge clk);
			ack <= 1'bx;
			data <= 1'b1;
		@(posedge clk);
			data <= 1'b0;
		@(posedge clk);
			data <= 1'b1;
		@(posedge clk);
			data <= 1'bx;
		repeat(4) @(posedge clk);
			done_counting <= 1'b0;
		repeat(4) @(posedge clk);
			done_counting <= 1'b1;
		@(posedge clk);

		if (failed)
			$display("Hint: Your FSM didn't pass the sample timing diagram posted with the problem statement. Perhaps try debugging that?");
		
		
	
		repeat(5000) @(posedge clk, negedge clk) begin
			reset <= !($random & 255);
			data <= $random;
			done_counting <= !($random & 31);
			ack <= !($random & 31);
		end

		#1 $finish;
	end
	
endmodule

module tb();

	typedef struct packed {
		int errors;
		int errortime;
		int errors_shift_ena;
		int errortime_shift_ena;
		int errors_counting;
		int errortime_counting;
		int errors_done;
		int errortime_done;

		int clocks;
	} stats;
	
	stats stats1;
	
	
	wire[511:0] wavedrom_title;
	wire wavedrom_enable;
	int wavedrom_hide_after_time;
	
	reg clk=0;
	initial forever
		#5 clk = ~clk;

	logic reset;
	logic data;
	logic done_counting;
	logic ack;
	logic shift_ena_ref;
	logic shift_ena_dut;
	logic counting_ref;
	logic counting_dut;
	logic done_ref;
	logic done_dut;

	initial begin 
		$dumpfile("wave.vcd");
		$dumpvars(1, stim1.clk, tb_mismatch ,clk,reset,data,done_counting,ack,shift_ena_ref,shift_ena_dut,counting_ref,counting_dut,done_ref,done_dut );
	end


	wire tb_match;		// Verification
	wire tb_mismatch = ~tb_match;
	
	stimulus_gen stim1 (
		.clk,
		.* ,
		.reset,
		.data,
		.done_counting,
		.ack );
	RefModule good1 (
		.clk,
		.reset,
		.data,
		.done_counting,
		.ack,
		.shift_ena(shift_ena_ref),
		.counting(counting_ref),
		.done(done_ref) );
		
	TopModule top_module1 (
		.clk,
		.reset,
		.data,
		.done_counting,
		.ack,
		.shift_ena(shift_ena_dut),
		.counting(counting_dut),
		.done(done_dut) );

	
	bit strobe = 0;
	task wait_for_end_of_timestep;
		repeat(5) begin
			strobe <= !strobe;  // Try to delay until the very end of the time step.
			@(strobe);
		end
	endtask	

	
	final begin
		if (stats1.errors_shift_ena) $display("Hint: Output '%s' has %0d mismatches. First mismatch occurred at time %0d.", "shift_ena", stats1.errors_shift_ena, stats1.errortime_shift_ena);
		else $display("Hint: Output '%s' has no mismatches.", "shift_ena");
		if (stats1.errors_counting) $display("Hint: Output '%s' has %0d mismatches. First mismatch occurred at time %0d.", "counting", stats1.errors_counting, stats1.errortime_counting);
		else $display("Hint: Output '%s' has no mismatches.", "counting");
		if (stats1.errors_done) $display("Hint: Output '%s' has %0d mismatches. First mismatch occurred at time %0d.", "done", stats1.errors_done, stats1.errortime_done);
		else $display("Hint: Output '%s' has no mismatches.", "done");

		$display("Hint: Total mismatched samples is %1d out of %1d samples\n", stats1.errors, stats1.clocks);
		$display("Simulation finished at %0d ps", $time);
		$display("Mismatches: %1d in %1d samples", stats1.errors, stats1.clocks);
	end
	
	// Verification: XORs on the right makes any X in good_vector match anything, but X in dut_vector will only match X.
	assign tb_match = ( { shift_ena_ref, counting_ref, done_ref } === ( { shift_ena_ref, counting_ref, done_ref } ^ { shift_ena_dut, counting_dut, done_dut } ^ { shift_ena_ref, counting_ref, done_ref } ) );
	// Use explicit sensitivity list here. @(*) causes NetProc::nex_input() to be called when trying to compute
	// the sensitivity list of the @(strobe) process, which isn't implemented.
	always @(posedge clk, negedge clk) begin

		stats1.clocks++;
		if (!tb_match) begin
			if (stats1.errors == 0) stats1.errortime = $time;
			stats1.errors++;
		end
		if (shift_ena_ref !== ( shift_ena_ref ^ shift_ena_dut ^ shift_ena_ref ))
		begin if (stats1.errors_shift_ena == 0) stats1.errortime_shift_ena = $time;
			stats1.errors_shift_ena = stats1.errors_shift_ena+1'b1; end
		if (counting_ref !== ( counting_ref ^ counting_dut ^ counting_ref ))
		begin if (stats1.errors_counting == 0) stats1.errortime_counting = $time;
			stats1.errors_counting = stats1.errors_counting+1'b1; end
		if (done_ref !== ( done_ref ^ done_dut ^ done_ref ))
		begin if (stats1.errors_done == 0) stats1.errortime_done = $time;
			stats1.errors_done = stats1.errors_done+1'b1; end

	end

   // add timeout after 100K cycles
   initial begin
     #1000000
     $display("TIMEOUT");
     $finish();
   end

endmodule