The PS/2 mouse protocol sends messages that are three bytes long.
However, within a continuous byte stream, it's not obvious where messages
start and end. The only indication is that the first byte of each three
byte message always has in[3]=1 (but in[3] of the other two bytes may be
1 or 0 depending on data). We want a finite state machine that will
search for message boundaries when given an input byte stream. The
algorithm we'll use is to discard bytes until we see one with in[3]=1. We
then assume that this is byte 1 of a message, and signal the receipt of a
message once all 3 bytes have been received (done). The FSM should signal
done in the cycle immediately after the third byte of each message was
successfully received. Reset should be active high synchronous.

module TopModule (
  input clk,
  input [7:0] in,
  input reset,
  output done
);


Reference solution:

module RefModule (
  input clk,
  input [7:0] in,
  input reset,
  output done
);

  parameter BYTE1=0, BYTE2=1, BYTE3=2, DONE=3;
  reg [1:0] state;
  reg [1:0] next;

  wire in3 = in[3];

  always_comb begin
    case (state)
      BYTE1: next = in3 ? BYTE2 : BYTE1;
      BYTE2: next = BYTE3;
      BYTE3: next = DONE;
      DONE: next = in3 ? BYTE2 : BYTE1;
    endcase
  end

  always @(posedge clk) begin
    if (reset) state <= BYTE1;
      else state <= next;
  end

  assign done = (state==DONE);

endmodule


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


module stimulus_gen (
	input clk,
	output logic [7:0] in,
	output logic reset
);

	initial begin
		repeat(200) @(negedge clk) begin
			in <= $random;
			reset <= !($random & 31);
		end

		#1 $finish;
	end
	
endmodule

module tb();

	typedef struct packed {
		int errors;
		int errortime;
		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 [7:0] in;
	logic reset;
	logic done_ref;
	logic done_dut;

	initial begin 
		$dumpfile("wave.vcd");
		$dumpvars(1, stim1.clk, tb_mismatch ,clk,in,reset,done_ref,done_dut );
	end


	wire tb_match;		// Verification
	wire tb_mismatch = ~tb_match;
	
	stimulus_gen stim1 (
		.clk,
		.* ,
		.in,
		.reset );
	RefModule good1 (
		.clk,
		.in,
		.reset,
		.done(done_ref) );
		
	TopModule top_module1 (
		.clk,
		.in,
		.reset,
		.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_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 = ( { done_ref } === ( { done_ref } ^ { done_dut } ^ { 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 (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