Build a decade counter that counts from 0 through 9, inclusive, with a
period of 10. The reset input is active high synchronous, and should
reset the counter to 0. We want to be able to pause the counter rather
than always incrementing every clock cycle, so the "slowena" input if
high indicates when the counter should increment.

module TopModule (
  input clk,
  input slowena,
  input reset,
  output reg [3:0] q
);


Reference solution:

module RefModule (
  input clk,
  input slowena,
  input reset,
  output reg [3:0] q
);

  always @(posedge clk)
    if (reset)
      q <= 0;
    else if (slowena) begin
      if (q == 9)
        q <= 0;
      else
        q <= q+1;
    end

endmodule


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


module stimulus_gen (
	input clk,
	output reg slowena,
	output reg reset,
	output reg[511:0] wavedrom_title,
	output reg wavedrom_enable,
	input tb_match	
);

	task reset_test(input async=0);
		bit arfail, srfail, datafail;
	
		@(posedge clk);
		@(posedge clk) reset <= 0;
		repeat(3) @(posedge clk);
	
		@(negedge clk) begin datafail = !tb_match ; reset <= 1; end
		@(posedge clk) arfail = !tb_match;
		@(posedge clk) begin
			srfail = !tb_match;
			reset <= 0;
		end
		if (srfail)
			$display("Hint: Your reset doesn't seem to be working.");
		else if (arfail && (async || !datafail))
			$display("Hint: Your reset should be %0s, but doesn't appear to be.", async ? "asynchronous" : "synchronous");
		// Don't warn about synchronous reset if the half-cycle before is already wrong. It's more likely
		// a functionality error than the reset being implemented asynchronously.
	
	endtask


// Add two ports to module stimulus_gen:
//    output [511:0] wavedrom_title
//    output reg wavedrom_enable

	task wavedrom_start(input[511:0] title = "");
	endtask
	
	task wavedrom_stop;
		#1;
	endtask	



	
	reg hint1;
	initial begin
		reset <= 1;
		slowena <= 1;
		wavedrom_start("Synchronous reset and counting.");
		reset_test();
		repeat(12) @(posedge clk);
		wavedrom_stop();
		@(posedge clk);

		//wavedrom_start("Testing.");
		reset <= 1;
		@(posedge clk);
		reset <= 0;
		repeat(9) @(posedge clk);
		slowena <= 0;
		@(negedge clk) hint1 = tb_match;
		repeat(3) @(posedge clk);
		if (hint1 && !tb_match) begin
			$display ("Hint: What is supposed to happen when the counter is 9 and not enabled?");
		end
		//wavedrom_stop();
		slowena <= 1;
		reset <= 1;
		@(posedge clk);
		reset <= 0;


		wavedrom_start("Enable/disable");
		repeat(15) @(posedge clk) slowena <= !($random & 1);
		wavedrom_stop();
		@(posedge clk);

		repeat(400) @(posedge clk, negedge clk) begin
			slowena <= !($random&3);
			reset <= !($random & 31);
		end
		#1 $finish;
	end
	
endmodule

module tb();

	typedef struct packed {
		int errors;
		int errortime;
		int errors_q;
		int errortime_q;

		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 slowena;
	logic reset;
	logic [3:0] q_ref;
	logic [3:0] q_dut;

	initial begin 
		$dumpfile("wave.vcd");
		$dumpvars(1, stim1.clk, tb_mismatch ,clk,slowena,reset,q_ref,q_dut );
	end


	wire tb_match;		// Verification
	wire tb_mismatch = ~tb_match;
	
	stimulus_gen stim1 (
		.clk,
		.* ,
		.slowena,
		.reset );
	RefModule good1 (
		.clk,
		.slowena,
		.reset,
		.q(q_ref) );
		
	TopModule top_module1 (
		.clk,
		.slowena,
		.reset,
		.q(q_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_q) $display("Hint: Output '%s' has %0d mismatches. First mismatch occurred at time %0d.", "q", stats1.errors_q, stats1.errortime_q);
		else $display("Hint: Output '%s' has no mismatches.", "q");

		$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 = ( { q_ref } === ( { q_ref } ^ { q_dut } ^ { q_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 (q_ref !== ( q_ref ^ q_dut ^ q_ref ))
		begin if (stats1.errors_q == 0) stats1.errortime_q = $time;
			stats1.errors_q = stats1.errors_q+1'b1; end

	end

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

endmodule