Problem: Prob024_hadd

Specification:

I would like you to implement a module named TopModule with the following
interface. All input and output ports are one bit unless otherwise
specified.

 - input  a
 - input  b
 - output sum
 - output cout

The module should implement a half adder. A half adder adds two bits
(with no carry-in) and produces a sum and carry-out.



Reference implementation:

module RefModule (
  input a,
  input b,
  output sum,
  output cout
);

  assign {cout, sum} = a+b;

endmodule



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


module stimulus_gen (
	input clk,
	output logic a,b
);

	always @(posedge clk, negedge clk)
		{a,b} <= $random;
	
	initial begin
		repeat(100) @(negedge clk);
		$finish;
	end
	
endmodule
module tb();

	typedef struct packed {
		int errors;
		int errortime;
		int errors_sum;
		int errortime_sum;
		int errors_cout;
		int errortime_cout;

		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 a;
	logic b;
	logic sum_ref;
	logic sum_dut;
	logic cout_ref;
	logic cout_dut;

	initial begin 
		$dumpfile("wave.vcd");
		$dumpvars(1, stim1.clk, tb_mismatch ,a,b,sum_ref,sum_dut,cout_ref,cout_dut );
	end


	wire tb_match;		// Verification
	wire tb_mismatch = ~tb_match;
	
	stimulus_gen stim1 (
		.clk,
		.* ,
		.a,
		.b );
	RefModule good1 (
		.a,
		.b,
		.sum(sum_ref),
		.cout(cout_ref) );
		
	TopModule top_module1 (
		.a,
		.b,
		.sum(sum_dut),
		.cout(cout_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_sum) $display("Hint: Output '%s' has %0d mismatches. First mismatch occurred at time %0d.", "sum", stats1.errors_sum, stats1.errortime_sum);
		else $display("Hint: Output '%s' has no mismatches.", "sum");
		if (stats1.errors_cout) $display("Hint: Output '%s' has %0d mismatches. First mismatch occurred at time %0d.", "cout", stats1.errors_cout, stats1.errortime_cout);
		else $display("Hint: Output '%s' has no mismatches.", "cout");

		$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 = ( { sum_ref, cout_ref } === ( { sum_ref, cout_ref } ^ { sum_dut, cout_dut } ^ { sum_ref, cout_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 (sum_ref !== ( sum_ref ^ sum_dut ^ sum_ref ))
		begin if (stats1.errors_sum == 0) stats1.errortime_sum = $time;
			stats1.errors_sum = stats1.errors_sum+1'b1; end
		if (cout_ref !== ( cout_ref ^ cout_dut ^ cout_ref ))
		begin if (stats1.errors_cout == 0) stats1.errortime_cout = $time;
			stats1.errors_cout = stats1.errors_cout+1'b1; end

	end

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

endmodule