tb: make tb_timer.v selftesting

- Compare against an expected result and count errors
- Exit with the right error code
- Lower write_word() to 1 clk cycle instead of two. It only requires one
  clock cycle to write, otherwise if it is two one have to compensate for it
  in the tests since we are counting cycles.

hw/application_fpga/core/timer/tb/tb_timer.v

@@ -197,7 +197,7 @@ module tb_timer ();
       tb_write_data = word;
       tb_cs = 1;
       tb_we = 1;
-      #(2 * CLK_PERIOD);
+      #(CLK_PERIOD);
       tb_cs = 0;
       tb_we = 0;
     end
@@ -250,37 +250,64 @@ module tb_timer ();
     begin : test1
       reg [31 : 0] time_start;
       reg [31 : 0] time_stop;
+      reg [31 : 0] time_expected;
+      reg [31 : 0] time_counted;
 
       tc_ctr = tc_ctr + 1;
       tb_monitor = 0;
 
       $display("");
       $display("--- test1: started.");
+      time_expected = 32'h6 * 32'h9 + 1;
+      // One extra clock cycle for the API
 
-      write_word(ADDR_PRESCALER, 8'h02);
+      write_word(ADDR_PRESCALER, 32'h6);
-      write_word(ADDR_TIMER, 8'h10);
+      write_word(ADDR_TIMER, 32'h9);
+      #(CLK_PERIOD);
 
+      write_word(ADDR_CTRL, 32'h1);
       time_start = cycle_ctr;
-      write_word(ADDR_CTRL, 8'h01);
 
-      #(2 * CLK_PERIOD);
+      #(CLK_PERIOD);
       read_word(ADDR_STATUS);
       while (read_data) begin
         read_word(ADDR_STATUS);
       end
 
       time_stop = cycle_ctr;
-      write_word(CTRL_START_BIT, 8'h02);
+      time_counted = time_stop - time_start;
+
+      if (time_counted == time_expected) begin
+        $display("--- test1: Correct number of cycles counted: %0d", time_counted);
+      end
+      else begin
+        $display("--- test1: Error, expected %0d cycles, counted cycles: %0d", time_expected,
+                 time_counted);
+        error_ctr = error_ctr + 1;
+      end
 
-      $display("--- test1: Cycles between start and stop: %d", (time_stop - time_start));
-      #(CLK_PERIOD);
-      tb_monitor = 0;
 
       $display("--- test1: completed.");
       $display("");
     end
   endtask  // tes1
 
+  //----------------------------------------------------------------
+  // exit_with_error_code()
+  //
+  // Exit with the right error code
+  //----------------------------------------------------------------
+  task exit_with_error_code;
+    begin
+      if (error_ctr == 0) begin
+        $finish(0);
+      end
+      else begin
+        $fatal(1);
+      end
+    end
+  endtask  // exit_with_error_code
+
 
   //----------------------------------------------------------------
   // timer_test
@@ -300,7 +327,7 @@ module tb_timer ();
     $display("   -= Testbench for timer completed =-");
     $display("     ===============================");
     $display("");
-    $finish;
+    exit_with_error_code();
   end  // timer_test
 endmodule  // tb_timer