Fixed homing fail alarm handling. Re-integrated software debouncing.

- [bug] Fixed a homing fail issue, where the alarm was not being set
right, not cleared correctly. It would report the wrong code and enter
an infinite alarm loop. This was due to how alarm codes were altered a
while back. Now updated and fixed to show the right codes.

- [feature] Re-installed optional software debouncing for hard limit
switches. By request.

grbl/limits.c

@@ -95,29 +95,46 @@ uint8_t limits_get_state()
 // homing cycles and will not respond correctly. Upon user request or need, there may be a
 // special pinout for an e-stop, but it is generally recommended to just directly connect
 // your e-stop switch to the Arduino reset pin, since it is the most correct way to do this.
-ISR(LIMIT_INT_vect) // DEFAULT: Limit pin change interrupt process.
-{
-  // Ignore limit switches if already in an alarm state or in-process of executing an alarm.
-  // When in the alarm state, Grbl should have been reset or will force a reset, so any pending
-  // moves in the planner and serial buffers are all cleared and newly sent blocks will be
-  // locked out until a homing cycle or a kill lock command. Allows the user to disable the hard
-  // limit setting if their limits are constantly triggering after a reset and move their axes.
-  if (sys.state != STATE_ALARM) {
-    if (!(sys_rt_exec_alarm)) {
-      #ifdef HARD_LIMIT_FORCE_STATE_CHECK
-        // Check limit pin state.
+#ifndef ENABLE_SOFTWARE_DEBOUNCE
+  ISR(LIMIT_INT_vect) // DEFAULT: Limit pin change interrupt process.
+  {
+    // Ignore limit switches if already in an alarm state or in-process of executing an alarm.
+    // When in the alarm state, Grbl should have been reset or will force a reset, so any pending
+    // moves in the planner and serial buffers are all cleared and newly sent blocks will be
+    // locked out until a homing cycle or a kill lock command. Allows the user to disable the hard
+    // limit setting if their limits are constantly triggering after a reset and move their axes.
+    if (sys.state != STATE_ALARM) {
+      if (!(sys_rt_exec_alarm)) {
+        #ifdef HARD_LIMIT_FORCE_STATE_CHECK
+          // Check limit pin state.
+          if (limits_get_state()) {
+            mc_reset(); // Initiate system kill.
+            system_set_exec_alarm(EXEC_ALARM_HARD_LIMIT); // Indicate hard limit critical event
+          }
+        #else
+          mc_reset(); // Initiate system kill.
+          system_set_exec_alarm(EXEC_ALARM_HARD_LIMIT); // Indicate hard limit critical event
+        #endif
+      }
+    }
+  }
+#else // OPTIONAL: Software debounce limit pin routine.
+  // Upon limit pin change, enable watchdog timer to create a short delay. 
+  ISR(LIMIT_INT_vect) { if (!(WDTCSR & (1<<WDIE))) { WDTCSR |= (1<<WDIE); } }
+  ISR(WDT_vect) // Watchdog timer ISR
+  {
+    WDTCSR &= ~(1<<WDIE); // Disable watchdog timer. 
+    if (sys.state != STATE_ALARM) {  // Ignore if already in alarm state. 
+      if (!(sys_rt_exec_alarm)) {
+        // Check limit pin state. 
         if (limits_get_state()) {
           mc_reset(); // Initiate system kill.
           system_set_exec_alarm(EXEC_ALARM_HARD_LIMIT); // Indicate hard limit critical event
         }
-      #else
-        mc_reset(); // Initiate system kill.
-        system_set_exec_alarm(EXEC_ALARM_HARD_LIMIT); // Indicate hard limit critical event
-      #endif
+      }  
     }
   }
-}
-
+#endif
 
 // Homes the specified cycle axes, sets the machine position, and performs a pull-off motion after
 // completing. Homing is a special motion case, which involves rapid uncontrolled stops to locate