v1.1c: New sleep mode. Laser mode and other bug fixes.

- New $SLP sleep mode that will disable spindle, coolant, and stepper
enable pins. Allows users to disable their steppers without having to
alter their settings. A reset is required to exit and re-initializes in
alarm state.

- Laser mode wasn’t updating the spindle PWM correctly (effected
spindle speed overrides) and not checking for modal states either.
Fixed both issues.

- While in laser mode, parking motions are ignored, since the power off
delay with the retract motion would burn the material. It will just
turn off and not move. A restore immediately powers up and resumes. No
delays.

- Changing rpm max and min settings did not update the spindle PWM
calculations. Now fixed.

- Increased default planner buffer from 16 to 17 block. It seems to be
stable, but need to monitor this carefully.

- Removed software debounce routine for limit pins. Obsolete.

- Fixed a couple parking motion bugs. One related to restoring
incorrectly and the other the parking rate wasn’t compatible with the
planner structs.

- Fixed a bug caused by refactoring the critical alarms in a recent
push. Soft limits weren’t invoking a critical alarm.

- Updated the documentation with the new sleep feature and added some
more details to the change summary.

grbl/limits.c

@@ -88,53 +88,35 @@ uint8_t limits_get_state()
 // limit switch can cause a lot of problems, like false readings and multiple interrupt calls.
 // If a switch is triggered at all, something bad has happened and treat it as such, regardless
 // if a limit switch is being disengaged. It's impossible to reliably tell the state of a
-// bouncing pin without a debouncing method. A simple software debouncing feature may be enabled
-// through the config.h file, where an extra timer delays the limit pin read by several milli-
-// seconds to help with, not fix, bouncing switches.
+// bouncing pin because the Arduino microcontroller does not retain any state information when
+// detecting a pin change. If we poll the pins in the ISR, you can miss the correct reading if the 
+// switch is bouncing.
 // NOTE: Do not attach an e-stop to the limit pins, because this interrupt is disabled during
 // 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.
-#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)) {
+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
     }
   }
-#endif
+}
 
 
 // Homes the specified cycle axes, sets the machine position, and performs a pull-off motion after