Hard limits, homing direction, pull-off limits after homing, status reports in mm or inches, system alarm, and more.

- Thank you statement added for Alden Hart of Synthetos.

- Hard limits option added, which also works with homing by pulling off
the switches to help prevent unintended triggering. Hard limits use a
interrupt to sense a falling edge pin change and immediately go into
alarm mode, which stops everything and forces the user to issue a reset
(Ctrl-x) or reboot.

- Auto cycle start now a configuration option.

- Alarm mode: A new method to kill all Grbl processes in the event of
something catastrophic or potentially catastropic. Just works with hard
limits for now, but will be expanded to include g-code errors (most
likely) and other events.

- Updated status reports to be configurable in inches or mm mode. Much
more to do here, but this is the first step.

- New settings: auto cycle start, hard limit enable, homing direction
mask (which works the same as the stepper mask), homing pulloff
distance (or distance traveled from homed machine zero to prevent
accidental limit trip).

- Minor memory liberation and calculation speed ups.

limits.c

@@ -38,8 +38,31 @@ void limits_init()
 {
   LIMIT_DDR &= ~(LIMIT_MASK); // Set as input pins
   LIMIT_PORT |= (LIMIT_MASK); // Enable internal pull-up resistors. Normal high operation.
+
+  if bit_istrue(settings.flags,BITFLAG_HARD_LIMIT_ENABLE) {
+    MCUCR = (1<<ISC01) | (0<<ISC00); //1 0 triggers at a falling edge.
+    LIMIT_PCMSK |= LIMIT_MASK;   // Enable specific pins of the Pin Change Interrupt
+    PCICR |= (1 << LIMIT_INT);   // Enable Pin Change Interrupt
+  }
 }
 
+// This is the Limit Pin Change Interrupt, which handles the hard limit feature. This is
+// called when Grbl detects a falling edge on a limit pin.
+// 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.
+ISR(LIMIT_INT_vect) 
+{
+  // Kill all processes upon hard limit event.
+  st_go_idle(); // Immediately stop stepper motion
+  spindle_stop(); // Stop spindle
+  sys.auto_start = false; // Disable auto cycle start.
+  sys.execute |= EXEC_ALARM;
+  // TODO: When Grbl system status is installed, update here to indicate loss of position.
+}
+
+
 // Moves all specified axes in same specified direction (positive=true, negative=false)
 // and at the homing rate. Homing is a special motion case, where there is only an 
 // acceleration followed by abrupt asynchronous stops by each axes reaching their limit 
@@ -85,6 +108,7 @@ static void homing_cycle(bool x_axis, bool y_axis, bool z_axis, int8_t pos_dir,
       
   // Set default out_bits. 
   uint8_t out_bits0 = settings.invert_mask;
+  out_bits0 ^= (settings.homing_dir_mask & DIRECTION_MASK); // Apply homing direction settings
   if (!pos_dir) { out_bits0 ^= DIRECTION_MASK; }   // Invert bits, if negative dir.
   
   // Initialize stepping variables
@@ -160,11 +184,8 @@ static void homing_cycle(bool x_axis, bool y_axis, bool z_axis, int8_t pos_dir,
 
 
 void limits_go_home() 
-{
-  plan_synchronize();  // Empty all motions in buffer.
-  
-  // Enable steppers by resetting the stepper disable port
-  STEPPERS_DISABLE_PORT &= ~(1<<STEPPERS_DISABLE_BIT);
+{  
+  STEPPERS_DISABLE_PORT &= ~(1<<STEPPERS_DISABLE_BIT); // Enable steppers, but not the cycle.
   
   // Jog all axes toward home to engage their limit switches at faster homing seek rate.
   homing_cycle(false, false, true, true, false, settings.homing_seek_rate);  // First jog the z axis
@@ -186,9 +207,5 @@ void limits_go_home()
     }
   }
 
-  delay_ms(settings.stepper_idle_lock_time);
-  // Disable steppers by setting stepper disable
-  if (settings.stepper_idle_lock_time != 0xff) {
-    STEPPERS_DISABLE_PORT |= (1<<STEPPERS_DISABLE_BIT);
-  }
+  st_go_idle(); // Call main stepper shutdown routine.  
 }