Introduce endstop interrupts

If ENDSTOP_INTERRUPTS_FEATURE is enabled this tries to set up interrupt routines
for all used endstop pins. If this worked without errors, `endstops.update()` is called
only if one of the endstops changed its state.

The new interrupt routines do not really check the endstops and react upon them. All what they
do, is to set a flag if it makes sense to call the endstop test we are used to.

This can be used on:
* ARM (DUE) based boards - all pins can raise interrupts,
* RAMPS - all 6 endstop pins plus some other on EXT-2 can raise interrupts,
* RAMPS based boards - as long the designers did not change the pins for the endstops or at least left enough,
* all boards, if there are enough pins that can raise interrupts, and you are willing/able to swap with pins dedicated to other purpose.

Marlin/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/Marlin_main.cpp

   #include "twibus.h"
 #endif
 
+#if ENABLED(ENDSTOP_INTERRUPTS_FEATURE)
+  #include "endstop_interrupts.h"
+#endif
 /**
  * Look here for descriptions of G-codes:
  *  - http://linuxcnc.org/handbook/gcode/g-code.html
     i2c.onReceive(i2c_on_receive);
     i2c.onRequest(i2c_on_request);
   #endif
+
+  #if ENABLED(ENDSTOP_INTERRUPTS_FEATURE)
+    setup_enstop_interrupts();
+  #endif
 }
 
 /**

Marlin/endstop_interrupts.h

+/**
+ * Marlin 3D Printer Firmware
+ * Copyright (C) 2016 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
+ *
+ * Based on Sprinter and grbl.
+ * Copyright (C) 2011 Camiel Gubbels / Erik van der Zalm
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+/**
+ *  Endstop interrupts
+ *  Without endstop interrups the stepper-ISR must always test all endstops when interested in their states (endstops.update()).
+ *  Most time the test will result in finding out nothing has changed.
+ *  With endstop interrupts endstops.update() is called only when we know that at least one endstop has changed its state.
+ *
+ *  This can work only if all __used__ endstop pins can provide ether an 'external interrupt' or a 'pin change interrupt'.
+ *  You can find out about pins issuing interrupts by running 'pin_interrupt_test.ino' (Marlin\buildroot\share\pin_interrupt_test\pin_interrupt_test.ino)
+ */
+
+ #ifndef _ENDSTOP_INTERRUPTS_H_
+   #define _ENDSTOP_INTERRUPTS_H_
+
+  /**
+   * Patch for pins_arduino.h (...\Arduino\hardware\arduino\avr\variants\mega\pins_arduino.h)  
+   *
+   * These macros for the Arduino MEGA do not include the two connected pins on Port J (D13, D14).
+   * So we extend them here because this are the normal pins for Y_MIN and Y_MAX on RAMPS.
+   * There are more PCI enabled processor pins on Port J, but they are not connected to Arduino MEGA.
+   */
+  #if defined(ARDUINO_AVR_MEGA2560) || defined(ARDUINO_AVR_MEGA)
+    #undef  digitalPinToPCICR
+    #define digitalPinToPCICR(p)    ( (((p) >= 10) && ((p) <= 15)) || \
+                                    (((p) >= 50) && ((p) <= 53)) || \
+                                    (((p) >= 62) && ((p) <= 69)) ? (&PCICR) : ((uint8_t *)0) )
+    #undef  digitalPinToPCICRbit
+    #define digitalPinToPCICRbit(p) ( (((p) >= 10) && ((p) <= 13)) || (((p) >= 50) && ((p) <= 53)) ? 0 : \
+                                    ( (((p) >= 14) && ((p) <= 15)) ? 1 : \
+                                    ( (((p) >= 62) && ((p) <= 69)) ? 2 : \
+                                    0 ) ) )
+    #undef  digitalPinToPCMSK
+    #define digitalPinToPCMSK(p)    ( (((p) >= 10) && ((p) <= 13)) || (((p) >= 50) && ((p) <= 53)) ? (&PCMSK0) : \
+                                    ( (((p) >= 14) && ((p) <= 15)) ? (&PCMSK1) : \
+                                    ( (((p) >= 62) && ((p) <= 69)) ? (&PCMSK2) : \
+                                    ((uint8_t *)0) ) ) )
+    #undef  digitalPinToPCMSKbit
+    #define digitalPinToPCMSKbit(p) ( (((p) >= 10) && ((p) <= 13)) ? ((p) - 6) : \
+                                    ( ((p) == 14) ? 2 : \
+                                    ( ((p) == 15) ? 1 : \
+                                    ( ((p) == 50) ? 3 : \
+                                    ( ((p) == 51) ? 2 : \
+                                    ( ((p) == 52) ? 1 : \
+                                    ( ((p) == 53) ? 0 : \
+                                    ( (((p) >= 62) && ((p) <= 69)) ? ((p) - 62) : \
+                                    0 ) ) ) ) ) ) ) )
+  #endif
+
+  volatile uint8_t e_hit = 0; // Different from 0 when the endstops shall be tested in detail. 
+                              // Must be reset to 0 by the test function when the tests are finished.
+
+  // Install Pin change interrupt for a pin, can be called multiple times
+  void pciSetup(byte pin) {
+    *digitalPinToPCMSK(pin) |= bit (digitalPinToPCMSKbit(pin));  // enable pin
+    PCIFR  |= bit (digitalPinToPCICRbit(pin)); // clear any outstanding interrupt
+    PCICR  |= bit (digitalPinToPCICRbit(pin)); // enable interrupt for the group
+  }
+
+  // This is what is really done inside the interrupts.
+  FORCE_INLINE void endstop_ISR_worker( void ) {
+    e_hit = 2; // Because the detection of a e-stop hit has a 1 step debouncer it has to be called at least twice.
+  }
+
+  // Use one Routine to handle each group
+  // One ISR for all EXT-Interrupts
+  void endstop_ISR(void) {
+    endstop_ISR_worker();
+  }
+
+  #ifdef PCINT0_vect
+    ISR(PCINT0_vect) { // handle pin change interrupt
+      endstop_ISR_worker();
+    }
+  #endif
+
+  #ifdef PCINT1_vect
+    ISR(PCINT1_vect) { // handle pin change interrupt
+      endstop_ISR_worker();
+    }
+  #endif
+
+  #ifdef PCINT2_vect
+    ISR(PCINT2_vect) { // handle pin change interrupt
+      endstop_ISR_worker();
+    }
+  #endif
+
+  #ifdef PCINT3_vect
+    ISR(PCINT3_vect) { // handle pin change interrupt
+      endstop_ISR_worker();
+    }
+  #endif
+
+  void setup_enstop_interrupts( void ) {
+
+    #if HAS_X_MAX
+      #if (digitalPinToInterrupt(X_MAX_PIN) != NOT_AN_INTERRUPT) // if pin has an external interrupt
+        attachInterrupt(digitalPinToInterrupt(X_MAX_PIN), endstop_ISR, CHANGE); // assign it
+      #else
+        // Not all used endstop/probe -pins can raise interrupts. Please deactivate ENDSTOP_INTERRUPTS or change the pin configuration!
+        static_assert(digitalPinToPCICR(X_MAX_PIN) != NULL, "ENDSTOP_INTERRUPT_ERROR"); // if pin has no pin change interrupt - error
+        pciSetup(X_MAX_PIN);                                                            // assign it
+      #endif
+    #endif
+
+    #if HAS_X_MIN
+      #if (digitalPinToInterrupt(X_MIN_PIN) != NOT_AN_INTERRUPT)
+        attachInterrupt(digitalPinToInterrupt(X_MIN_PIN), endstop_ISR, CHANGE);
+      #else
+        // Not all used endstop/probe -pins can raise interrupts. Please deactivate ENDSTOP_INTERRUPTS or change the pin configuration!
+        static_assert(digitalPinToPCICR(X_MIN_PIN) != NULL, "ENDSTOP_INTERRUPT_ERROR");
+        pciSetup(X_MIN_PIN);
+      #endif
+    #endif
+
+    #if HAS_Y_MAX
+      #if (digitalPinToInterrupt(Y_MAX_PIN) != NOT_AN_INTERRUPT)
+        attachInterrupt(digitalPinToInterrupt(Y_MAX_PIN), endstop_ISR, CHANGE);
+      #else
+        // Not all used endstop/probe -pins can raise interrupts. Please deactivate ENDSTOP_INTERRUPTS or change the pin configuration!
+        static_assert(digitalPinToPCICR(Y_MAX_PIN) != NULL, "ENDSTOP_INTERRUPT_ERROR");
+        pciSetup(Y_MAX_PIN);
+      #endif
+    #endif
+
+    #if HAS_Y_MIN
+      #if (digitalPinToInterrupt(Y_MIN_PIN) != NOT_AN_INTERRUPT)
+        attachInterrupt(digitalPinToInterrupt(Y_MIN_PIN), endstop_ISR, CHANGE);
+      #else
+        // Not all used endstop/probe -pins can raise interrupts. Please deactivate ENDSTOP_INTERRUPTS or change the pin configuration!
+        static_assert(digitalPinToPCICR(Y_MIN_PIN) != NULL, "ENDSTOP_INTERRUPT_ERROR");
+        pciSetup(Y_MIN_PIN);
+      #endif
+    #endif
+
+    #if HAS_Z_MAX
+      #if (digitalPinToInterrupt(Z_MAX_PIN) != NOT_AN_INTERRUPT)
+        attachInterrupt(digitalPinToInterrupt(Z_MAX_PIN), endstop_ISR, CHANGE);
+      #else
+        // Not all used endstop/probe -pins can raise interrupts. Please deactivate ENDSTOP_INTERRUPTS or change the pin configuration!
+        static_assert(digitalPinToPCICR(Z_MAX_PIN) != NULL, "ENDSTOP_INTERRUPT_ERROR");
+        pciSetup(Z_MAX_PIN);
+      #endif
+    #endif
+
+    #if HAS_Z_MIN
+      #if (digitalPinToInterrupt(Z_MIN_PIN) != NOT_AN_INTERRUPT)
+        attachInterrupt(digitalPinToInterrupt(Z_MIN_PIN), endstop_ISR, CHANGE);
+      #else
+        // Not all used endstop/probe -pins can raise interrupts. Please deactivate ENDSTOP_INTERRUPTS or change the pin configuration!
+        static_assert(digitalPinToPCICR(Z_MIN_PIN) != NULL, "ENDSTOP_INTERRUPT_ERROR");
+        pciSetup(Z_MIN_PIN);
+      #endif
+    #endif
+
+    #if HAS_Z2_MAX
+      #if (digitalPinToInterrupt(Z2_MAX_PIN) != NOT_AN_INTERRUPT)
+        attachInterrupt(digitalPinToInterrupt(Z2_MAX_PIN), endstop_ISR, CHANGE);
+      #else
+        // Not all used endstop/probe -pins can raise interrupts. Please deactivate ENDSTOP_INTERRUPTS or change the pin configuration!
+        static_assert(digitalPinToPCICR(Z2_MAX_PIN) != NULL, "ENDSTOP_INTERRUPT_ERROR");
+        pciSetup(Z2_MAX_PIN);
+      #endif
+    #endif
+
+    #if HAS_Z2_MIN
+      #if (digitalPinToInterrupt(Z2_MIN_PIN) != NOT_AN_INTERRUPT)
+        attachInterrupt(digitalPinToInterrupt(Z2_MIN_PIN), endstop_ISR, CHANGE);
+      #else
+        // Not all used endstop/probe -pins can raise interrupts. Please deactivate ENDSTOP_INTERRUPTS or change the pin configuration!
+        static_assert(digitalPinToPCICR(Z2_MIN_PIN) != NULL, "ENDSTOP_INTERRUPT_ERROR");
+        pciSetup(Z2_MIN_PIN);
+      #endif
+    #endif
+
+    #if HAS_Z_MIN_PROBE_PIN
+      #if (digitalPinToInterrupt(Z_MIN_PROBE_PIN) != NOT_AN_INTERRUPT)
+        attachInterrupt(digitalPinToInterrupt(Z_MIN_PROBE_PIN), endstop_ISR, CHANGE);
+      #else
+        // Not all used endstop/probe -pins can raise interrupts. Please deactivate ENDSTOP_INTERRUPTS or change the pin configuration!
+        static_assert(digitalPinToPCICR(Z_MIN_PROBE_PIN) != NULL, "ENDSTOP_INTERRUPT_ERROR");
+        pciSetup(Z_MIN_PROBE_PIN);
+      #endif
+    #endif
+
+    // When we arive here without error each pin has ether a EXT-interrupt or a PCI.
+  }
+
+
+#endif //_ENDSTOP_INTERRUPTS_H_

Marlin/example_configurations/Cartesio/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/Felix/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/Felix/DUAL/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/Hephestos/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/Hephestos_2/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/K8200/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/K8400/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/K8400/Dual-head/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/RepRapWorld/Megatronics/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/RigidBot/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/SCARA/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/TAZ4/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/WITBOX/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/adafruit/ST7565/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/delta/biv2.5/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/delta/generic/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/delta/kossel_mini/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/delta/kossel_pro/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/delta/kossel_xl/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/makibox/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING false // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/example_configurations/tvrrug/Round2/Configuration.h

 #define Z_MAX_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 #define Z_MIN_PROBE_ENDSTOP_INVERTING true  // set to true to invert the logic of the endstop.
 
+// If all used endstop pins are able to cause interrupts, you can enable ENDSTOP_INTERRUPTS_FEATURE.
+// Then the function testing the endstops will only be called, if the state of one of the endstops changed.
+//#define ENDSTOP_INTERRUPTS_FEATURE
 
 //=============================================================================
 //============================== Movement Settings ============================

Marlin/stepper.cpp

   #endif // !ADVANCE && !LIN_ADVANCE
 }
 
+#if ENABLED(ENDSTOP_INTERRUPTS_FEATURE)
+  extern volatile uint8_t e_hit;
+#endif
+
 /**
  * Stepper Driver Interrupt
  *
     #if HAS_BED_PROBE
       || endstops.z_probe_enabled
     #endif
-  ) endstops.update();
+  )
+  #if ENABLED(ENDSTOP_INTERRUPTS_FEATURE)
+    if(e_hit) {
+  #endif
+      endstops.update();
+  #if ENABLED(ENDSTOP_INTERRUPTS_FEATURE)
+      e_hit--;
+    }
+  #endif
 
   // Take multiple steps per interrupt (For high speed moves)
   bool all_steps_done = false;

buildroot/share/pin_interrupt_test/pin_interrupt_test.ino

+// Search pins uasable for endstop-interupts
+// Compile with the same settings you'd use with Marlin.
+
+#if defined(ARDUINO_AVR_MEGA2560) || defined(ARDUINO_AVR_MEGA)
+    #undef  digitalPinToPCICR
+    #define digitalPinToPCICR(p)    ( (((p) >= 10) && ((p) <= 15)) || \
+                                    (((p) >= 50) && ((p) <= 53)) || \
+                                    (((p) >= 62) && ((p) <= 69)) ? (&PCICR) : ((uint8_t *)0) )
+#endif
+
+void setup() {
+  Serial.begin(9600);
+  Serial.println("PINs causing interrups are:");
+  for(int i=2; i<NUM_DIGITAL_PINS; i++){
+    if( digitalPinToPCICR(i) != NULL || (int)digitalPinToInterrupt(i) != -1 ) {
+      for (int j= 0; j<NUM_ANALOG_INPUTS; j++){
+        if(analogInputToDigitalPin(j) == i) {
+          Serial.print("A");
+          Serial.print(j);
+          Serial.print(" = ");
+        }
+      }
+      Serial.print("D");
+      Serial.println(i);
+    }
+  }
+  Serial.println("Arduino pin numbering!");
+}
+
+void loop() {
+  // put your main code here, to run repeatedly:
+}