Add SPINDLE_SERVO option (#19971)

Marlin/Configuration_adv.h

 
   #define SPINDLE_LASER_FREQUENCY       2500   // (Hz) Spindle/laser frequency (only on supported HALs: AVR and LPC)
 
+  //#define SPINDLE_SERVO         // A servo converting an angle to spindle power
+  #ifdef SPINDLE_SERVO
+    #define SPINDLE_SERVO_NR   0  // Index of servo used for spindle control
+    #define SPINDLE_SERVO_MIN 10  // Minimum angle for servo spindle
+  #endif
+
   /**
    * Speed / Power can be set ('M3 S') and displayed in terms of:
    *  - PWM255  (S0 - S255)
    *  - PERCENT (S0 - S100)
    *  - RPM     (S0 - S50000)  Best for use with a spindle
+   *  - SERVO   (S0 - S180)
    */
   #define CUTTER_POWER_UNIT PWM255
 

Marlin/src/feature/spindle_laser.cpp

 
 #include "spindle_laser.h"
 
+#if ENABLED(SPINDLE_SERVO)
+  #include "../module/servo.h"
+#endif
+
 SpindleLaser cutter;
 uint8_t SpindleLaser::power;
 bool SpindleLaser::isReady;                                           // Ready to apply power setting from the UI to OCR
 // Init the cutter to a safe OFF state
 //
 void SpindleLaser::init() {
-  OUT_WRITE(SPINDLE_LASER_ENA_PIN, !SPINDLE_LASER_ACTIVE_STATE);      // Init spindle to off
+  #if ENABLED(SPINDLE_SERVO)
+    MOVE_SERVO(SPINDLE_SERVO_NR, SPINDLE_SERVO_MIN);
+  #else
+    OUT_WRITE(SPINDLE_LASER_ENA_PIN, !SPINDLE_LASER_ACTIVE_STATE);    // Init spindle to off
+  #endif
   #if ENABLED(SPINDLE_CHANGE_DIR)
     OUT_WRITE(SPINDLE_DIR_PIN, SPINDLE_INVERT_DIR ? 255 : 0);         // Init rotation to clockwise (M3)
   #endif
       ocr_off();
       isReady = false;
     }
+  #elif ENABLED(SPINDLE_SERVO)
+    MOVE_SERVO(SPINDLE_SERVO_NR, power);
   #else
     WRITE(SPINDLE_LASER_ENA_PIN, enabled() ? SPINDLE_LASER_ACTIVE_STATE : !SPINDLE_LASER_ACTIVE_STATE);
     isReady = true;

Marlin/src/feature/spindle_laser.h

 #endif
 
 #define PCT_TO_PWM(X) ((X) * 255 / 100)
+#define PCT_TO_SERVO(X) ((X) * 180 / 100)
 
 #ifndef SPEED_POWER_INTERCEPT
   #define SPEED_POWER_INTERCEPT 0
   }
 
   // Convert a cpower (e.g., SPEED_POWER_STARTUP) to unit power (upwr, upower),
-  // which can be PWM, Percent, or RPM (rel/abs).
+  // which can be PWM, Percent, Servo angle, or RPM (rel/abs).
   static const inline cutter_power_t cpwr_to_upwr(const cutter_cpower_t cpwr) { // STARTUP power to Unit power
     const cutter_power_t upwr = (
       #if ENABLED(SPINDLE_FEATURE)
           cpwr                            // to RPM
         #elif CUTTER_UNIT_IS(PERCENT)     // to PCT
           cpwr_to_pct(cpwr)
+        #elif CUTTER_UNIT_IS(SERVO)       // to SERVO angle
+          PCT_TO_SERVO(cpwr_to_pct(cpwr))
         #else                             // to PWM
           PCT_TO_PWM(cpwr_to_pct(cpwr))
         #endif

Marlin/src/gcode/control/M3-M5.cpp

   auto get_s_power = [] {
     if (parser.seenval('S')) {
       const float spwr = parser.value_float();
-      cutter.unitPower = TERN(SPINDLE_LASER_PWM,
+      #if ENABLED(SPINDLE_SERVO)
+        cutter.unitPower = spwr;
+      #else
+        cutter.unitPower = TERN(SPINDLE_LASER_PWM,
                               cutter.power_to_range(cutter_power_t(round(spwr))),
                               spwr > 0 ? 255 : 0);
+      #endif
     }
     else
       cutter.unitPower = cutter.cpwr_to_upwr(SPEED_POWER_STARTUP);
     }
     else
       cutter.set_power(cutter.upower_to_ocr(get_s_power()));
+  #elif ENABLED(SPINDLE_SERVO)
+    cutter.set_power(get_s_power()); 
   #else
     cutter.set_enabled(true);
   #endif

Marlin/src/inc/Conditionals_adv.h

 // Determine NUM_SERVOS if none was supplied
 #ifndef NUM_SERVOS
   #define NUM_SERVOS 0
-  #if ANY(CHAMBER_VENT, HAS_Z_SERVO_PROBE, SWITCHING_EXTRUDER, SWITCHING_NOZZLE)
+  #if ANY(HAS_Z_SERVO_PROBE, CHAMBER_VENT, SWITCHING_TOOLHEAD, SWITCHING_EXTRUDER, SWITCHING_NOZZLE, SPINDLE_SERVO)
     #if NUM_SERVOS <= Z_PROBE_SERVO_NR
       #undef NUM_SERVOS
       #define NUM_SERVOS (Z_PROBE_SERVO_NR + 1)
       #undef NUM_SERVOS
       #define NUM_SERVOS (SWITCHING_EXTRUDER_E23_SERVO_NR + 1)
     #endif
+    #if NUM_SERVOS <= SPINDLE_SERVO_NR
+      #undef NUM_SERVOS
+      #define NUM_SERVOS (SPINDLE_SERVO_NR + 1)
+    #endif
   #endif
 #endif
 

Marlin/src/inc/SanityCheck.h

 #if HAS_CUTTER
   #ifndef CUTTER_POWER_UNIT
     #error "CUTTER_POWER_UNIT is required with a spindle or laser. Please update your Configuration_adv.h."
-  #elif !CUTTER_UNIT_IS(PWM255) && !CUTTER_UNIT_IS(PERCENT) && !CUTTER_UNIT_IS(RPM)
-    #error "CUTTER_POWER_UNIT must be PWM255, PERCENT, or RPM. Please update your Configuration_adv.h."
+  #elif !CUTTER_UNIT_IS(PWM255) && !CUTTER_UNIT_IS(PERCENT) && !CUTTER_UNIT_IS(RPM) && !CUTTER_UNIT_IS(SERVO)
+    #error "CUTTER_POWER_UNIT must be PWM255, PERCENT, RPM, or SERVO. Please update your Configuration_adv.h."
   #endif
 
   #if ENABLED(LASER_POWER_INLINE)
   #define _PIN_CONFLICT(P) (PIN_EXISTS(P) && P##_PIN == SPINDLE_LASER_PWM_PIN)
   #if BOTH(SPINDLE_FEATURE, LASER_FEATURE)
     #error "Enable only one of SPINDLE_FEATURE or LASER_FEATURE."
-  #elif !PIN_EXISTS(SPINDLE_LASER_ENA)
-    #error "(SPINDLE|LASER)_FEATURE requires SPINDLE_LASER_ENA_PIN."
+  #elif !PIN_EXISTS(SPINDLE_LASER_ENA) && DISABLED(SPINDLE_SERVO)
+    #error "(SPINDLE|LASER)_FEATURE requires SPINDLE_LASER_ENA_PIN or SPINDLE_SERVO to control the power."
   #elif ENABLED(SPINDLE_CHANGE_DIR) && !PIN_EXISTS(SPINDLE_DIR)
     #error "SPINDLE_DIR_PIN is required for SPINDLE_CHANGE_DIR."
   #elif ENABLED(SPINDLE_LASER_PWM)