Use uint8 instead of float for backlash correction

- Saves 3 bytes of SRAM.
- Consistent with fan speed.

Marlin/src/gcode/calibrate/G425.cpp

@@ -56,7 +56,8 @@
 #define HAS_Y_CENTER BOTH(CALIBRATION_MEASURE_FRONT, CALIBRATION_MEASURE_BACK)
 
 #if ENABLED(BACKLASH_GCODE)
-  extern float backlash_distance_mm[], backlash_correction, backlash_smoothing_mm;
+  extern float backlash_distance_mm[], backlash_smoothing_mm;
+  extern uint8_t backlash_correction;
 #endif
 
 enum side_t : uint8_t { TOP, RIGHT, FRONT, LEFT, BACK, NUM_SIDES };
@@ -446,7 +447,7 @@ inline void calibrate_backlash(measurements_t &m, const float uncertainty) {
 
   {
     // New scope for TEMPORARY_BACKLASH_CORRECTION
-    TEMPORARY_BACKLASH_CORRECTION(0.0f);
+    TEMPORARY_BACKLASH_CORRECTION(all_off);
     TEMPORARY_BACKLASH_SMOOTHING(0.0f);
 
     probe_sides(m, uncertainty);
@@ -478,7 +479,7 @@ inline void calibrate_backlash(measurements_t &m, const float uncertainty) {
 
     {
       // New scope for TEMPORARY_BACKLASH_CORRECTION
-      TEMPORARY_BACKLASH_CORRECTION(1.0f);
+      TEMPORARY_BACKLASH_CORRECTION(all_on);
       TEMPORARY_BACKLASH_SMOOTHING(0.0f);
       move_to(
         X_AXIS, current_position[X_AXIS] + 3,
@@ -513,7 +514,7 @@ inline void update_measurements(measurements_t &m, const AxisEnum axis) {
  *    - Call calibrate_backlash() beforehand for best accuracy
  */
 inline void calibrate_toolhead(measurements_t &m, const float uncertainty, const uint8_t extruder) {
-  TEMPORARY_BACKLASH_CORRECTION(1.0f);
+  TEMPORARY_BACKLASH_CORRECTION(all_on);
   TEMPORARY_BACKLASH_SMOOTHING(0.0f);
 
   #if HOTENDS > 1
@@ -556,7 +557,7 @@ inline void calibrate_toolhead(measurements_t &m, const float uncertainty, const
  *   uncertainty    in     - How far away from the object to begin probing
  */
 inline void calibrate_all_toolheads(measurements_t &m, const float uncertainty) {
-  TEMPORARY_BACKLASH_CORRECTION(1.0f);
+  TEMPORARY_BACKLASH_CORRECTION(all_on);
   TEMPORARY_BACKLASH_SMOOTHING(0.0f);
 
   HOTEND_LOOP() calibrate_toolhead(m, uncertainty, e);
@@ -588,7 +589,7 @@ inline void calibrate_all() {
     reset_hotend_offsets();
   #endif
 
-  TEMPORARY_BACKLASH_CORRECTION(1.0f);
+  TEMPORARY_BACKLASH_CORRECTION(all_on);
   TEMPORARY_BACKLASH_SMOOTHING(0.0f);
 
   // Do a fast and rough calibration of the toolheads

Marlin/src/gcode/calibrate/M425.cpp

@@ -26,8 +26,8 @@
 
 #include "../../module/planner.h"
 
-float backlash_distance_mm[XYZ] = BACKLASH_DISTANCE_MM,
-      backlash_correction = BACKLASH_CORRECTION;
+float   backlash_distance_mm[XYZ] = BACKLASH_DISTANCE_MM;
+uint8_t backlash_correction = BACKLASH_CORRECTION * all_on;
 
 #ifdef BACKLASH_SMOOTHING_MM
   float backlash_smoothing_mm = BACKLASH_SMOOTHING_MM;
@@ -74,7 +74,7 @@ void GcodeSuite::M425() {
 
   if (parser.seen('F')) {
     planner.synchronize();
-    backlash_correction = MAX(0, MIN(1.0, parser.value_linear_units()));
+    backlash_correction = MAX(0, MIN(1.0, parser.value_float())) * all_on;
     noArgs = false;
   }
 
@@ -90,7 +90,7 @@ void GcodeSuite::M425() {
     SERIAL_ECHOPGM("Backlash correction is ");
     if (!backlash_correction) SERIAL_ECHOPGM("in");
     SERIAL_ECHOLNPGM("active:");
-    SERIAL_ECHOPAIR("  Correction Amount/Fade-out:     F", backlash_correction);
+    SERIAL_ECHOPAIR("  Correction Amount/Fade-out:     F", float(ui8_to_percent(backlash_correction))/100);
     SERIAL_ECHOLNPGM("     (F1.0 = full, F0.0 = none)");
     SERIAL_ECHOPGM("  Backlash Distance (mm):        ");
     LOOP_XYZ(a) {

Marlin/src/lcd/extensible_ui/ui_api.cpp

@@ -82,7 +82,8 @@
 #include "ui_api.h"
 
 #if ENABLED(BACKLASH_GCODE)
-  extern float backlash_distance_mm[XYZ], backlash_correction;
+  extern float backlash_distance_mm[XYZ];
+  extern uint8_t backlash_correction;
   #ifdef BACKLASH_SMOOTHING_MM
     extern float backlash_smoothing_mm;
   #endif
@@ -686,8 +687,8 @@ namespace ExtUI {
     void setAxisBacklash_mm(const float value, const axis_t axis)
                                                       { backlash_distance_mm[axis] = clamp(value,0,5); }
 
-    float getBacklashCorrection_percent()             { return backlash_correction * 100; }
-    void setBacklashCorrection_percent(const float value) { backlash_correction = clamp(value, 0, 100) / 100.0f; }
+    float getBacklashCorrection_percent()             { return ui8_to_percent(backlash_correction); }
+    void setBacklashCorrection_percent(const float value) { backlash_correction = map(clamp(value, 0, 100), 0, 100, 0, 255); }
 
     #ifdef BACKLASH_SMOOTHING_MM
       float getBacklashSmoothing_mm()                 { return backlash_smoothing_mm; }

Marlin/src/module/planner.cpp

@@ -1572,13 +1572,14 @@ void Planner::synchronize() {
  */
 #if ENABLED(BACKLASH_COMPENSATION)
   #if ENABLED(BACKLASH_GCODE)
-    extern float backlash_distance_mm[], backlash_correction;
+    extern float backlash_distance_mm[];
+    extern uint8_t backlash_correction;
     #ifdef BACKLASH_SMOOTHING_MM
       extern float backlash_smoothing_mm;
     #endif
   #else
     constexpr float backlash_distance_mm[XYZ] = BACKLASH_DISTANCE_MM,
-                    backlash_correction = BACKLASH_CORRECTION;
+    constexpr uint8_t backlash_correction = BACKLASH_CORRECTION * 255;
     #ifdef BACKLASH_SMOOTHING_MM
       constexpr float backlash_smoothing_mm = BACKLASH_SMOOTHING_MM;
     #endif
@@ -1612,13 +1613,15 @@ void Planner::synchronize() {
       if (!changed_dir) return;
     #endif
 
+    const float f_corr = float(backlash_correction)/255.0f;
+
     LOOP_XYZ(axis) {
       if (backlash_distance_mm[axis]) {
         const bool reversing = TEST(dm,axis);
 
         // When an axis changes direction, add axis backlash to the residual error
         if (TEST(changed_dir, axis))
-          residual_error[axis] += backlash_correction * (reversing ? -1.0f : 1.0f) * backlash_distance_mm[axis] * planner.settings.axis_steps_per_mm[axis];
+          residual_error[axis] += (reversing ? -f_corr : f_corr) * backlash_distance_mm[axis] * planner.settings.axis_steps_per_mm[axis];
 
         // Decide how much of the residual error to correct in this segment
         int32_t error_correction = residual_error[axis];