Apply const, spacing, etc.

Marlin/M100_Free_Mem_Chk.cpp

    *  This is useful to check the correctness of the M100 D and the M100 F commands.
    */
   void corrupt_free_memory(char *ptr, const uint16_t size) {
-    if (parser.seen('C')) {
-      ptr += 8;
-      const uint16_t near_top = top_of_stack() - ptr - 250, // -250 to avoid interrupt activity that's altered the stack.
-                     j = near_top / (size + 1);
-
-      SERIAL_ECHOLNPGM("Corrupting free memory block.\n");
-      for (uint16_t i = 1; i <= size; i++) {
-        char * const addr = ptr + i * j;
-        *addr = i;
-        SERIAL_ECHOPAIR("\nCorrupting address: ", hex_address(addr));
-      }
-      SERIAL_EOL();
+    ptr += 8;
+    const uint16_t near_top = top_of_stack() - ptr - 250, // -250 to avoid interrupt activity that's altered the stack.
+                   j = near_top / (size + 1);
+
+    SERIAL_ECHOLNPGM("Corrupting free memory block.\n");
+    for (uint16_t i = 1; i <= size; i++) {
+      char * const addr = ptr + i * j;
+      *addr = i;
+      SERIAL_ECHOPAIR("\nCorrupting address: ", hex_address(addr));
     }
+    SERIAL_EOL();
   }
 #endif // M100_FREE_MEMORY_CORRUPTOR
 

Marlin/Marlin_main.cpp

  *
  * Returns TRUE if the target is invalid
  */
-bool get_target_extruder_from_command(int code) {
+bool get_target_extruder_from_command(const uint16_t code) {
   if (parser.seen('T')) {
-    if (parser.value_byte() >= EXTRUDERS) {
+    const int8_t e = parser.value_byte();
+    if (e >= EXTRUDERS) {
       SERIAL_ECHO_START();
       SERIAL_CHAR('M');
       SERIAL_ECHO(code);
-      SERIAL_ECHOLNPAIR(" " MSG_INVALID_EXTRUDER " ", parser.value_byte());
+      SERIAL_ECHOLNPAIR(" " MSG_INVALID_EXTRUDER " ", e);
       return true;
     }
-    target_extruder = parser.value_byte();
+    target_extruder = e;
   }
   else
     target_extruder = active_extruder;
         #if HAS_POSITION_SHIFT
           const float p = current_position[i];
         #endif
-        float v = parser.value_axis_units((AxisEnum)i);
+        const float v = parser.value_axis_units((AxisEnum)i);
 
         current_position[i] = v;
 
 inline void gcode_M42() {
   if (!parser.seen('S')) return;
 
-  int pin_status = parser.value_int();
+  const int pin_status = parser.value_int();
   if (!WITHIN(pin_status, 0, 255)) return;
 
   int pin_number = parser.seen('P') ? parser.value_int() : LED_PIN;
    */
   inline void gcode_M43() {
 
-    if (parser.seen('T')) {   // must be first or else it's "S" and "E" parameters will execute endstop or servo test
+    if (parser.seen('T')) {   // must be first or else its "S" and "E" parameters will execute endstop or servo test
       toggle_pins();
       return;
     }
       home_offset[Z_AXIS] = parser.value_linear_units() - DELTA_HEIGHT;
       update_software_endstops(Z_AXIS);
     }
-    if (parser.seen('L')) delta_diagonal_rod = parser.value_linear_units();
-    if (parser.seen('R')) delta_radius = parser.value_linear_units();
-    if (parser.seen('S')) delta_segments_per_second = parser.value_float();
-    if (parser.seen('B')) delta_calibration_radius = parser.value_float();
+    if (parser.seen('L')) delta_diagonal_rod             = parser.value_linear_units();
+    if (parser.seen('R')) delta_radius                   = parser.value_linear_units();
+    if (parser.seen('S')) delta_segments_per_second      = parser.value_float();
+    if (parser.seen('B')) delta_calibration_radius       = parser.value_float();
     if (parser.seen('X')) delta_tower_angle_trim[A_AXIS] = parser.value_float();
     if (parser.seen('Y')) delta_tower_angle_trim[B_AXIS] = parser.value_float();
     if (parser.seen('Z')) { // rotate all 3 axis for Z = 0
    */
   inline void gcode_M280() {
     if (!parser.seen('P')) return;
-    int servo_index = parser.value_int();
+    const int servo_index = parser.value_int();
     if (WITHIN(servo_index, 0, NUM_SERVOS - 1)) {
       if (parser.seen('S'))
         MOVE_SERVO(servo_index, parser.value_int());
    *       M302 S170 P1 ; set min extrude temp to 170 but leave disabled
    */
   inline void gcode_M302() {
-    bool seen_S = parser.seen('S');
+    const bool seen_S = parser.seen('S');
     if (seen_S) {
       thermalManager.extrude_min_temp = parser.value_celsius();
       thermalManager.allow_cold_extrude = (thermalManager.extrude_min_temp == 0);
    * M405: Turn on filament sensor for control
    */
   inline void gcode_M405() {
-    // This is technically a linear measurement, but since it's quantized to centimeters and is a different unit than
-    // everything else, it uses parser.value_int() instead of parser.value_linear_units().
-    if (parser.seen('D')) meas_delay_cm = parser.value_byte();
-    NOMORE(meas_delay_cm, MAX_MEASUREMENT_DELAY);
+    // This is technically a linear measurement, but since it's quantized to centimeters and is a different
+    // unit than everything else, it uses parser.value_byte() instead of parser.value_linear_units().
+    if (parser.seen('D')) {
+      meas_delay_cm = parser.value_byte();
+      NOMORE(meas_delay_cm, MAX_MEASUREMENT_DELAY);
+    }
 
     if (filwidth_delay_index[1] == -1) { // Initialize the ring buffer if not done since startup
       const uint8_t temp_ratio = thermalManager.widthFil_to_size_ratio() - 100; // -100 to scale within a signed byte

Marlin/SdFatStructs.h

   uint8_t  reservedNT;
           /**
            * The granularity of the seconds part of creationTime is 2 seconds
-           * so this field is a count of tenths of a second and its valid
+           * so this field is a count of tenths of a second and it's valid
            * value range is 0-199 inclusive. (WHG note - seems to be hundredths)
            */
   uint8_t  creationTimeTenths;

Marlin/example_configurations/K8400/README.md

 http://www.k8400.eu/
 
 Configuration files for the K8400, ported upstream from the official Velleman firmware.
-Like it's predecessor, (K8200), the K8400 is a 3Drag clone. There are some minor differences, documented in pins_K8400.h.
+Like its predecessor, (K8200), the K8400 is a 3Drag clone. There are some minor differences, documented in pins_K8400.h.
 
 Single and dual head configurations provided. Copy the correct Configuration.h and Configuration_adv.h to the /Marlin/ directory.
 

Marlin/servo.cpp

   if (SERVO_INDEX(timer, Channel[timer]) < ServoCount && Channel[timer] < SERVOS_PER_TIMER) {
     *OCRnA = *TCNTn + SERVO(timer, Channel[timer]).ticks;
     if (SERVO(timer, Channel[timer]).Pin.isActive)    // check if activated
-      digitalWrite(SERVO(timer, Channel[timer]).Pin.nbr, HIGH); // its an active channel so pulse it high
+      digitalWrite(SERVO(timer, Channel[timer]).Pin.nbr, HIGH); // it's an active channel so pulse it high
   }
   else {
     // finished all channels so wait for the refresh period to expire before starting over

Marlin/twibus.h

 /**
  * TWIBUS class
  *
- * This class implements a wrapper around the two wire (I2C) bus, it allows
- * Marlin to send and request data from any slave device on the bus. This is
- * an experimental feature and it's inner workings as well as public facing
- * interface are prune to change in the future.
+ * This class implements a wrapper around the two wire (I2C) bus, allowing
+ * Marlin to send and request data from any slave device on the bus.
  *
- * The two main consumers of this class are M260 and M261, where M260 allows
- * Marlin to send a I2C packet to a device (please be aware that no repeated
- * starts are possible), this can be done in caching method by calling multiple
- * times M260 B<byte-1 value in base 10> or a one liner M260, have a look at
- * the gcode_M260() function for more information. M261 allows Marlin to
- * request data from a device, the received data is then relayed into the serial
- * line for host interpretation.
+ * The two main consumers of this class are M260 and M261. M260 provides a way
+ * to send an I2C packet to a device (no repeated starts) by caching up to 32
+ * bytes in a buffer and then sending the buffer.
+ * M261 requests data from a device. The received data is relayed to serial out
+ * for the host to interpret.
+ *
+ *  For more information see
+ *    - http://marlinfw.org/docs/gcode/M260.html 
+ *    - http://marlinfw.org/docs/gcode/M261.html 
  *
  */
 class TWIBus {