limits.c

/*
  limits.c - code pertaining to limit-switches and performing the homing cycle
  Part of Grbl

  Copyright (c) 2012-2016 Sungeun K. Jeon for Gnea Research LLC
  Copyright (c) 2009-2011 Simen Svale Skogsrud
  
  Grbl 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.

  Grbl 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 Grbl.  If not, see <http://www.gnu.org/licenses/>.
*/
  
#include "grbl.h"


// Homing axis search distance multiplier. Computed by this value times the cycle travel.
#ifndef HOMING_AXIS_SEARCH_SCALAR
  #define HOMING_AXIS_SEARCH_SCALAR  1.5 // Must be > 1 to ensure limit switch will be engaged.
#endif
#ifndef HOMING_AXIS_LOCATE_SCALAR
  #define HOMING_AXIS_LOCATE_SCALAR  5.0 // Must be > 1 to ensure limit switch is cleared.
#endif

void limits_init()
{
  #ifdef DEFAULTS_RAMPS_BOARD
    // Set as input pins
    MIN_LIMIT_DDR(0) &= ~(1<<MIN_LIMIT_BIT(0));
    MIN_LIMIT_DDR(1) &= ~(1<<MIN_LIMIT_BIT(1));
    MIN_LIMIT_DDR(2) &= ~(1<<MIN_LIMIT_BIT(2));
    MAX_LIMIT_DDR(0) &= ~(1<<MAX_LIMIT_BIT(0));
    MAX_LIMIT_DDR(1) &= ~(1<<MAX_LIMIT_BIT(1));
    MAX_LIMIT_DDR(2) &= ~(1<<MAX_LIMIT_BIT(2));

    #ifdef DISABLE_LIMIT_PIN_PULL_UP
      MIN_LIMIT_PORT(0) &= ~(1<<MIN_LIMIT_BIT(0)); // Normal low operation. Requires external pull-down.
      MIN_LIMIT_PORT(1) &= ~(1<<MIN_LIMIT_BIT(1)); // Normal low operation. Requires external pull-down.
      MIN_LIMIT_PORT(2) &= ~(1<<MIN_LIMIT_BIT(2)); // Normal low operation. Requires external pull-down.
      MAX_LIMIT_PORT(0) &= ~(1<<MAX_LIMIT_BIT(0)); // Normal low operation. Requires external pull-down.
      MAX_LIMIT_PORT(1) &= ~(1<<MAX_LIMIT_BIT(1)); // Normal low operation. Requires external pull-down.
      MAX_LIMIT_PORT(2) &= ~(1<<MAX_LIMIT_BIT(2)); // Normal low operation. Requires external pull-down.
    #else
      MIN_LIMIT_PORT(0) |= (1<<MIN_LIMIT_BIT(0));  // Enable internal pull-up resistors. Normal high operation.
      MIN_LIMIT_PORT(1) |= (1<<MIN_LIMIT_BIT(1));  // Enable internal pull-up resistors. Normal high operation.
      MIN_LIMIT_PORT(2) |= (1<<MIN_LIMIT_BIT(2));  // Enable internal pull-up resistors. Normal high operation.
      MAX_LIMIT_PORT(0) |= (1<<MAX_LIMIT_BIT(0));  // Enable internal pull-up resistors. Normal high operation.
      MAX_LIMIT_PORT(1) |= (1<<MAX_LIMIT_BIT(1));  // Enable internal pull-up resistors. Normal high operation.
      MAX_LIMIT_PORT(2) |= (1<<MAX_LIMIT_BIT(2));  // Enable internal pull-up resistors. Normal high operation.
    #endif
    #ifndef DISABLE_HW_LIMITS
      if (bit_istrue(settings.flags,BITFLAG_HARD_LIMIT_ENABLE)) {
        LIMIT_PCMSK |= LIMIT_MASK; // Enable specific pins of the Pin Change Interrupt
        PCICR |= (1 << LIMIT_INT); // Enable Pin Change Interrupt
      } else {
        limits_disable();
      }
      #ifdef ENABLE_SOFTWARE_DEBOUNCE
        MCUSR &= ~(1<<WDRF);
        WDTCSR |= (1<<WDCE) | (1<<WDE);
        WDTCSR = (1<<WDP0); // Set time-out at ~32msec.
      #endif
    #endif // DISABLE_HW_LIMITS
  #else
    LIMIT_DDR &= ~(LIMIT_MASK); // Set as input pins

    #ifdef DISABLE_LIMIT_PIN_PULL_UP
      LIMIT_PORT &= ~(LIMIT_MASK); // Normal low operation. Requires external pull-down.
    #else
      LIMIT_PORT |= (LIMIT_MASK);  // Enable internal pull-up resistors. Normal high operation.
    #endif

    if (bit_istrue(settings.flags,BITFLAG_HARD_LIMIT_ENABLE)) {
      LIMIT_PCMSK |= LIMIT_MASK; // Enable specific pins of the Pin Change Interrupt
      PCICR |= (1 << LIMIT_INT); // Enable Pin Change Interrupt
    } else {
      limits_disable();
    }
  
    #ifdef ENABLE_SOFTWARE_DEBOUNCE
      MCUSR &= ~(1<<WDRF);
      WDTCSR |= (1<<WDCE) | (1<<WDE);
      WDTCSR = (1<<WDP0); // Set time-out at ~32msec.
    #endif
  #endif // DEFAULTS_RAMPS_BOARD
}


// Disables hard limits.
void limits_disable()
{
  #ifdef DEFAULTS_RAMPS_BOARD
    #ifndef DISABLE_HW_LIMITS
     LIMIT_PCMSK &= ~LIMIT_MASK;  // Disable specific pins of the Pin Change Interrupt
     PCICR &= ~(1 << LIMIT_INT);  // Disable Pin Change Interrupt
    #endif
  #else
    LIMIT_PCMSK &= ~LIMIT_MASK;  // Disable specific pins of the Pin Change Interrupt
    PCICR &= ~(1 << LIMIT_INT);  // Disable Pin Change Interrupt
  #endif // DEFAULTS_RAMPS_BOARD
}
#ifdef DEFAULTS_RAMPS_BOARD  
  static volatile uint8_t * const max_limit_pins[N_AXIS] = {&MAX_LIMIT_PIN(0), &MAX_LIMIT_PIN(1), &MAX_LIMIT_PIN(2)};
  static volatile uint8_t * const min_limit_pins[N_AXIS] = {&MIN_LIMIT_PIN(0), &MIN_LIMIT_PIN(1), &MIN_LIMIT_PIN(2)};
  static const uint8_t max_limit_bits[N_AXIS] = {MAX_LIMIT_BIT(0), MAX_LIMIT_BIT(1), MAX_LIMIT_BIT(2)};
  static const uint8_t min_limit_bits[N_AXIS] = {MIN_LIMIT_BIT(0), MIN_LIMIT_BIT(1), MIN_LIMIT_BIT(2)};
#endif // DEFAULTS_RAMPS_BOARD

// Returns limit state as a bit-wise uint8 variable. Each bit indicates an axis limit, where 
// triggered is 1 and not triggered is 0. Invert mask is applied. Axes are defined by their
// number in bit position, i.e. Z_AXIS is (1<<2) or bit 2, and Y_AXIS is (1<<1) or bit 1.
uint8_t limits_get_state()
{
  uint8_t limit_state = 0;
  #ifdef DEFAULTS_RAMPS_BOARD
    uint8_t pin;
    uint8_t idx;
    #ifdef INVERT_LIMIT_PIN_MASK
      #error "INVERT_LIMIT_PIN_MASK is not implemented"
    #endif
    for (idx=0; idx<N_AXIS; idx++) {
      pin = *max_limit_pins[idx] & (1<<max_limit_bits[idx]);
      pin = !!pin;
      if (bit_isfalse(settings.flags,BITFLAG_INVERT_LIMIT_PINS)) { pin = !pin; }
      #ifdef INVERT_MAX_LIMIT_PIN_MASK
        if (bit_istrue(INVERT_MAX_LIMIT_PIN_MASK, bit(idx))) { pin = !pin; }
      #endif
      if (pin)
        limit_state |= (1 << idx);
      pin = *min_limit_pins[idx] & (1<<min_limit_bits[idx]);
      pin = !!pin;
      if (bit_isfalse(settings.flags,BITFLAG_INVERT_LIMIT_PINS)) { pin = !pin; }
      #ifdef INVERT_MIN_LIMIT_PIN_MASK
        if (bit_istrue(INVERT_MIN_LIMIT_PIN_MASK, bit(idx))) { pin = !pin; }
      #endif
      if (pin)
        limit_state |= (1 << idx);
    } 
    return(limit_state);
  #else
    uint8_t pin = (LIMIT_PIN & LIMIT_MASK);
    #ifdef INVERT_LIMIT_PIN_MASK
      pin ^= INVERT_LIMIT_PIN_MASK;
    #endif
    if (bit_isfalse(settings.flags,BITFLAG_INVERT_LIMIT_PINS)) { pin ^= LIMIT_MASK; }
    if (pin) {  
      uint8_t idx;
      for (idx=0; idx<N_AXIS; idx++) {
        if (pin & get_limit_pin_mask(idx)) { limit_state |= (1 << idx); }
      }
    }
    return(limit_state);
  #endif //DEFAULTS_RAMPS_BOARD
}

#ifdef DEFAULTS_RAMPS_BOARD
  #ifndef DISABLE_HW_LIMITS
    #error "HW limits are not implemented"
  #endif
#else
// This is the Limit Pin Change Interrupt, which handles the hard limit feature. A bouncing 
// limit switch can cause a lot of problems, like false readings and multiple interrupt calls.
// If a switch is triggered at all, something bad has happened and treat it as such, regardless
// if a limit switch is being disengaged. It's impossible to reliably tell the state of a
// bouncing pin because the Arduino microcontroller does not retain any state information when
// detecting a pin change. If we poll the pins in the ISR, you can miss the correct reading if the 
// switch is bouncing.
// 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.
  #ifndef ENABLE_SOFTWARE_DEBOUNCE
    ISR(LIMIT_INT_vect) // DEFAULT: Limit pin change interrupt process. 
    {
      // Ignore limit switches if already in an alarm state or in-process of executing an alarm.
      // When in the alarm state, Grbl should have been reset or will force a reset, so any pending 
      // moves in the planner and serial buffers are all cleared and newly sent blocks will be 
      // locked out until a homing cycle or a kill lock command. Allows the user to disable the hard
      // limit setting if their limits are constantly triggering after a reset and move their axes.
      if (sys.state != STATE_ALARM) { 
        if (!(sys_rt_exec_alarm)) {
          #ifdef HARD_LIMIT_FORCE_STATE_CHECK
            // Check limit pin state. 
            if (limits_get_state()) {
              mc_reset(); // Initiate system kill.
              system_set_exec_alarm(EXEC_ALARM_HARD_LIMIT); // Indicate hard limit critical event
            }
          #else
            mc_reset(); // Initiate system kill.
            system_set_exec_alarm(EXEC_ALARM_HARD_LIMIT); // Indicate hard limit critical event
          #endif
        }
      }
    }  
  #else // OPTIONAL: Software debounce limit pin routine.
    // Upon limit pin change, enable watchdog timer to create a short delay. 
    ISR(LIMIT_INT_vect) { if (!(WDTCSR & (1<<WDIE))) { WDTCSR |= (1<<WDIE); } }
    ISR(WDT_vect) // Watchdog timer ISR
    {
      WDTCSR &= ~(1<<WDIE); // Disable watchdog timer. 
      if (sys.state != STATE_ALARM) {  // Ignore if already in alarm state. 
        if (!(sys_rt_exec_alarm)) {
          // Check limit pin state. 
          if (limits_get_state()) {
            mc_reset(); // Initiate system kill.
            system_set_exec_alarm(EXEC_ALARM_HARD_LIMIT); // Indicate hard limit critical event
          }
        }  
      }
    }
  #endif
#endif // DEFAULTS_RAMPS_BOARD

#ifdef DEFAULTS_RAMPS_BOARD
  static uint8_t axislock_active(uint8_t *axislock)
  {
    uint8_t res = 0;
    uint8_t idx;
 
    for (idx = 0; idx < N_AXIS; idx++)
      if (axislock[idx]) {
        res = 1;
        break;
      }
 
    return res;
  }
#endif // DEFAULTS_RAMPS_BOARD

 
// Homes the specified cycle axes, sets the machine position, and performs a pull-off motion after
// completing. Homing is a special motion case, which involves rapid uncontrolled stops to locate
// the trigger point of the limit switches. The rapid stops are handled by a system level axis lock
// mask, which prevents the stepper algorithm from executing step pulses. Homing motions typically
// circumvent the processes for executing motions in normal operation.
// NOTE: Only the abort realtime command can interrupt this process.
// TODO: Move limit pin-specific calls to a general function for portability.
void limits_go_home(uint8_t cycle_mask)
{
  if (sys.abort) { return; } // Block if system reset has been issued.

  // Initialize plan data struct for homing motion. Spindle and coolant are disabled.
  plan_line_data_t plan_data;
  plan_line_data_t *pl_data = &plan_data;
  memset(pl_data,0,sizeof(plan_line_data_t));
  pl_data->condition = (PL_COND_FLAG_SYSTEM_MOTION|PL_COND_FLAG_NO_FEED_OVERRIDE);
  pl_data->line_number = HOMING_CYCLE_LINE_NUMBER;

  // Initialize variables used for homing computations.
  uint8_t n_cycle = (2*N_HOMING_LOCATE_CYCLE+1);
  uint8_t step_pin[N_AXIS];
  float target[N_AXIS];
  float max_travel = 0.0;
  uint8_t idx;
  for (idx=0; idx<N_AXIS; idx++) {
    // Initialize step pin masks
    step_pin[idx] = get_step_pin_mask(idx);
    #ifdef COREXY
      if ((idx==A_MOTOR)||(idx==B_MOTOR)) { step_pin[idx] = (get_step_pin_mask(X_AXIS)|get_step_pin_mask(Y_AXIS)); }
    #endif

    if (bit_istrue(cycle_mask,bit(idx))) {
      // Set target based on max_travel setting. Ensure homing switches engaged with search scalar.
      // NOTE: settings.max_travel[] is stored as a negative value.
      max_travel = max(max_travel,(-HOMING_AXIS_SEARCH_SCALAR)*settings.max_travel[idx]);
    }
  }
  // Set search mode with approach at seek rate to quickly engage the specified cycle_mask limit switches.
  bool approach = true;
  float homing_rate = settings.homing_seek_rate;
  #ifdef DEFAULTS_RAMPS_BOARD
    uint8_t limit_state, n_active_axis;
    uint8_t axislock[N_AXIS];
    do {

      system_convert_array_steps_to_mpos(target,sys_position);

      // Initialize and declare variables needed for homing routine.
      n_active_axis = 0;
      for (idx=0; idx<N_AXIS; idx++) {
        axislock[idx]=0;
        // Set target location for active axes and setup computation for homing rate.
        if (bit_istrue(cycle_mask,bit(idx))) {
          n_active_axis++;
          #ifdef COREXY
            if (idx == X_AXIS) {
              int32_t axis_position = system_convert_corexy_to_y_axis_steps(sys_position);
              sys_position[A_MOTOR] = axis_position;
              sys_position[B_MOTOR] = -axis_position;
            } else if (idx == Y_AXIS) {
              int32_t axis_position = system_convert_corexy_to_x_axis_steps(sys_position);
              sys_position[A_MOTOR] = sys_position[B_MOTOR] = axis_position;
            } else {
              sys_position[Z_AXIS] = 0;
            }
          #else
            sys_position[idx] = 0;
          #endif
          // Set target direction based on cycle mask and homing cycle approach state.
          // NOTE: This happens to compile smaller than any other implementation tried.
          if (bit_istrue(settings.homing_dir_mask,bit(idx))) {
            if (approach) { target[idx] = -max_travel; }
            else { target[idx] = max_travel; }
          } else {
            if (approach) { target[idx] = max_travel; }
            else { target[idx] = -max_travel; }
          }
          // Apply axislock to the step port pins active in this cycle.
          axislock[idx] = step_pin[idx];
          sys.homing_axis_lock[idx] = axislock[idx];
        }

      }
      homing_rate *= sqrt(n_active_axis); // [sqrt(N_AXIS)] Adjust so individual axes all move at homing rate.
      

      // Perform homing cycle. Planner buffer should be empty, as required to initiate the homing cycle.
      pl_data->feed_rate = homing_rate; // Set current homing rate.
      plan_buffer_line(target, pl_data); // Bypass mc_line(). Directly plan homing motion.

      sys.step_control = STEP_CONTROL_EXECUTE_SYS_MOTION; // Set to execute homing motion and clear existing flags.
      st_prep_buffer(); // Prep and fill segment buffer from newly planned block.
      st_wake_up(); // Initiate motion
      do {
        if (approach) {
          // Check limit state. Lock out cycle axes when they change.
          limit_state = limits_get_state();
          for (idx=0; idx<N_AXIS; idx++) {
            if (axislock[idx] & step_pin[idx]) {
              if (limit_state & (1 << idx)) {
                #ifdef COREXY
                  if (idx==Z_AXIS) { axislock[idx] &= ~(step_pin[Z_AXIS]); }
                  else { axislock[idx] &= ~(step_pin[A_MOTOR]|step_pin[B_MOTOR]); }
                #else
                  axislock[idx] &= ~(step_pin[idx]);
                #endif
              }
            }
            sys.homing_axis_lock[idx] = axislock[idx];
          }
        }

        st_prep_buffer(); // Check and prep segment buffer. NOTE: Should take no longer than 200us.

        // Exit routines: No time to run protocol_execute_realtime() in this loop.
        if (sys_rt_exec_state & (EXEC_SAFETY_DOOR | EXEC_RESET | EXEC_CYCLE_STOP)) {
          uint8_t rt_exec = sys_rt_exec_state;
          // Homing failure condition: Reset issued during cycle.
          if (rt_exec & EXEC_RESET) { system_set_exec_alarm(EXEC_ALARM_HOMING_FAIL_RESET); }
          // Homing failure condition: Safety door was opened.
          if (rt_exec & EXEC_SAFETY_DOOR) { system_set_exec_alarm(EXEC_ALARM_HOMING_FAIL_DOOR); }
          // Homing failure condition: Limit switch still engaged after pull-off motion
          if (!approach && (limits_get_state() & cycle_mask)) { system_set_exec_alarm(EXEC_ALARM_HOMING_FAIL_PULLOFF); }
          // Homing failure condition: Limit switch not found during approach.
          if (approach && (rt_exec & EXEC_CYCLE_STOP)) { system_set_exec_alarm(EXEC_ALARM_HOMING_FAIL_APPROACH); }
          if (sys_rt_exec_alarm) {
            mc_reset(); // Stop motors, if they are running.
            protocol_execute_realtime();
            return;
          } else {
            // Pull-off motion complete. Disable CYCLE_STOP from executing.
            system_clear_exec_state_flag(EXEC_CYCLE_STOP);
            break;
          }
        }

      } while (axislock_active(axislock));
      st_reset(); // Immediately force kill steppers and reset step segment buffer.
      delay_ms(settings.homing_debounce_delay); // Delay to allow transient dynamics to dissipate.

      // Reverse direction and reset homing rate for locate cycle(s).
      approach = !approach;

      // After first cycle, homing enters locating phase. Shorten search to pull-off distance.
      if (approach) {
        max_travel = settings.homing_pulloff*HOMING_AXIS_LOCATE_SCALAR;
        homing_rate = settings.homing_feed_rate;
      } else {
        max_travel = settings.homing_pulloff;
        homing_rate = settings.homing_seek_rate;
      }
    } while (n_cycle-- > 0);
  #else
    uint8_t limit_state, axislock, n_active_axis;
    do {

      system_convert_array_steps_to_mpos(target,sys_position);

      // Initialize and declare variables needed for homing routine.
      axislock = 0;
      n_active_axis = 0;
      for (idx=0; idx<N_AXIS; idx++) {
        // Set target location for active axes and setup computation for homing rate.
        if (bit_istrue(cycle_mask,bit(idx))) {
          n_active_axis++;
          #ifdef COREXY
            if (idx == X_AXIS) {
              int32_t axis_position = system_convert_corexy_to_y_axis_steps(sys_position);
              sys_position[A_MOTOR] = axis_position;
              sys_position[B_MOTOR] = -axis_position;
            } else if (idx == Y_AXIS) {
              int32_t axis_position = system_convert_corexy_to_x_axis_steps(sys_position);
              sys_position[A_MOTOR] = sys_position[B_MOTOR] = axis_position;
            } else {
              sys_position[Z_AXIS] = 0;
            }
          #else
            sys_position[idx] = 0;
          #endif
          // Set target direction based on cycle mask and homing cycle approach state.
          // NOTE: This happens to compile smaller than any other implementation tried.
          if (bit_istrue(settings.homing_dir_mask,bit(idx))) {
            if (approach) { target[idx] = -max_travel; }
            else { target[idx] = max_travel; }
          } else {
            if (approach) { target[idx] = max_travel; }
            else { target[idx] = -max_travel; }
          }
          // Apply axislock to the step port pins active in this cycle.
          axislock |= step_pin[idx];
        }

      }
      homing_rate *= sqrt(n_active_axis); // [sqrt(N_AXIS)] Adjust so individual axes all move at homing rate.
      sys.homing_axis_lock = axislock;

      // Perform homing cycle. Planner buffer should be empty, as required to initiate the homing cycle.
      pl_data->feed_rate = homing_rate; // Set current homing rate.
      plan_buffer_line(target, pl_data); // Bypass mc_line(). Directly plan homing motion.

      sys.step_control = STEP_CONTROL_EXECUTE_SYS_MOTION; // Set to execute homing motion and clear existing flags.
      st_prep_buffer(); // Prep and fill segment buffer from newly planned block.
      st_wake_up(); // Initiate motion
      do {
        if (approach) {
          // Check limit state. Lock out cycle axes when they change.
          limit_state = limits_get_state();
          for (idx=0; idx<N_AXIS; idx++) {
            if (axislock & step_pin[idx]) {
              if (limit_state & (1 << idx)) {
                #ifdef COREXY
                  if (idx==Z_AXIS) { axislock &= ~(step_pin[Z_AXIS]); }
                  else { axislock &= ~(step_pin[A_MOTOR]|step_pin[B_MOTOR]); }
                #else
                  axislock &= ~(step_pin[idx]);
                #endif
              }
            }
          }
          sys.homing_axis_lock = axislock;
        }

        st_prep_buffer(); // Check and prep segment buffer. NOTE: Should take no longer than 200us.

        // Exit routines: No time to run protocol_execute_realtime() in this loop.
        if (sys_rt_exec_state & (EXEC_SAFETY_DOOR | EXEC_RESET | EXEC_CYCLE_STOP)) {
          uint8_t rt_exec = sys_rt_exec_state;
          // Homing failure condition: Reset issued during cycle.
          if (rt_exec & EXEC_RESET) { system_set_exec_alarm(EXEC_ALARM_HOMING_FAIL_RESET); }
          // Homing failure condition: Safety door was opened.
          if (rt_exec & EXEC_SAFETY_DOOR) { system_set_exec_alarm(EXEC_ALARM_HOMING_FAIL_DOOR); }
          // Homing failure condition: Limit switch still engaged after pull-off motion
          if (!approach && (limits_get_state() & cycle_mask)) { system_set_exec_alarm(EXEC_ALARM_HOMING_FAIL_PULLOFF); }
          // Homing failure condition: Limit switch not found during approach.
          if (approach && (rt_exec & EXEC_CYCLE_STOP)) { system_set_exec_alarm(EXEC_ALARM_HOMING_FAIL_APPROACH); }
          if (sys_rt_exec_alarm) {
            mc_reset(); // Stop motors, if they are running.
            protocol_execute_realtime();
            return;
          } else {
            // Pull-off motion complete. Disable CYCLE_STOP from executing.
            system_clear_exec_state_flag(EXEC_CYCLE_STOP);
            break;
          }
        }

      } while (STEP_MASK & axislock);
      st_reset(); // Immediately force kill steppers and reset step segment buffer.
      delay_ms(settings.homing_debounce_delay); // Delay to allow transient dynamics to dissipate.

      // Reverse direction and reset homing rate for locate cycle(s).
      approach = !approach;

      // After first cycle, homing enters locating phase. Shorten search to pull-off distance.
      if (approach) {
        max_travel = settings.homing_pulloff*HOMING_AXIS_LOCATE_SCALAR;
        homing_rate = settings.homing_feed_rate;
      } else {
        max_travel = settings.homing_pulloff;
        homing_rate = settings.homing_seek_rate;
      }
    } while (n_cycle-- > 0);
  #endif // DEFAULTS_RAMPS_BOARD

  // The active cycle axes should now be homed and machine limits have been located. By
  // default, Grbl defines machine space as all negative, as do most CNCs. Since limit switches
  // can be on either side of an axes, check and set axes machine zero appropriately. Also,
  // set up pull-off maneuver from axes limit switches that have been homed. This provides
  // some initial clearance off the switches and should also help prevent them from falsely
  // triggering when hard limits are enabled or when more than one axes shares a limit pin.
  int32_t set_axis_position;
  // Set machine positions for homed limit switches. Don't update non-homed axes.
  for (idx=0; idx<N_AXIS; idx++) {
    // NOTE: settings.max_travel[] is stored as a negative value.
    if (cycle_mask & bit(idx)) {
      #ifdef HOMING_FORCE_SET_ORIGIN
        set_axis_position = 0;
      #else
        if ( bit_istrue(settings.homing_dir_mask,bit(idx)) ) {
          set_axis_position = lround((settings.max_travel[idx]+settings.homing_pulloff)*settings.steps_per_mm[idx]);
        } else {
          set_axis_position = lround(-settings.homing_pulloff*settings.steps_per_mm[idx]);
        }
      #endif

      #ifdef COREXY
        if (idx==X_AXIS) {
          int32_t off_axis_position = system_convert_corexy_to_y_axis_steps(sys_position);
          sys_position[A_MOTOR] = set_axis_position + off_axis_position;
          sys_position[B_MOTOR] = set_axis_position - off_axis_position;
        } else if (idx==Y_AXIS) {
          int32_t off_axis_position = system_convert_corexy_to_x_axis_steps(sys_position);
          sys_position[A_MOTOR] = off_axis_position + set_axis_position;
          sys_position[B_MOTOR] = off_axis_position - set_axis_position;
        } else {
          sys_position[idx] = set_axis_position;
        }
      #else
        sys_position[idx] = set_axis_position;
      #endif

    }
  }
  sys.step_control = STEP_CONTROL_NORMAL_OP; // Return step control to normal operation.
}


// Performs a soft limit check. Called from mc_line() only. Assumes the machine has been homed,
// the workspace volume is in all negative space, and the system is in normal operation.
// NOTE: Used by jogging to limit travel within soft-limit volume.
void limits_soft_check(float *target)
{
  if (system_check_travel_limits(target)) {
    sys.soft_limit = true;
    // Force feed hold if cycle is active. All buffered blocks are guaranteed to be within
    // workspace volume so just come to a controlled stop so position is not lost. When complete
    // enter alarm mode.
    if (sys.state == STATE_CYCLE) {
      system_set_exec_state_flag(EXEC_FEED_HOLD);
      do {
        protocol_execute_realtime();
        if (sys.abort) { return; }
      } while ( sys.state != STATE_IDLE );
    }
    mc_reset(); // Issue system reset and ensure spindle and coolant are shutdown.
    system_set_exec_alarm(EXEC_ALARM_SOFT_LIMIT); // Indicate soft limit critical event
    protocol_execute_realtime(); // Execute to enter critical event loop and system abort
    return;
  }
}