stepper.c

/*
  stepper.c - stepper motor driver: executes motion plans using stepper motors
  Part of Grbl

  Copyright (c) 2009-2011 Simen Svale Skogsrud
  Copyright (c) 2011-2012 Sungeun K. Jeon
  Copyright (c) 2012 Chuck Harrison for http://opensourceecology.org/wiki/CNC_Torch_Table
 
  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/>.
*/

/* The timer calculations of this module informed by the 'RepRap cartesian firmware' by Zack Smith
   and Philipp Tiefenbacher. */

#include <avr/interrupt.h>
#include "stepper.h"
#include "config.h"
#include "settings.h"
#include <math.h>
#include <stdlib.h>
#include <util/delay.h>
#include "nuts_bolts.h"
#include <avr/interrupt.h>
#include "planner.h"
#include "motion_control.h"
#include "protocol.h"
#include "limits.h"

#include "print.h"
#include <avr/pgmspace.h>


// Some useful constants
#define TICKS_PER_MICROSECOND (F_CPU/1000000)
#define CYCLES_PER_ACCELERATION_TICK ((TICKS_PER_MICROSECOND*1000000)/ACCELERATION_TICKS_PER_SECOND)

// Stepper state variable. Contains running data and trapezoid variables for coordinated motion.
typedef struct {
  // Used by the bresenham line algorithm
  int32_t counter_x,        // Counter variables for the bresenham line tracer
          counter_y, 
          counter_z;
  uint32_t event_count;
  uint32_t step_events_completed;  // The number of step events left in current motion

  // Used by the trapezoid generator
  uint32_t cycles_per_step_event;        // The number of machine cycles between each step event
  uint32_t trapezoid_tick_cycle_counter; // The cycles since last trapezoid_tick. Used to generate ticks at a steady
                                              // pace without allocating a separate timer
  uint32_t trapezoid_adjusted_rate;      // The current rate of step_events according to the trapezoid generator
  uint32_t min_safe_rate;  // Minimum safe rate for full deceleration rate reduction step. Otherwise halves step_rate.
} stepper_t;

static stepper_t st;
static block_t *current_block;  // A pointer to the block currently being traced

// Used by independent_axis mode (e.g. homing)
static indep_t_ptr indep_frame;
bool indep_mode;

// Used by the stepper driver interrupt
static uint8_t step_pulse_time; // Step pulse reset time after step rise
static uint8_t out_bits;        // The next stepping-bits to be output
uint8_t out_bits0;              // The stepping-bit state between pulses    
static volatile uint8_t busy;   // True when SIG_OUTPUT_COMPARE1A is being serviced. Used to avoid retriggering that handler.

#if STEP_PULSE_DELAY > 0
  static uint8_t step_bits;  // Stores out_bits output to complete the step pulse delay
#endif

//         __________________________
//        /|                        |\     _________________         ^
//       / |                        | \   /|               |\        |
//      /  |                        |  \ / |               | \       s
//     /   |                        |   |  |               |  \      p
//    /    |                        |   |  |               |   \     e
//   +-----+------------------------+---+--+---------------+----+    e
//   |               BLOCK 1            |      BLOCK 2          |    d
//
//                           time ----->
// 
//  The trapezoid is the shape the speed curve over time. It starts at block->initial_rate, accelerates by block->rate_delta
//  during the first block->accelerate_until step_events_completed, then keeps going at constant speed until 
//  step_events_completed reaches block->decelerate_after after which it decelerates until the trapezoid generator is reset.
//  The slope of acceleration is always +/- block->rate_delta and is applied at a constant rate following the midpoint rule
//  by the trapezoid generator, which is called ACCELERATION_TICKS_PER_SECOND times per second.

static void set_step_events_per_minute(uint32_t steps_per_minute);

static void set_motion_state_indep(indep_t_ptr it); // forward declaration

void inline disable_steppers() {
  // TBD: respect settings flag BITFLAG_INVERT_ST_ENABLE
  STEPPERS_DISABLE_PORT = (STEPPERS_DISABLE_PORT & ~STEPPERS_DISABLE_MASK)
                          | (~STEPPERS_DISABLE_INVERT_MASK & STEPPERS_DISABLE_MASK); }
static void inline enable_steppers() {
  STEPPERS_DISABLE_PORT = (STEPPERS_DISABLE_PORT & ~STEPPERS_DISABLE_MASK)
                          | (STEPPERS_DISABLE_INVERT_MASK & STEPPERS_DISABLE_MASK); }


// Stepper state initialization. Motion should only start if cycle or homing is in progress
// Startup init and limits call this function but shouldn't start the cycle.
// Caller (st_cycle_start or st_indep_start) should enable steppers first.
void st_wake_up() 
{
  if (sys.state == STATE_CYCLE || sys.state == STATE_HOMING) {
    // Initialize stepper output bits
    out_bits = out_bits0; 
    // Initialize step pulse timing from settings. Here to ensure updating after re-writing.
    #ifdef STEP_PULSE_DELAY
      // Set total step pulse time after direction pin set. Ad hoc computation from oscilloscope.
      step_pulse_time = -(((settings.pulse_microseconds+STEP_PULSE_DELAY-2)*TICKS_PER_MICROSECOND) >> 3);
      // Set delay between direction pin write and step command.
      OCR2A = -(((settings.pulse_microseconds)*TICKS_PER_MICROSECOND) >> 3);
    #else // Normal operation
      // Set step pulse time. Ad hoc computation from oscilloscope. Uses two's complement.
      step_pulse_time = -(((settings.pulse_microseconds-2)*TICKS_PER_MICROSECOND) >> 3);
    #endif
    // Enable stepper driver interrupt
    TIMSK1 |= (1<<OCIE1A);
  }
}

// Independent-move start
void st_indep_start(indep_t_ptr frame) 
{
  if(1) {  // ok to move. TODO: insert some guard tests here
    indep_frame = frame;
    indep_mode = true;
    set_step_events_per_minute(HOME_EVENTS_PER_SECOND*60);
    st.event_count = INDEP_EVENT_COUNT;
    st.counter_x = -(INDEP_EVENT_COUNT >> 1);
    st.counter_y = st.counter_x;
    st.counter_z = st.counter_x;
    set_motion_state_indep(frame); // for hard limits
    st_wake_up();
  }
}


// 'axes_moving' & 'axes_dir' bitfields describe the current motion state, needed for hard limits
uint8_t axes_moving, axes_dir;
// update motion state, called at beginning of new block
static void set_motion_state_block(block_t* block)
{
  axes_moving = 0;
  axes_dir = 0;
  uint8_t dir = block->direction_bits;
  if(block->steps_x != 0) {
    axes_moving |= X_HOME_BIT;
    if(dir & X_DIRECTION_BIT) {
      axes_dir |= X_HOME_BIT;
    }
  }
   if(block->steps_y != 0) {
    axes_moving |= Y_HOME_BIT;
    if(dir & Y_DIRECTION_BIT) {
      axes_dir |= Y_HOME_BIT;
    }
  }
   if(block->steps_z != 0) {
    axes_moving |= Z_HOME_BIT;
    if(dir & Z_DIRECTION_BIT) {
      axes_dir |= Z_HOME_BIT;
    }
  }
}

// update motion state, called at beginning of new independent-mode movement
static void set_motion_state_indep(indep_t_ptr it)
{
  axes_moving = 0;
  axes_dir = 0;
  uint8_t ob0 = out_bits0^settings.invert_mask;
  while(it) {
    switch(it->axis) {
      case X_AXIS:
        if(ob0 & (1<<X_DIRECTION_BIT)) {
          axes_dir |= it->home_mask;
        }
        break;
      case Y_AXIS:
        if(ob0 & (1<<Y_DIRECTION_BIT)) {
          axes_dir |= it->home_mask;
        }
        break;
      case Z_AXIS:
        if(ob0 & (1<<Z_DIRECTION_BIT)) {
          axes_dir |= it->home_mask;
        }
        break;
    }
    if((it->flags & INDEP_HOMING) 
       && (it->home_mask & (home_limit_state()^it->home_nullstate)) ) {
      it->state = done;
    } else {
      axes_moving |= it->home_mask;
    }
    it = it->next_axis;
  }
}


// Stepper shutdown
void st_go_idle() 
{
  // Disable stepper driver interrupt
  TIMSK1 &= ~(1<<OCIE1A); 
  axes_moving = 0;
  //printPgmString(PSTR("st_go_idle\r\n"));

  // Disable steppers only upon system alarm activated or by user setting to not be kept enabled.
  if ((settings.stepper_idle_lock_time != 0xff) || bit_istrue(sys.execute,EXEC_ALARM)) {
    // Force stepper dwell to lock axes for a defined amount of time to ensure the axes come to a complete
    // stop and not drift from residual inertial forces at the end of the last movement.
    delay_ms(settings.stepper_idle_lock_time);
    disable_steppers();
  }
}

// Hard limit test is called inside the stepper interrupt
//  This version is for a configuration with mid-span home switch, end-of-travel limit switch
// As coded here, no axis can move towards its limit until all axes are clear of their limits.
static inline void test_hard_limits()
{
  if ( settings.flags & (1<<BITFLAG_HARD_LIMIT_ENABLE) ) { 
    uint8_t home_lim = home_limit_state()^LIMITS_INVERT_MASK;
    if( ((home_lim^axes_dir) & axes_moving) // we are moving towards a limit switch
        && (home_lim & LIMIT_MASK) ) { // we hit one
      mc_reset(); // Initiate system kill.
      sys.execute |= EXEC_CRIT_EVENT; // Indicate hard limit critical event
    }
  }
}
// This function determines an acceleration velocity change every CYCLES_PER_ACCELERATION_TICK by
// keeping track of the number of elapsed cycles during a de/ac-celeration. The code assumes that 
// step_events occur significantly more often than the acceleration velocity iterations.
inline static uint8_t iterate_trapezoid_cycle_counter() 
{
  st.trapezoid_tick_cycle_counter += st.cycles_per_step_event;  
  if(st.trapezoid_tick_cycle_counter > CYCLES_PER_ACCELERATION_TICK) {
    st.trapezoid_tick_cycle_counter -= CYCLES_PER_ACCELERATION_TICK;
    return(true);
  } else {
    return(false);
  }
}          

// "The Stepper Driver Interrupt" - This timer interrupt is the workhorse of Grbl. It is executed at the rate set with
// config_step_timer. It pops blocks from the block_buffer and executes them by pulsing the stepper pins appropriately. 
// It is supported by The Stepper Port Reset Interrupt which it uses to reset the stepper port after each pulse. 
// The bresenham line tracer algorithm controls all three stepper outputs simultaneously with these two interrupts.
ISR(TIMER1_COMPA_vect)
{        
  if (busy) { return; } // The busy-flag is used to avoid reentering this interrupt
  
  PORTC |= 0x08;  // diagnostic timing test
  
  // Set the direction pins a couple of nanoseconds before we step the steppers
  STEPPING_PORT = (STEPPING_PORT & ~DIRECTION_MASK) | (out_bits & DIRECTION_MASK);
  // Then pulse the stepping pins
  #ifdef STEP_PULSE_DELAY
    step_bits = (STEPPING_PORT & ~STEP_MASK) | out_bits; // Store out_bits to prevent overwriting.
  #else  // Normal operation
    STEPPING_PORT = (STEPPING_PORT & ~STEP_MASK) | out_bits;
  #endif
  // Enable step pulse reset timer so that The Stepper Port Reset Interrupt can reset the signal after
  // exactly settings.pulse_microseconds microseconds, independent of the main Timer1 prescaler.
  TCNT2 = step_pulse_time; // Reload timer counter
  TCCR2B = (1<<CS21); // Begin timer2. Full speed, 1/8 prescaler

  busy = true;
  // Re-enable interrupts to allow ISR_TIMER2_OVERFLOW to trigger on-time and allow serial communications
  // regardless of time in this handler. The following code prepares the stepper driver for the next
  // step interrupt compare and will always finish before returning to the main program.
  sei();
  out_bits = out_bits0;

  // If there is no current block, attempt to pop one from the buffer
  if (current_block == NULL && !indep_mode) {
    // Anything in the buffer? If so, initialize next motion.
    current_block = plan_get_current_block();
    if (current_block != NULL) {
      if (sys.state == STATE_CYCLE) {
        // During feed hold, do not update rate and trap counter. Keep decelerating.
        st.trapezoid_adjusted_rate = current_block->initial_rate;
        set_step_events_per_minute(st.trapezoid_adjusted_rate); // Initialize cycles_per_step_event
        st.trapezoid_tick_cycle_counter = CYCLES_PER_ACCELERATION_TICK/2; // Start halfway for midpoint rule.
      }
      st.min_safe_rate = current_block->rate_delta + (current_block->rate_delta >> 1); // 1.5 x rate_delta
      st.counter_x = -(current_block->step_event_count >> 1);
      st.counter_y = st.counter_x;
      st.counter_z = st.counter_x;
      st.event_count = current_block->step_event_count;
      st.step_events_completed = 0;  
      out_bits0 = (out_bits0 & ~DIRECTION_MASK)
                  | ((current_block->direction_bits ^ settings.invert_mask) & DIRECTION_MASK);
      set_motion_state_block(current_block); // for hard limits
    } else {
      st_go_idle();
      bit_true(sys.execute,EXEC_CYCLE_STOP); // Flag main program for cycle end
    }    
  } 

  if (indep_mode) {
    indep_t_ptr it = indep_frame;
    while(it) {
      if(indep_increment(it)) {
        *(&st.counter_x + it->axis) += it->dpdt;
      }
      it = it->next_axis;
    }
  } else if (current_block != NULL) {
    st.counter_x += current_block->steps_x;
    st.counter_y += current_block->steps_y;
    st.counter_z += current_block->steps_z;
  }
  if (current_block != NULL || indep_mode) {
    // Execute step displacement profile by bresenham line algorithm
    // note: unnecessary to set direction here; it is set at new block & retained in out_bits0

    if (st.counter_x > 0) {
      out_bits ^= (1<<X_STEP_BIT);
      st.counter_x -= st.event_count;
      if ((out_bits^settings.invert_mask) & (1<<X_DIRECTION_BIT)) { sys.position[X_AXIS]--; }
      else { sys.position[X_AXIS]++; }
    }
    if (st.counter_y > 0) {
      out_bits ^= (1<<Y_STEP_BIT);
      st.counter_y -= st.event_count;
      if ((out_bits^settings.invert_mask) & (1<<Y_DIRECTION_BIT)) { sys.position[Y_AXIS]--; }
      else { sys.position[Y_AXIS]++; }
    }
    if (st.counter_z > 0) {
      out_bits ^= (1<<Z_STEP_BIT);
      st.counter_z -= st.event_count;
      if ((out_bits^settings.invert_mask) & (1<<Z_DIRECTION_BIT)) { sys.position[Z_AXIS]--; }
      else { sys.position[Z_AXIS]++; }
    }
    // check home & limit switches for hard limit situation 
    if(out_bits != out_bits0) { // if we are taking a motor step
      test_hard_limits();
    }
                   
    if(!indep_mode) {      
      st.step_events_completed++; // Iterate step events

      // While in block steps, check for de/ac-celeration events and execute them accordingly.
      if (st.step_events_completed < current_block->step_event_count) {
        if (sys.state == STATE_HOLD) {
          // Check for and execute feed hold by enforcing a steady deceleration from the moment of 
          // execution. The rate of deceleration is limited by rate_delta and will never decelerate
          // faster or slower than in normal operation. If the distance required for the feed hold 
          // deceleration spans more than one block, the initial rate of the following blocks are not
          // updated and deceleration is continued according to their corresponding rate_delta.
          // NOTE: The trapezoid tick cycle counter is not updated intentionally. This ensures that 
          // the deceleration is smooth regardless of where the feed hold is initiated and if the
          // deceleration distance spans multiple blocks.
          if ( iterate_trapezoid_cycle_counter() ) {                    
            // If deceleration complete, set system flags and shutdown steppers.
            if (st.trapezoid_adjusted_rate <= current_block->rate_delta) {
              // Just go idle. Do not NULL current block. The bresenham algorithm variables must
              // remain intact to ensure the stepper path is exactly the same. Feed hold is still
              // active and is released after the buffer has been reinitialized.
              st_go_idle();
              bit_true(sys.execute,EXEC_CYCLE_STOP); // Flag main program that feed hold is complete.
            } else {
              st.trapezoid_adjusted_rate -= current_block->rate_delta;
              set_step_events_per_minute(st.trapezoid_adjusted_rate);
            }      
          }
          
        } else {
          // The trapezoid generator always checks step event location to ensure de/ac-celerations are 
          // executed and terminated at exactly the right time. This helps prevent over/under-shooting
          // the target position and speed. 
          // NOTE: By increasing the ACCELERATION_TICKS_PER_SECOND in config.h, the resolution of the 
          // discrete velocity changes increase and accuracy can increase as well to a point. Numerical 
          // round-off errors can effect this, if set too high. This is important to note if a user has 
          // very high acceleration and/or feedrate requirements for their machine.
          if (st.step_events_completed < current_block->accelerate_until) {
            // Iterate cycle counter and check if speeds need to be increased.
            if ( iterate_trapezoid_cycle_counter() ) {
              st.trapezoid_adjusted_rate += current_block->rate_delta;
              if (st.trapezoid_adjusted_rate >= current_block->nominal_rate) {
                // Reached nominal rate a little early. Cruise at nominal rate until decelerate_after.
                st.trapezoid_adjusted_rate = current_block->nominal_rate;
              }
              set_step_events_per_minute(st.trapezoid_adjusted_rate);
            }
          } else if (st.step_events_completed >= current_block->decelerate_after) {
            // Reset trapezoid tick cycle counter to make sure that the deceleration is performed the
            // same every time. Reset to CYCLES_PER_ACCELERATION_TICK/2 to follow the midpoint rule for
            // an accurate approximation of the deceleration curve.
            if (st.step_events_completed == current_block-> decelerate_after) {
              st.trapezoid_tick_cycle_counter = CYCLES_PER_ACCELERATION_TICK/2;
            } else {
              // Iterate cycle counter and check if speeds need to be reduced.
              if ( iterate_trapezoid_cycle_counter() ) {  
                // NOTE: We will only do a full speed reduction if the result is more than the minimum safe 
                // rate, initialized in trapezoid reset as 1.5 x rate_delta. Otherwise, reduce the speed by
                // half increments until finished. The half increments are guaranteed not to exceed the 
                // CNC acceleration limits, because they will never be greater than rate_delta. This catches
                // small errors that might leave steps hanging after the last trapezoid tick or a very slow
                // step rate at the end of a full stop deceleration in certain situations. The half rate 
                // reductions should only be called once or twice per block and create a nice smooth 
                // end deceleration.
                if (st.trapezoid_adjusted_rate > st.min_safe_rate) {
                  st.trapezoid_adjusted_rate -= current_block->rate_delta;
                } else {
                  st.trapezoid_adjusted_rate >>= 1; // Bit shift divide by 2
                }
                if (st.trapezoid_adjusted_rate < current_block->final_rate) {
                  // Reached final rate a little early. Cruise to end of block at final rate.
                  st.trapezoid_adjusted_rate = current_block->final_rate;
                }
                set_step_events_per_minute(st.trapezoid_adjusted_rate);
              }
            }
          } else {
            // No accelerations. Make sure we cruise exactly at the nominal rate.
            if (st.trapezoid_adjusted_rate != current_block->nominal_rate) {
              st.trapezoid_adjusted_rate = current_block->nominal_rate;
              set_step_events_per_minute(st.trapezoid_adjusted_rate);
            }
          }
        }            
      } else {   
        // If current block is finished, reset pointer 
        current_block = NULL;
        plan_discard_current_block();
      }
    }
  }
  busy = false;
  PORTC &= ~(0x08); // diagnostic timing test
  
}

// This interrupt is set up by ISR_TIMER1_COMPAREA when it sets the motor port bits. It resets
// the motor port after a short period (settings.pulse_microseconds) completing one step cycle.
// NOTE: Interrupt collisions between the serial and stepper interrupts can cause delays by
// a few microseconds, if they execute right before one another. Not a big deal, but can
// cause issues at high step rates if another high frequency asynchronous interrupt is 
// added to Grbl.
ISR(TIMER2_OVF_vect)
{
  // Reset stepping pins (leave the direction pins)
  STEPPING_PORT = out_bits0; 
  TCCR2B = 0; // Disable Timer2 to prevent re-entering this interrupt when it's not needed. 
}

#ifdef STEP_PULSE_DELAY
  // This interrupt is used only when STEP_PULSE_DELAY is enabled. Here, the step pulse is
  // initiated after the STEP_PULSE_DELAY time period has elapsed. The ISR TIMER2_OVF interrupt
  // will then trigger after the appropriate settings.pulse_microseconds, as in normal operation.
  // The new timing between direction, step pulse, and step complete events are setup in the
  // st_wake_up() routine.
  ISR(TIMER2_COMPA_vect) 
  { 
    STEPPING_PORT = step_bits; // Begin step pulse.
  }
#endif

// Reset and clear stepper subsystem variables
void st_reset()
{
  memset(&st, 0, sizeof(st));
  set_step_events_per_minute(MINIMUM_STEPS_PER_MINUTE);
  current_block = NULL;
  busy = false;
  indep_mode=false;
  axes_moving = 0;
  disable_steppers();
}

// Initialize and start the stepper motor subsystem
void st_init()
{
  // Configure directions of interface pins
  STEPPING_DDR |= STEPPING_MASK;
  out_bits0 = (STEPPING_PORT & ~STEPPING_MASK) | settings.invert_mask;
  // nothing ever changes X_STEP_BIT, Y_STEP_BIT, Z_STEP_BIT locations in out_bits0
  STEPPING_PORT = out_bits0;
  STEPPERS_DISABLE_DDR |= STEPPERS_DISABLE_MASK;
  
  // for diagnostic timing test
  DDRC |= 0x08;
  
  // waveform generation = 0100 = CTC
  TCCR1B &= ~(1<<WGM13);
  TCCR1B |=  (1<<WGM12);
  TCCR1A &= ~(1<<WGM11); 
  TCCR1A &= ~(1<<WGM10);

  // output mode = 00 (disconnected)
  TCCR1A &= ~(3<<COM1A0); 
  TCCR1A &= ~(3<<COM1B0); 
	
  // Configure Timer 2
  TCCR2A = 0; // Normal operation
  TCCR2B = 0; // Disable timer until needed.
  TIMSK2 |= (1<<TOIE2); // Enable Timer2 Overflow interrupt     
  #ifdef STEP_PULSE_DELAY
    TIMSK2 |= (1<<OCIE2A); // Enable Timer2 Compare Match A interrupt
  #endif

  // Start in the idle state, but first wake up to check for keep steppers enabled option.
  enable_steppers();
  st_wake_up();
  st_go_idle();
}

// Configures the prescaler and ceiling of timer 1 to produce the given rate as accurately as possible.
// Returns the actual number of cycles per interrupt
static uint32_t config_step_timer(uint32_t cycles) // cycles = desired clock ticks per interrupt
{
  uint16_t ceiling;
  uint8_t prescaler;
  uint32_t actual_cycles;
  if (cycles <= 0xffffL) {
    ceiling = cycles;
    prescaler = 1; // prescaler: 0
    actual_cycles = ceiling;
  } else if (cycles <= 0x7ffffL) {
    ceiling = cycles >> 3;
    prescaler = 2; // prescaler: 8
    actual_cycles = ceiling * 8L;
  } else if (cycles <= 0x3fffffL) {
    ceiling =  cycles >> 6;
    prescaler = 3; // prescaler: 64
    actual_cycles = ceiling * 64L;
  } else if (cycles <= 0xffffffL) {
    ceiling =  (cycles >> 8);
    prescaler = 4; // prescaler: 256
    actual_cycles = ceiling * 256L;
  } else if (cycles <= 0x3ffffffL) {
    ceiling = (cycles >> 10);
    prescaler = 5; // prescaler: 1024
    actual_cycles = ceiling * 1024L;    
  } else {
    // Okay, that was slower than we actually go. Just set the slowest speed
    ceiling = 0xffff;
    prescaler = 5;
    actual_cycles = 0xffff * 1024;
  }
  // Set prescaler
  TCCR1B = (TCCR1B & ~(0x07<<CS10)) | (prescaler<<CS10);
  // Set ceiling
  OCR1A = ceiling;
  return(actual_cycles);
}

static void set_step_events_per_minute(uint32_t steps_per_minute) 
{
  if (steps_per_minute < MINIMUM_STEPS_PER_MINUTE) { steps_per_minute = MINIMUM_STEPS_PER_MINUTE; }
  st.cycles_per_step_event = config_step_timer((TICKS_PER_MICROSECOND*1000000*60)/steps_per_minute);
}

// Planner external interface to start stepper interrupt and execute the blocks in queue. Called
// by the main program functions: planner auto-start and run-time command execution.
// TODO: Update sys.cycle_start and feed_hold variables to a sys.state variable. This state
// variable will manage all of Grbl's processes and keep them separate.
void st_cycle_start() 
{
  if (sys.state == STATE_QUEUED) {
    sys.state = STATE_CYCLE;
    enable_steppers();
    st_wake_up();
  }
}

// Execute a feed hold with deceleration, only during cycle. Called by main program.
void st_feed_hold() 
{
  if (sys.state == STATE_CYCLE) {
    sys.state = STATE_HOLD;
    sys.auto_start = false; // Disable planner auto start upon feed hold.
  }
}

// Reinitializes the cycle plan and stepper system after a feed hold for a resume. Called by 
// runtime command execution in the main program, ensuring that the planner re-plans safely.
// NOTE: Bresenham algorithm variables are still maintained through both the planner and stepper
// cycle reinitializations. The stepper path should continue exactly as if nothing has happened.
// Only the planner de/ac-celerations profiles and stepper rates have been updated.
void st_cycle_reinitialize()
{
  if (current_block != NULL) {
    // Replan buffer from the feed hold stop location.
    plan_cycle_reinitialize(current_block->step_event_count - st.step_events_completed);
    // Update initial rate and timers after feed hold.
    st.trapezoid_adjusted_rate = 0; // Resumes from rest
    set_step_events_per_minute(st.trapezoid_adjusted_rate);
    st.trapezoid_tick_cycle_counter = CYCLES_PER_ACCELERATION_TICK/2; // Start halfway for midpoint rule.
    st.step_events_completed = 0;
    sys.state = STATE_QUEUED;
  } else {
    sys.state = STATE_IDLE;
  }
}