homingnotes

1) in stepper always check homing_axis_lock to enable steps.  Rename axis_lock to axis_engage
   `st.step_outbits &= sys.axis_engage;`

Homing Setup:
   sys.limit_leaving = 0;
   travel = max travel in approprate direction.
   axis_engage = HOMING_CYCLE_N.
   Start.

In limit routine, On pin change:
    sys.limit_state = LIMIT_PORT^limit_invert
    
    //If the limit is on or we are leaving (but not both) Stop axis.  Never Start axis here:
    axis_engage &= ~(sys.limit_state^sys.limit_leaving)

     //check for real alarm
    if (state & !(sys.state & (STATE_ALARM|STATE_HOMING)) 
           mc_reset(). sys.execute |= (ALARM| CRIT_EVENT)

homing loop becomes:
  for cycles {
   do {
      st_prep_buffer();
      chheck reset()
      }
   while (sys.axis_engage & HOMING_CYCLE_N);  all bits are clear.

   if sys.limit.leaving: sys.limit_leaving = 0
   else  sys.limit_leaving = HOMING_CYCLE_N
}
  
Which may let us move it into the main loop instead of interrupting it.


 


1a) So initialize to all active axes
2) In limit routine, always toggle axis lock when limit state changes. according to rule below
 -- It would be lovely if the limit pins were in the same order as the step pins.
 but anyway:
    LIMIT high means on pin
    Start Homing.
      AxisLock = HOMING_AXES & LIMIT.  (only move the axes that are not homed).
    Interrupt: 
      -will only occur on change: 
      A) expected: Limit goes high
          If homing, AxisLock = (AxisLock ^ LIMIT) & HOMING_AXES.  
                1^1=0 0^1=1



HA LIM AL  -> AL
1  1   1   -> 0  #hit limit
1  1   0      0  #stay on limit
1  0   1      1  #approahing li,mit
1  0   0      0  #Limit released when not moving.  Don't start.
0  1   1      0 #stop on limit always
0  1   0      0 #"even if we weren't moving
0  0   1      1 # alllow to keep moving
0  0   0      0 # no change

AL = AL&~LIM.  OK.  Now: How do we modify to allow commanded stepoff

LE = LEAVING

LE LIM AL ANL -> AL
1  1   1  0 -> 1  #keep going away
1  1   0  0    0   #don't start
1  0   1  1    0 #STOP  
1  0   0  0    0  #stay stopped  
0  1   1  0    0  #stop on limit always
0  1   0  0    0  #"even if we weren't moving
0  0   1  1    1  # alllow to keep moving
0  0   0  0    0  # no change


AL = A&~LIM 

AL&=~(LIM^LE).   

If the limit is on or we are leaving (but not both) Stop A.  Never Start A.



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