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+This document describes Klipper's stepper phase adjusted endstop
+system. This functionality can improve the accuracy of traditional
+endstop switches. It is most useful when using a Trinamic stepper
+motor driver that has run-time configuration.
+
+A typical endstop switch has an accuracy of around 100 microns. (Each
+time an axis is homed the switch may trigger slightly earlier or
+slightly later.) Although this is a relatively small error, it can
+result in unwanted artifacts. In particular, this positional deviation
+may be noticeable when printing the first layer of an object. In
+contrast, typical stepper motors can obtain significantly higher
+precision.
+
+The stepper phase adjusted endstop mechanism can use the precision of
+the stepper motors to improve the precision of the endstop switches.
+When a stepper motor moves it cycles through a series of phases until
+in completes four "full steps". So, a stepper motor using 16
+micro-steps would have 64 phases and when moving in a positive
+direction it would cycle through phases: 0, 1, 2, ... 61, 62, 63, 0,
+1, 2, etc. Crucially, when the stepper motor is at a particular
+position on a linear rail it should always be at the same stepper
+phase. Thus, when a carriage triggers the endstop switch the stepper
+controlling that carriage should always be at the same stepper motor
+phase. Klipper's endstop phase system combines the stepper phase with
+the endstop trigger to improve the accuracy of the endstop.
+
+In order to use this functionality it is necessary to be able to
+identify the phase of the stepper motor. If one is using Trinamic
+TMC2130, TMC2208, or TMC2224 drivers in run-time configuration mode
+(ie, not stand-alone mode) then Klipper can query the stepper phase
+from the driver. (It is also possible to use this system on
+traditional stepper drivers if one can reliably reset the stepper
+drivers - see below for details.)
+
+Calibrating endstop phases
+==========================
+
+If using Trinamic stepper motor drivers with run-time configuration
+then one can calibrate the endstop phases using the
+ENDSTOP_PHASE_CALIBRATE command. Start by adding the following to the
+config file:
+```
+[endstop_phase]
+```
+
+Then RESTART the printer and run a `G28` command followed by an
+`ENDSTOP_PHASE_CALIBRATE` command. Then move the toolhead to a new
+location and run `G28` again. Try moving the toolhead to several
+different locations and rerun `G28` from each position. Run at least
+five `G28` commands.
+
+After performing the above, the `ENDSTOP_PHASE_CALIBRATE` command will
+often report the same (or nearly the same) phase for the stepper. This
+phase can be saved in the config file so that all future G28 commands
+use that phase. (So, in future homing operations, Klipper will obtain
+the same position even if the endstop triggers a little earlier or a
+little later.)
+
+To save the endstop phase for a particular stepper motor, run
+something like the following:
+```
+ENDSTOP_PHASE_CALIBRATE STEPPER=stepper_z
+```
+
+Run the above for all the steppers one wishes to save. Typically, one
+would use this on stepper_z for cartesian and corexy printers, and for
+stepper_a, stepper_b, and stepper_c on delta printers. Finally, run
+the following to update the configuration file with the data:
+```
+SAVE_CONFIG
+```
+
+Additional notes
+----------------
+
+* After calibrating the endstop phase, if the endstop is later moved
+  or adjusted then it will be necessary to recalibrate the endstop.
+  Remove the calibration data from the config file and rerun the steps
+  above.
+
+* In order to use this system the endstop must be accurate enough to
+  identify the stepper position within two "full steps". So, for
+  example, if a stepper is using 16 micro-steps with a step distance
+  of 0.005mm then the endstop must have an accuracy of at least
+  0.160mm. If one gets "Endstop stepper_z incorrect phase" type error
+  messages than in may be due to an endstop that is not sufficiently
+  accurate. If recalibration does not help then disable endstop phase
+  adjustments by removing them from the config file.
+
+* If one is using a traditional stepper controlled Z axis (as on a
+  cartesian or corexy printer) along with traditional bed leveling
+  screws then it is also possible to use this system to arrange for
+  each print layer to be performed on a "full step" boundary. To
+  enable this feature be sure the G-Code slicer is configured with a
+  layer height that is a multiple of a "full step", manually enable
+  the endstop_align_zero option in the endstop_phase config section
+  (see config/example-extras.cfg for further details), and then
+  re-level the bed screws.
+
+* It is possible to use this system with traditional (non-Trinamic)
+  stepper motor drivers. However, doing this requires making sure that
+  the stepper motor drivers are reset every time the micro-controller
+  is reset. (If the two are always reset together then Klipper can
+  determine the stepper phase by tracking the total number of steps it
+  has commanded the stepper to move.) Currently, the only way to do
+  this reliably is if both the micro-controller and stepper motor
+  drivers are powered solely from USB and that USB power is provided
+  from a host running on a Raspberry Pi. In this situation one can
+  specify an mcu config with "restart_method: rpi_usb" - that option
+  will arrange for the micro-controller to always be reset via a USB
+  power reset, which would arrange for both the micro-controller and
+  stepper motor drivers to be reset together. If using this mechanism,
+  one would then need to manually configure the "endstop_phase" config
+  sections (see config/example-extras.cfg for the details).