stepper: Change default max_error from 50us to 25us

Change the default compression error window (max_error) from 50us to
25us - it's common for stepper motor drivers to have 30us for their
"pwm fixed off time" and it would be good for the steps to be
scheduled within that time.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/Features.md

@@ -7,12 +7,11 @@ compelling features:
   stepper motor, it compresses those events, transmits them to the
   micro-controller, and then the micro-controller executes each event
   at the requested time.  Each stepper event is scheduled with a
-  precision of no less than 50 micro-seconds. The software does not
-  use kinematic estimations (such as the Bresenham algorithm) -
-  instead it calculates precise step times based on the physics of
-  acceleration and the physics of the machine kinematics. More precise
-  stepper movement translates to quieter and more stable printer
-  operation.
+  precision of 25 micro-seconds or better. The software does not use
+  kinematic estimations (such as the Bresenham algorithm) - instead it
+  calculates precise step times based on the physics of acceleration
+  and the physics of the machine kinematics. More precise stepper
+  movement translates to quieter and more stable printer operation.
 
 * Best in class performance. Klipper is able to achieve high stepping
   rates on both new and old micro-controllers. Even an old 8bit AVR

klippy/stepper.py

@@ -51,7 +51,7 @@ class PrinterStepper:
         self.min_stop_interval = (math.sqrt(3.*inv_max_step_accel + jc**2)
                                   - math.sqrt(inv_max_step_accel + jc**2))
     def build_config(self):
-        max_error = self.config.getfloat('max_error', 0.000050)
+        max_error = self.config.getfloat('max_error', 0.000025)
         step_pin = self.config.get('step_pin')
         dir_pin = self.config.get('dir_pin')
         min_stop_interval = max(0., self.min_stop_interval - max_error)