stepper: Support step_pulse_duration even on TMC drivers in UART/SPI mode

Commit 689231df changed the default step_pulse_duration to 100ns for
TMC drivers that are configured in UART/SPI mode, but it failed to
document that change in Config_Changes.md and Config_Reference.md.
Update those documents to reflect that change.

Update the code so that a user may specify a custom
step_pulse_duration even on TMC drivers that are configured in
UART/SPI mode.  This may be useful for drivers that use a level
shifter on the step pin that requires more than a ~300ns to detect a
level change.

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

1 files changed, 4 insertions, 3 deletions

diff --git a/docs/Config_Changes.md b/docs/Config_Changes.md
index e91ff465..11447d4d 100644
--- a/docs/Config_Changes.md
+++ b/docs/Config_Changes.md
@@ -16,9 +16,10 @@ use the older (typically less accurate) definition, define a custom
 `resistance1: 100000`, and `beta: 3950`.
 
 20211104: The "step pulse duration" option in "make menuconfig" has
-been removed. A new `step_pulse_duration` setting in the
-[stepper config section](Config_Reference.md#stepper) should be set
-for all steppers that need a custom pulse duration.
+been removed. The default step duration for TMC drivers configured in
+UART or SPI mode is now 100ns. A new `step_pulse_duration` setting in
+the [stepper config section](Config_Reference.md#stepper) should be
+set for all steppers that need a custom pulse duration.
 
 20211102: Several deprecated features have been removed.  The stepper
 `step_distance` option has been removed (deprecated on 20201222).  The