Steptimeout is used both inside and outside
of interrupts, and as such it needs special attention.
Specifically, the increment outside of the interrupt
context needs to occur when interrupts are disabled,
or a clear of the variable can be missed.
The variable was also made volatile. This is not strictly necessary
given the current code, but it is the more conservative approach
to dealing with the variable (and costs nothing in the current code).
Jim McGee
Michael Moon
Markus Amsler
Markus Hitter