Disable the timer and clear any pending bit, then setup the new counter
register values, then restart the timer.  This prevents a situation where
an old event fires just as we're about to load a new value into the timer,
when the start routine is called to change the time of the current event.

Also re-nest the parens properly for casting the result of converting
time and frequency to a count.  This doesn't actually change the result of
the calcs, but will some day prevent a loss-of-precision warning on the
assignment, if that warning gets enabled.