On Apr 19, 2007, at 7:37 AM, Steve McKown wrote:
H
In case you're wondering why it's set up this way: converting
between powers
of 2 needs only fast bit shifts. Far more efficient than, say,
dividing by
1000.
Actually, the reason has more to do with error and the effort/cost it
would take to handle it. If you want 1kHz rather than 1.024kHz units,
then you need to convert. Here's a modified version of a post I sent
to the EmStar list a while back, when someone suggested just doing
this ((timer * 1024) / 1000):
The issue is whether you want precise 1024Hz timers or imprecise
1000Hz timers. You can get reasonably precise 1000Hz timers, but only
if you are willing to do a bit of bookkeeping to keep track of
partial ticks.
E.g., imagine a user specifies a 999ms timer. The closest thing in
1024Hz (the underlying clock) is 1023 ticks. So your timer starts
firing every 1023/1024 ticks. Now imagine another component asks for
a 111ms timer. The closest thing is 114 ticks. But 114 * 9 = 1026;
the two timers will slowly move out of phase, although 999 mod 111 = 0.
Now imagine a frequency hopping scheme that (for some reason...) has
a period of 999ms and breaks it up into 9 slots of 111ms. A node that
is active for all slots and so fires a periodic timer every 111 ms
will have time progress differently than a node which is wake for one
slot and fires a timer every 999ms. Even if you could calibrate the
crystals and figure out the temperature-to-skew function (poster on
this at SenSys this year), you still have timer shifts that are
*dependent on the values you pass to the timer* rather than the nodes
themselves.
You can get rid of these imprecisions by tabulating the fact that
111ms is actually 123.64 ticks, and so keep track of partial ticks.
But for the complex cases, I would expect that this is enough cycles
that it slows down your maximum timer firing rate. And any tiny
errors will creep over time.
Besides interoperability with devices that operate with true
millisecond granularity, it's not clear to me what the benefit of
having a interval-1000 timer is, besides the comfort of thinking in
decimal rather than binary.
Phil
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