Subject:
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Re: Lego Network Protocol questions
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Newsgroups:
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lugnet.robotics
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Date:
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Thu, 6 May 1999 21:34:30 GMT
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Original-From:
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Ben Laurie <ben@algroup*StopSpam*.co.uk>
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Viewed:
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2865 times
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Kekoa Proudfoot wrote:
>
> Ben Laurie <ben@algroup.co.uk> wrote:
> > Sure, but you should be able to get the edge of the stop bit(s) to
> > within 1/16th bit+interrupt latency (which I'd guess is tiny in
> > comparison to the bit timing) which gives you the position of any other
> > bit just as accurately (give or take clock drift - but that should be
> > tiny). So, allowing a whole bit for slop is generous, IMO. This should
> > make a slotted protocol quite achievable and efficient. Some fun in
> > getting started, but after that, no problem.
>
> I was under the impression you wanted to sync a clock to the rhythm of bit
> arrivals. You cannot do this.
That was someone else. I came in in the middle :-)
> You can definitely sync a clock to the
> rhythm of byte arrivals. I was trying to point out this distinction to
> you. You must have been assuming measurement of the byte arrival times
> even though you were talking exclusively about bits. No matter, we were
> talking about the same thing then.
See above.
> You mention measurement accuracy to 1/16th bit + interrupt latency, and
> that interrupt latency was small in comparison. Certainly interrupt
> latency is sub-millisecond. The system timer has a resolution of 1 ms, but
> the underlying H8 timer has resolution to 1/500 ms. The accuracy you will
> be able to get will fall somewhere in there. 1 ms accuracy is pretty good,
> I think.
>
> When I said that the latency would be unreliable, it was because the OCIA
> interrupt (used for scheduling, motor control, etc.) has priority over the
> RXI interrupt. Timing might be accurate most of the time, but in the rare
> case when the OCIA interrupt handler is running when a byte is received,
> you will get a bogus measurement, which you may or may not be able to
> detect given an expected maximum amount of clock skew.
Will it be big enough to matter in the slotted protocol context?
> All of this does not matter much. I agree with you that a slotted protocol
> is possible to implement. If messages are sent at 4800 baud using e.g. a
> three byte 55 ff 00 leader, 8 data bytes with complements, plus checksum
> with complement, the message adds up to 23 bytes and something less than 5
> ms; assuming a sender always sends something at the start of its slot, I
> bet you can find a way to keep clocks enough in sync to get away with (in
> this example) 5 or 6 ms slots no problem.
No problem at all, I'd guess.
Cheers,
Ben.
--
http://www.apache-ssl.org/ben.html
"My grandfather once told me that there are two kinds of people: those
who work and those who take the credit. He told me to try to be in the
first group; there was less competition there."
- Indira Gandhi
--
Did you check the web site first?: http://www.crynwr.com/lego-robotics
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Message is in Reply To:
 | | Re: Lego Network Protocol questions
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| (...) I was under the impression you wanted to sync a clock to the rhythm of bit arrivals. You cannot do this. You can definitely sync a clock to the rhythm of byte arrivals. I was trying to point out this distinction to you. You must have been (...) (27 years ago, 6-May-99, to lugnet.robotics)
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