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PICList Thread
'Clock Accuracy'
1999\10\12@195823 by Thomas Brandon

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What sort of accuracy does one get with various clocks (not including RC)
for a PIC? For MIDI serial, the specs call for 1% timing accuracy. What sort
of difficulty will there be in getting a clock to this accuracy? Is there
any difference in accuracy (or other specs) between a resonator and a
crystal (or diff. types of crystal? What about higher accuracies? What is
the maximum (practical) achievable accuracy (i.e. in production, not like 1
in a 1000 will get it) for various High speed clocks? What about driving
multiple PICs at reliable high speed, anything good for this (be nice not to
have to redesign the circuit when another PIC (and hence load
resistance\capacitance\inductance) are added?

Thanks,
Tom.

1999\10\13@051409 by Russell McMahon

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>What sort of accuracy does one get with various clocks (not including RC)
>for a PIC? For MIDI serial, the specs call for 1% timing accuracy. What
sort
>of difficulty will there be in getting a clock to this accuracy?

A resonator will easily meet these specs.
Murata and others provide details on web.
Off the cuff resonators give you about 0.1% across temperature and crystals
are 10+ times better across temperature.
Try

http://www.murata.com/develop/index.htm



> Is there
>any difference in accuracy (or other specs) between a resonator and a
>crystal (or diff. types of crystal?

Cheapest available "microprocessor crystals" will be entirely adequate.
Resonators are (usually) cheaper and can eliminate 2 external capacitors.
Murata have had good feedback on this list as being responsive to customer
needs and understanding specific microprocessor oscillator circuit demands.


What about higher accuracies? What is
>the maximum (practical) achievable accuracy (i.e. in production, not like 1
>in a 1000 will get it) for various High speed clocks? What about driving
>multiple PICs at reliable high speed, anything good for this (be nice not
to
>have to redesign the circuit when another PIC (and hence load
>resistance\capacitance\inductance) are added?

You should be able to get essentially 100% compliance at this (low) accuracy
requirement.
What do you mean by "high speed"
Changes in crystal parameters can make a particular capacitor/microprocessor
combination suddenly not work. However, any crystal manufacturer worth their
salt will be able to guarantee to meet a prior spec given a sample and
circuit description.

Try Rakon for starters. AFAIK they were originally a NZ based firm but they
now supply some very smart crystal based products worldwide to some very
large users. There are many many more reputable suppliers.





     Russell McMahon
_____________________________

>From another world - http://www.easttimor.com

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-----Original Message-----
From: Thomas Brandon <spam_OUTtomTakeThisOuTspamPSY.UNSW.EDU.AU>
To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU <PICLISTspamKILLspamMITVMA.MIT.EDU>
Date: Wednesday, 13 October 1999 12:58
Subject: Clock Accuracy


>Thanks,
>Tom.
>

1999\10\13@125348 by Robin Abbott

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Thomas

Your question raises some interesting points. In any asynchronous serial
transfer such as RS232 the timing of an entire frame (byte) is derived from
the falling edge of a start bit. You need to consider that a 10% error in
clock might cause the first bit to be read 10% too soon - not a problem,
however by the tenth bit a 10% error from the first bit is now an entire bit
period, consider that both ends may use a clock with an error, and it's soon
clear that accuracy needs to be at least 2-3% for such a transfer to be
reliable with a safety margin.

In your case a resonator will be fine I think.

Robin Abbott - .....robin.abbottKILLspamspam.....dial.pipex.com

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* Forest Electronic Developments
* http://dspace.dial.pipex.com/robin.abbott/FED
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{Original Message removed}

1999\10\13@131028 by Wagner Lipnharski

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Robin Abbott wrote:
>
> Thomas
>
> Your question raises some interesting points. In any asynchronous serial
> transfer such as RS232 the timing of an entire frame (byte) is derived from
> the falling edge of a start bit. You need to consider that a 10% error in
> clock might cause the first bit to be read 10% too soon - not a problem,
> however by the tenth bit a 10% error from the first bit is now an entire bit
> period, consider that both ends may use a clock with an error, and it's soon
> clear that accuracy needs to be at least 2-3% for such a transfer to be
> reliable with a safety margin.
>
> In your case a resonator will be fine I think.

Just to clarify better the idea, take a look at:
http://www.ustr.net/8051pc/starting.htm
from the middle of the page, "Synchronization"... I put some graphs and
text about this possible "clock slack".

Wagner

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