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'[PIC]:Significance of 19.6608MHz crystal'
2002\11\11@093952 by Jim Main

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I notice on the picdem.net board, a 19.6608MHz crystal is used.

What's the significance of this particular frequency?  It's readily
available from stockists.

Jim

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2002\11\11@095617 by Jan-Erik Soderholm

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Note that  19.6608 Mhz  is evenly dividable
with all powers of 2 up to (at least) 2**16.

E.g. 19.6608 Mhz  /  65536  =  300 Hz.

(This is not so for 20.0000 Mhz)

This can be very handly when writing such
things as timed loops or setting the values for the timers...

Best regards

Jan-Erik Svderholm
S:t Anna Data
tel : +46 121 42161
mob : +46 70 5241690

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2002\11\11@105922 by Alan B. Pearce

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>I notice on the picdem.net board, a 19.6608MHz crystal is used.

>What's the significance of this particular frequency?  It's readily
>available from stockists.

Often used for baud rate generation. It will divide nicely by powers of 2 to
get the required frequencies.

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2002\11\11@124906 by Donovan Parks

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If you are trying to find a crystal for baud rate generation consider a
14.7456 MHz crystal.  It will give you 0% error on all standard baud rates.
I persume this is the crystal speed used in PC UART though haven't verified
this.  The use of a 19.6608MHz crystal for baud rate generation works well
for speeds under 115200kbps (which is as high as most people will go).

Donovan

> >I notice on the picdem.net board, a 19.6608MHz crystal is used.
>
> >What's the significance of this particular frequency?  It's readily
> >available from stockists.
>
> Often used for baud rate generation. It will divide nicely by powers of 2
to
> get the required frequencies.
>
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>

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2002\11\11@125547 by Andrew Warren

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Donovan Parks <PICLISTspamspam_OUTmitvma.mit.edu> wrote:

> The use of a 19.6608MHz crystal for baud rate generation works
> well for speeds under 115200kbps (which is as high as most people
> will go).

   I agree; very few people will go faster than 115 megabits per
   second on a PIC.

   -Andrew

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2002\11\11@125959 by Dave Tweed

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"Alan B. Pearce" <RemoveMEA.B.PearceTakeThisOuTspamRL.AC.UK> wrote:
> >I notice on the picdem.net board, a 19.6608MHz crystal is used.
> >What's the significance of this particular frequency?  It's readily
> >available from stockists.
>
> Often used for baud rate generation. It will divide nicely by powers
> of 2 to get the required frequencies.

Except, of course, for the higher rates that have an additional factor
of 3 in them: 57.6k, 115.2k, 230.4k, etc. The highest standard rate you
can get with this crystal is 38.4k.

18.432 MHz is a better choice for higher data rates, or 22.1184 MHz if
you have a processor that can go that fast.

-- Dave Tweed

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2002\11\11@142248 by Peter L. Peres

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On Mon, 11 Nov 2002, Jim Main wrote:

*>I notice on the picdem.net board, a 19.6608MHz crystal is used.
*>
*>What's the significance of this particular frequency?  It's readily
*>available from stockists.

= 2^18 * 75

allows all the standard baud rates used in serial communications to be
derived from it using simple binary dividers (excepting the odd 45.5 etc
bauds).

Peter

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2002\11\11@142906 by Olin Lathrop

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> If you are trying to find a crystal for baud rate generation consider a
> 14.7456 MHz crystal.  It will give you 0% error on all standard baud
rates.

So does 18.432 MHz, which is the highest such frequency at 20MHz and
under.


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2002\11\11@143111 by Olin Lathrop

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>     I agree; very few people will go faster than 115 megabits per
>     second on a PIC.

None actually.  Although many might at 115.2 *kilo* bits per second..


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2002\11\11@143526 by Andrew Warren

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Olin Lathrop <EraseMEPICLISTspammitvma.mit.edu> wrote:

> >     I agree; very few people will go faster than 115 megabits per
> >     second on a PIC.
>
> None actually.  Although many might at 115.2 *kilo* bits per second..

   Yeah, that was sorta my point.

   -Andy

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2002\11\11@211051 by Donovan Parks

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Why do you say nobody will use speeds higher than 115.2Kbps?  Is it not
possible to run at higher frequency with a PIC?

I'm curious, because you meantioned that a 18.432 MHz gives 0% error for the
standard baud rates.  It does upto 230400bps, but has significan error at
460800bps.  Is 460800bps not a standard baud rate (I am basically assuming
it is because it is supported by hyperterminal which is what I am using).

Thanks,
Donovan


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2002\11\11@221816 by Herbert Graf

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Read carefully what was written, the original poster said: very few people
will go faster than 115 megabits per second on a PIC." Notice the "mega bits
per second"? Let me know if you've found a way to transfer more then 115Mbps
with a PIC. TTYL

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2002\11\12@015815 by Brendan Moran

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At 09:47 AM 11/11/2002 -0800, you wrote:
>If you are trying to find a crystal for baud rate generation consider a
>14.7456 MHz crystal.  It will give you 0% error on all standard baud rates.
>I persume this is the crystal speed used in PC UART though haven't verified
>this.

Pretty sure that the PC UART is a 1.8432MHz crystal.

--Brendan

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2002\11\12@073516 by Olin Lathrop

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> Why do you say nobody will use speeds higher than 115.2Kbps?  Is it not
> possible to run at higher frequency with a PIC?

Yes it is.  However, 115.2Kbaud is the highest speed you can pretty much
rely on the other end being able to do.  Also note that the common Maxxim
RS-232 converter chips and their knockoffs are only rated for 120Kbaud.

> I'm curious, because you meantioned that a 18.432 MHz gives 0% error for
the
> standard baud rates.  It does upto 230400bps, but has significan error
at
> 460800bps.  Is 460800bps not a standard baud rate (I am basically
assuming
> it is because it is supported by hyperterminal which is what I am
using).

I was considering the "standard" baud rates to be 115.2Kbaud and under,
although there is no such official standard.  I guess "nearly universally
supported" would have been a better term than "standard".


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2002\11\13@165940 by Peter L. Peres

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On Mon, 11 Nov 2002, Brendan Moran wrote:

*>At 09:47 AM 11/11/2002 -0800, you wrote:
*>>If you are trying to find a crystal for baud rate generation consider a
*>>14.7456 MHz crystal.  It will give you 0% error on all standard baud rates.
*>>I persume this is the crystal speed used in PC UART though haven't verified
*>>this.
*>
*>Pretty sure that the PC UART is a 1.8432MHz crystal.

No, but it arranged such that it looks as if it had such a crystal at
least while in normal uart mode.

Peter

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