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'High freq. external clock for RC or XT type'
1997\02\27@204803 by Zemin Liu

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Hi everyone,

I am designing a project which needs very high (>= 20MHz) running speed. I want
to use external clock to drive the RC or XT type PICs because of their easy
availability. Is there any problem in this method? Thank all those who may give
me advices.

Z. liu


'High freq. external clock for RC or XT type'
1997\03\03@131348 by mike
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In message  <spam_OUT9702280144.AA12689TakeThisOuTspamzsulink.zsu.edu.cn> .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
writes:
> Hi everyone,
>
> I am designing a project which needs very high (>= 20MHz) running speed. I
want
>  to use external clock to drive the RC or XT type PICs because of their easy
>  availability. Is there any problem in this method? Thank all those who may
give
>  me advices.
>

I know people who use the 16C5x range at 50MHz with an external
oscillator.

Their application had a limited temperature range and they were
prepared to select on test and throw away any pics that didn't
work. In practice, all the pics have worked at that frequency.

It is way outside the PIC spec, so you are on your own, but it
does work, reliably too.

Incidently, they pay more for their canned oscillator than they
do for the PICs.

Regards,


Mike Watson

1997\03\06@124334 by Zemin Liu

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>I know people who use the 16C5x range at 50MHz with an external
>oscillator.
>
>Their application had a limited temperature range and they were
>prepared to select on test and throw away any pics that didn't
>work. In practice, all the pics have worked at that frequency.
>
>It is way outside the PIC spec, so you are on your own, but it
>does work, reliably too.
>
>Incidently, they pay more for their canned oscillator than they
>do for the PICs.
>
>Regards,
>
>
>Mike Watson
>

Mike Watson,

Thank you for the reply.

One more thing. Can both RC type and XT type PICs work at 50MHz? It seems that
the RC or XT just denote their oscilator types, the PIC's cores are the same.
Am I right?

Thanks any one who gives me help.

Zemin Liu

1997\03\06@141937 by Osama ALASSIRY

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What happens if I use a 30MHz crystal with a 16c84 (10MHz max?)

of course it will be set to use a HS crystal...

will it work?
what will happen?
will it work with an external clock?
how accurate will the timing be?
...


thanks,
               Osama

_____________________________________________________
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1997\03\06@143424 by Byron A Jeff

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>
> What happens if I use a 30MHz crystal with a 16c84 (10MHz max?)
>
> of course it will be set to use a HS crystal...
>
> will it work?

Nope.

> what will happen?

It will not run.

> will it work with an external clock?

Nope.

> how accurate will the timing be?

Not accurate at all considering that it doesn't run.

I've found that the best I can do with 16C84's at room temp is about 16 Mhz.
That's with a crystal oscialltor as an external clock and HS clock setting.

The only hope you have of running at that speed is to use a EPROM based PIC.
Then be prepared to take a stack of them and throw out the ones that
don't work.

Sorry that you idea isn't going to fly.

BAJ

1997\03\06@150056 by Don McKenzie

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Byron A Jeff wrote:
{Quote hidden}

You just about stole the words out of my mouth BAJ.

I have found 16Mhz the limit.
12Mhz Crystal Oscillator will run most, 16Mhz some, however if you just
add a 10Meg resistor across just a crystal, 16Mhz may be possible.
I don't know how flakey the data eeprom in the 84 will be at these
speeds.

Don McKenzie  EraseMEdonspam_OUTspamTakeThisOuTdontronics.com   http://www.dontronics.com

Send a blank message to helpspamspam_OUTdontronics.com for more info.
SLI, the serial LCD that auto detects baud rates from 100 to 125K bps.
SimmStick(tm) A PIC proto PCB the size of a 30 pin Simm Memory Module.

1997\03\06@150106 by Kalle Pihlajasaari

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Hi Osama,

> What happens if I use a 30MHz crystal with a 16c84 (10MHz max?)
> of course it will be set to use a HS crystal...
> will it work?
> what will happen?
> will it work with an external clock?
> how accurate will the timing be?

Nothing
of course
No
Still nothing
within limits
about the same as you would expect normally


A typical 16C84 will start to ocillate in HS mode reliably upto
about 12 MHz I have found.  Driving it with an external ocillator
works usually upto about 16 NHz.  When you exceed the specific devices
tollerance to 'pain' it just stops doing anything usefull.

The numbers for a typical 16C5X are about 30MHz for a crystal and
50 MHz for an external occilator.

If you daisy chain PICs with one OSCout to the next OSCin you can
get to about 25MHz for a 16C5X device.  The ocillator in the 16C84 seems
able to buffer the signal at 12 or 16 MHz through to one more PIC
at least.  With multiple PICs in a chain you notice the buffering
weakness at about 25MHz for the 16C5X devices at teh 3rd device
in the sequence and if you look at the signals on a scope you can see that
the amplitude suffers above 20MHz.

You cannot decently use a PIC in /RC mode with a crystal or external ocillator.

You can typically program the same /RC PIC fuses for /HS or /XT mode to
make use of the 'fractionally' cheaper /RC device if you have
non-mission-critical uses for the PIC.  Also usefull if there is no /HS
parts in stock at your favourite supplier.

Cheers
--
Kalle Pihlajasaari   @spam@kalleKILLspamspamip.co.za   http://www.ip.co.za/ip
Interface Products   P O Box 15775, DOORNFONTEIN, 2028, South Africa
+ 27 (11) 402-7750   Fax: 402-7751    http://www.ip.co.za/people/kalle

DonTronics, Silicon Studio and Wirz Electronics uP Product Dealer

1997\03\06@152328 by Todd Peterson

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At 10:14 PM 3/6/97 +0300, you wrote:
>What happens if I use a 30MHz crystal with a 16c84 (10MHz max?)

I haven't been able to push the '84 past about 16MHz.  Past that and it
fails entirely.  of course, for a product for resale you would actually want
to stay right near the rated 10MHz spec.

It is possible, you know, to port '84 code to something like a 16C554; these
can be run up to 20MHz and I suspect some might operate at up to 30MHz, but
I'd hate to rely on that for a product.  Would 20MHz suffice?

Also, you might have better luck with the high speeds using a TTL oscillator
as opposed to a crystal.  I am not sure, but it might be worth a try.

-Todd



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1997\03\06@154420 by Vishram sarurkar

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On Thu, 6 Mar 1997, Osama ALASSIRY wrote:

> What happens if I use a 30MHz crystal with a 16c84 (10MHz max?)
>
> of course it will be set to use a HS crystal...
>
> will it work?
> what will happen?
> will it work with an external clock?
> how accurate will the timing be?
> ...
>
>
> thanks,
>                 Osama
>
> _____________________________________________________
> Osama ALASSIRY  KILLspamosamaKILLspamspamqatar.net.qa RemoveMEosamaTakeThisOuTspamalassiry.com
>                               http://www.alassiry.com
>
hello,
       why don't you just try it and find out what happens...
vishram.

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               |            Vishram A. Sarurkar           |
               |           ^^^^^^^^^^^^^^^^^^^^^          |
               |     another hobbyist turned researcher   |
               |   slogging @ Indian Institute of Science |
               |               -----------                |
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               |    Phone:+91 (080) 3092487.              |
               +------------------------------------------+

1997\03\06@155127 by Mike

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At 02:12 PM 6/03/97 -0600, you wrote:
>At 10:14 PM 3/6/97 +0300, you wrote:
>>What happens if I use a 30MHz crystal with a 16c84 (10MHz max?)
>
>I haven't been able to push the '84 past about 16MHz.  Past that and it
>fails entirely.  of course, for a product for resale you would actually want
>to stay right near the rated 10MHz spec.
>
>It is possible, you know, to port '84 code to something like a 16C554; these
>can be run up to 20MHz and I suspect some might operate at up to 30MHz, but
>I'd hate to rely on that for a product.  Would 20MHz suffice?
>
>Also, you might have better luck with the high speeds using a TTL oscillator
>as opposed to a crystal.  I am not sure, but it might be worth a try.
>-Todd

In the very old days of the MC1468705F2S which had a linear relationship between
clock frequency and voltage - we found we could proportionately up the frequency
at higher voltages.

A 5v regulator had a diode (pointing) to ground (cathode) so it would raise the
5v reg o/p to about 5.75 volts. This would give us a bit more by the ratio
5.75/4.75 times the clock - then we would select the best ones to go a bit
higher
still (Oh 4.75 was a better fit than 5.0v).
(PS we also put a 4K7 resistor between 5v o/p and anode of diode, which made the
o/p voltage a bit more steady at varying temperatures).

Funnily enough this also seemed to work on the MC68705P3S even though this was
an HMOS and not CMOS device - we could happily run this at 6.5v without any
failures - though it ran quite warm at 2 Mhz (It was a 5v device and very
robust).

Rgds

Mike

Anybody got internet via cable TV fibre optics - yet ?

Some say there is no magic but, all things begin with thought then it becomes
academic, then some poor slob works out a practical way to implement all that
theory, this is called Engineering - for most people another form of magic.
                                                                      Massen

1997\03\06@160524 by Mike
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>hello,
>        why don't you just try it and find out what happens...
>vishram.

Good and obvious question, though you were first :)

It seems to be easier to press buttons and ask questions than do it.

Before I got connected to internet I couldn't ask any questions and tried all
sorts of things myself - I am amazed sometimes at the questions I've seen
since I subscribed. Oh and I'm not having a go at the current originator of
this thread - just suggesting a more intelligent use of our gestalt.

Maybe simple things should really be tried first - then comments questions
posted after we have failed to reason things out for ourselves. This way
some of us might learn something instead of getting habituated to being spoon
fed and might save some exasperation on the more experienced members etc.

But I suppose there is always going to be an issue over just what is simple
in one persons mind - I think that if you've got the bits in your lab, then
just save us bandwidth and try the damn thing - then we can have an
intelligent discussion as to imrovements or explanations if it doesn't work
when we think it really should !

Perhaps I'm wrong - don't we all have meters, power supplies and Ohms Law ?

If we are lucky to have a CRO or logic analyser then our posts will be more
grounded in reality than conjecture and spoon fed reasoning.

Just my few electrons worth...

Rgds

Mike
Perth, Western Australia

Some say there is no magic but, all things begin with thought then it becomes
academic, then some poor slob works out a practical way to implement all that
theory, this is called Engineering - for most people another form of magic.
                                                                      Massen

1997\03\07@081034 by Osama ALASSIRY

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----------
> From: Mike <TakeThisOuTerazmusEraseMEspamspam_OUTWANTREE.COM.AU>
> To: RemoveMEPICLISTspamTakeThisOuTMITVMA.MIT.EDU
> Subject: Re: High freq. external clock for RC or XT type
> Date: 7 March 1997 0:02
>
> >hello,
> >        why don't you just try it and find out what happens...
> >vishram.
>
> Good and obvious question, though you were first :)
>
> It seems to be easier to press buttons and ask questions than do it.

actually it is much easier and cheaper, I am just a student, crystals are
expensive here and I thought I'd ask if it's possible (maybe somebody did
it before) before spending US$80 on a crystal.

1997\03\07@110404 by Mike

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>> It seems to be easier to press buttons and ask questions than do it.
>
>actually it is much easier and cheaper, I am just a student, crystals are
>expensive here and I thought I'd ask if it's possible (maybe somebody did
>it before) before spending US$80 on a crystal.

I've found lots of crystals and TTL oscillator types from old 286/386/486
motherboards - that sold at auction for $2 to $10 each. Also speak to some
of the repair companies - they throw a whole board out and can't be bothered
desoldering the crystals.

I've now got a collection of about 100 or so TTL oscillators from 8Mhz to
80Mhz with all sorts of unusual values in-between. SOme at 36Mhz which are
great if you are into overclocking a 33Mhz PC, these came from old HP-IB
equipment and a HP1000 interface board - the whole cabinet was $20 at auction.

Some more electrons resulting in matter...

Rgds

Mike

Some say there is no magic but, all things begin with thought then it becomes
academic, then some poor slob works out a practical way to implement all that
theory, this is called Engineering - for most people another form of magic.
                                                                      Massen

1997\03\07@110407 by Mike

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>> It seems to be easier to press buttons and ask questions than do it.
>
>actually it is much easier and cheaper, I am just a student, crystals are
>expensive here and I thought I'd ask if it's possible (maybe somebody did
>it before) before spending US$80 on a crystal.

Besides - who sells crystals at $80 when a normal 24Mhz to 30Mhz is around
$5 to $8 each ????

Rgds

Mike

Some say there is no magic but, all things begin with thought then it becomes
academic, then some poor slob works out a practical way to implement all that
theory, this is called Engineering - for most people another form of magic.
                                                                      Massen

1997\03\07@123330 by Martin McCormick

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       I am sure we all have our favorite components we like to look for,
but crystals, crystal oscillators, and connectors are some of the best
treasures I look for when scrounging surplus circuit boards at a ham fest.

       There are some applications in which any clock from 1 to 10 MHZ
will do.  If there are any time-critical actions, just calculate according
to your clock.  Obviously, if one is going to write a magazine article and
produce a schematic for others to duplicate, then a standard frequency needs
to be used, but the main thing is that the frequency source be stable and
just about any crystal will do that.

Martin McCormick

1997\03\07@224502 by lmclaren

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Osama ALASSIRY wrote:
>
> ----------
> > From: Mike <erazmusEraseMEspam.....WANTREE.COM.AU>
> > To: EraseMEPICLISTspamMITVMA.MIT.EDU
> > Subject: Re: High freq. external clock for RC or XT type
> > Date: 7 March 1997 0:02
> >
> > >hello,
> > >        why don't you just try it and find out what happens...
> > >vishram.
> >
> > Good and obvious question, though you were first :)
> >
> > It seems to be easier to press buttons and ask questions than do it.
>
> actually it is much easier and cheaper, I am just a student, crystals are
> expensive here and I thought I'd ask if it's possible (maybe somebody did
> it before) before spending US$80 on a crystal.


You can get 20Mhz xtals of old network cards (some have osc blocks and
others are crystals)

--
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