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PICList Thread
'16c622A RC oscillator problems'
1999\09\10@124934 by Harold M Hallikainen

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       I've got a design that works with the 16c61, but when I move it
over to the 16c622A, the RC oscillator does not seem to run.  Here are
the details:

       1.  PIC running on +3V
       2.  100K from pin 16 to +3V, 330pF to ground.
       3.  Config word set as below:
2007 3FF3           00038         __CONFIG        _CP_OFF & _RC_OSC &
_PWRTE_ON & _WDT_OFF

       4.  Square wave visible on pin 15 (osc out) when using 16c61, but
pin sits low with 16c622A.
       5.  Tried reducing 100K down to 50K.  Same results.
       6.  With 50K, both 16c61 and 16c622A have 1Vpp triangle wave at
67 kHz.


       Any ideas, listmembers?  I REALLY appreciate the help!  This is
supposed to ship today!

Harold



Harold Hallikainen
spam_OUTharoldTakeThisOuTspamhallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

___________________________________________________________________
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1999\09\10@141408 by Barry King

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>         1.  PIC running on +3V
Is your 16C622 rated for 3V?  Not all PICs are.  Try temporarily at
5V, then you'll know it that's it.

>         3.  Config word set as below:
> 2007 3FF3           00038         __CONFIG        _CP_OFF & _RC_OSC &
> _PWRTE_ON & _WDT_OFF
Make SURE you've re-compiled correctly for the '622 target.  The
bits in the Config word may be different, and the address of the
Config word may be different.

>         4.  Square wave visible on pin 15 (osc out) when using
16c61, but
> pin sits low with 16c622A.

Which proves the osc is not working right- wrong config burn is the
most likely reason, IMHO.

-Barry
------------
Barry King, KA1NLH
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Hinesburg, Vermont, USA
http://www.nrgsystems.com

1999\09\10@155743 by William K. Borsum

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At 12:46 PM 9/10/99 EDT, you wrote:
>        I've got a design that works with the 16c61, but when I move it
>over to the 16c622A, the RC oscillator does not seem to run.  Here are
>the details:

Sometimes on the newer die shrink parts, there is not enough oomph in the
oscillator to start the thing going.  Had serious problems with the A, B, &
C revs of the 509 part--among others.  Our solution was to use a CARBON
COMPOSITION resistor in series--100K for the watch crystals we were using.
The carbon resistors are inherently noisy, and provided enough noise to
kick the thing into oscillation very reliably.  Don't know how this would
work with an RC oscillator, but it couldn't hurt.

Cheap fix and fast--should be able to get the parts locally, or in a day
from mouser or digikey.  Only available in through hole however.  Suggest
ordering a few of each value from about 20K to 150K so you can experiment.
kelly


William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems
<.....borsumKILLspamspam@spam@dascor.com> & <http://www.dascor.com>

1999\09\10@165617 by Harold M Hallikainen

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Thanks for the comments!  Reply comments and more info below...


On Fri, 10 Sep 1999 14:11:37 -0500 Barry King <barryspamKILLspamNRGSYSTEMS.COM>
writes:
>>         1.  PIC running on +3V
>Is your 16C622 rated for 3V?  Not all PICs are.  Try temporarily at
>5V, then you'll know it that's it.

       The 16C62X is rated 2.5V to 6.0V.  However, bumping the voltage
from 3.0 up to 4.5 DOES make the OSC2 pin run.  Note that at 2.5V, a
triangle wave can be seen on the OSC1 pin, which is what I would expect
with this oscillator circuit.  My GUESS is that the output of the schmitt
in the RC oscillator is not swinging far enough when the power supply is
3V to drive the following dividers.  I thought that maybe the pulse width
(during capacitor discharge) out of the oscillator was perhaps too short
for the divider, so I increased the capacitor size (decreasing frequency,
but also increasing the amount of time it takes the FET to discharge the
capacitor, increasing the pulse width coming out of the oscillator and
driving the dividers).  This, however, resulted in no change on OSC2.
So, it appears to me that it is indeed a voltage threshold problem
between the schmitt oscillator and the divider.  I do have Microchip
engineers checking into it for me, but I thought I'd do a little
"parallel processing" here and seek solutions from everywhere
simultaneously.

>
>>         3.  Config word set as below:
>> 2007 3FF3           00038         __CONFIG        _CP_OFF & _RC_OSC
>&
>> _PWRTE_ON & _WDT_OFF
>Make SURE you've re-compiled correctly for the '622 target.  The
>bits in the Config word may be different, and the address of the
>Config word may be different.

       Addresses and config contents are the same for the 61 and the
622.

>
>>         4.  Square wave visible on pin 15 (osc out) when using
>16c61, but
>> pin sits low with 16c622A.
>
>Which proves the osc is not working right- wrong config burn is the
>most likely reason, IMHO.
>

       As discussed above, the fact that there's a triangle on OSC1
indicates to me that the oscillator is RUNNING, but the dividers are not
(nothing on OSC2).
       I'm going to do some more measurements for Microchip on Monday.
We'll see what we come up with!  Maybe I need to go to a UJT oscillator!

       Again, thanks for the comments!

Harold



Harold Hallikainen
.....haroldKILLspamspam.....hallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

___________________________________________________________________
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Free software, free e-mail, and free Internet access for a month!
Try Juno Web: dl.http://www.juno.com/dynoget/tagj.

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