> You haven't looked hard enough. Just about every CMOS clock circuit I've
seen
> has a feedback resistor across its input/output typically 1 meg to 10 meg.
This
> ensures a reliable startup. The PIC16f is a CMOS device with a high input
> impedance pin and a low output impedance pin (typical of CMOS clock
> generators). The data sheet for the 75T202 chip recommends a 10 meg
resistor.
> The Basic Micro http://www.basicmicro.com/  MBasic manual page 146
recommends a
> 10 meg resistor across the 16f628 using a resonator. The Motorola CMOS
handbook
> is infested with clock circuits with this feedback resistor. I have
designed
> equipment with CMOS logic in commercial/broadcast circuits that use this
> resistor. VDO specifies this resistor in their automotive applications in
their
> instrument clusters. The Piccon at http://www.qsl.net/n6bg/piccon/ shows a
1
> meg across the xtal. The Simon Bus radio alert receiver uses a PIC, 4mhz
xtal
> with a 10 meg across its clock. I could go on and on.....

But not one of these references are from Microchip specifically pertaining
to the PIC crystal driver circuit, which was topic of discussion.

Such resistors are often used as negative feedback to keep CMOS logic gates
in the "linear" region.  However, the PIC oscillator circuit was designed
specifically for that purpose and needs no such resistor.  Adding such a
resistor to the PIC crystal oscillator is merely superstition.  You might
just as well wave a dead fish over it during a full moon.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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