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DPRG: 80C51FA for the New Micro's NMIY-0031 SBC (!!! or ???)

Subject: DPRG: 80C51FA for the New Micro's NMIY-0031 SBC (!!! or ???)
From: Eric B. Olsen robojoc at worldnet.att.net
Date: Mon Jul 21 22:10:48 CDT 1997

> Kipton Moravec wrote:
> > 
> > O.K. Found the extractor in my Digikey Catalog. Thanks Eric.  Am I going
> > to need an inserter too?  I will get the extractor on order in the
> > morning.  I guess the only way to get experience is to bust up a couple
> > in the learning process. :(

I insert the parts by hand.  

> > 
> > I found the crystal in the schematic they provided it is a standard
> > 11.0592 MHz as I expected.  What is curious for me is that while they
> > have the 22pF capacitors on both sides of the crystal going to ground
> > like the Intel book shows, they also have a 10 M Ohm resistor across the
> > crystal.  Has anyone seen that before, and why would you want to do
> > that?
> > 

The 10 M Ohm resistor is a parallel bias resistor commonly used in
crystal oscillator circuits.  Some manufacturers require them, some
don't;  I suspect some circuits need them, some don't.  The resistor is
also called a feedback resistor since it's connected from input to
output of the oscillator AMP.  It biases the amplifier at threshold,
allowing the oscillator to start-up.

Sometimes you'll find a series bias resistor (in series with the
crystal), this resistor is used to change the "feedback loop" as well,
i.e., to control phase-shift, add output impedance, and to match crystal
resistance correctly.  Also important are the different types of
oscillators designed using crystals in various modes. In my Intel data
book, the CHMOS versions of the 8051 employed a "positive reactance
oscillator" using the crystal in it's "fundamental response mode as an
inductive reactance in parallel resonance with capacitance external to
the crystal" !!!!  

Eric Olsen
Las Vegas, NV  


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