In article ,
Jeff Liebermann wrote:

On Wed, 19 Aug 2009 13:15:30 -0700, David Nebenzahl
wrote:

On 8/19/2009 12:49 PM Jeff Liebermann spake thus:

Since 1981, I've looked inside literally hundreds of computahs and
SBC's. Not a single one has a tunable clock oscillator. One or two
used replaceable modular oscillators, which could pre purchased as a
TCXO, but which were usually supplied as a commodity clock oscillator.

So I wonder if the lowly SX28, one of my favorite little machines to
program (a PIC-like li'l guy) is an exception to this seeming rule?

Is that the Ubicom or Parallax SX28 processor? Dunno, I've never
worked with these. (Reminder: I are not a programmist).

I ask because, looking at the specs for this CPU, it has some
configuration bits (marked IRCTRIM0-2) that trim the internal RC
oscillator frequency, supposedly in steps of about 3%, up to a maximum
of +/- 8% (yeah, I know, doesn't add up, but whatever). Is this what you
would call a "tunable oscillator"?

I can't tell for su
http://www.parallax.com/dl/docs/prod/datast/SX20AC-SX28AC-Data-v1.6.pdf
See Section 9.0
I don't see any internal or external compensation for temperature
drift. It does have a real time clock, but again, no stabilization.
There is a section in the RC oscillator (FUSE register) which sets the
divider ratio from the RC oscillator. This is really a coarse
adjustment to set the divider ratio to generate an assortment of
frequencies between 31KHz and 4MHz. No way is it intended for fine
tuning for temp compensation.

These daze, the way to stabilize a TCXO is to first pre-age (beat-up)
the crystal to reduce long term drift. The crystal oscillator is then
characterized over the required temperature range. A table of
frequency versus temperature is generated and saved in a PROM. A PIC
controller on the oscillator takes the measured temperature, reads the
table, and applies the necessary correcting voltage to a varactor to
stabilize the oscillator over a very wide temp range. With this
method, you can take a really awful crystal, and compensate it to
impressive accuracies.

So presumably what I just described is a varactor built into the SX28.

I don't think so. I couldn't see such a feature on the data sheet.
Varactors are also chip real estate hogs, and would usually require
substantial documentation and explanation to impliment. I don't see
any of that in the data sheet.


I sorta blundered across this:
"NTP temperature compensation"
http://www.ijs.si/time/temp-compensation/

Putting a crystal in a temperature-stabilized "oven" is a well known
technique for generating a stable frequency (the telco folks and the
broadcast folks have been doing that for over 75 years, at least).

I have thought for a long time that it would be "neat" to glue a
resistor to the crystal case, and use heat to control the frequency.
You'd pulse-width modulate the power going to the resistor...

Isaac