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?

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"?

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.


--
Found--the gene that causes belief in genetic determinism