Stratum one autonomy and assumptions about GPS

[Achim Gratz]

Hal Murray writes:
> Most CPU chips include a temperature sensor.

The temperature sensor for the CPU is close to useless unless you have
an SoC with the XO integrated.  It tells you a lot about the average
load of the CPU, but not much else.

> For Linux PCs, you need the coretemp module loaded.  The lm-sensors package 
> does that.

The motherboard temp sensor usually is located near the Southbridge if
there is still one or near the PoL converters.  Both these locations
don't necessarily measure a temperature that tells you anything useful
about the XO, either.  There are systems with more temp sensors, if
there's one that measures the cooling fan intake then that would be
useful I guess.

If we're talking about the rasPi, there's a not too expensive
temperature sensor from SiLabs (Si7053) with an I²C interface that gives
you one sample per second and has 14bit resolution.  The XO on the rasPi
is on the underside of the board, but it should be possible to glue that
sensor onto the XO with some thermal compound and get thin wires around
the board to the GPIO header with most cases.  You'd have to be careful
not to short something of course.  That'd give you the actual crystal
temperature, which might enable forward temp compensation (there's a
blog post somewhere from someone who's done that with x86 hardware, I
can't find that link quickly enough).  It seems that at least some rasPi
actually use an TCXO that is pretty good, so I'm not sure how much of an
improvement that would be for the typical indoor environment with
limited temperature swings.


Regards,
Achim.
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