Bear with me here.  I have an idea for "fixing" the CPU clock drift problem
that may not require any software change.

What is the relative priority of the serial ports and the main timer?  I
seem to recall someone saying that the main clock was hardwired at the
lowest priority level.  Therefore a timing source on a serial port would be
more reliable than the main timer.

I see that there is an option PPS_SYNC available in 1.3.

Is it possible to set the baud rate of a serial port to 1 (that is 1
bit/second)?

Conclusion:  maybe a proper loopback plug combined with a kernel with both
options NTP
options PPS_SYNC
and a proper NTP and serial port configuration could be taught to keep
proper time?

FWIW I see that both are off in GENERIC kernels.  Perhaps this accounts for
why NTP doesn't work with the standard distribution (at least for me).

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h.b.hotz@jpl.nasa.gov, or hbhotz@oxy.edu