[Elecraft] K3 RTC accuracy

[Carl Clawson]

Oooohh this brings back painful memories, but thankfully they are somewhat
vague after a couple decades. MS-DOS could show similar problems because 1)
it only read the (fairly accurate) RTC at boot and used the much less
accurate CPU clock to keep time thereafter, and 2) if you used the clock
interrupt for your own purposes -- driving stepper motors in our case -- you
could lose additional clock time if you dawdled too much. We dawdled -- a
lot. Eventually there came available aftermarket clock drivers that would
read the RTC for everything and not depend on an inaccurate and potentially
unreliable clock. I think we ended up writing our own driver after first
discovering several other ultimately flawed "solutions".

A typical spec for a CPU clock used to be 0.01%, but that would only have
you off by at worst about 9 seconds per day. This is still far worse than a
typical quartz watch which is good to more like one second a week.

73, Carl WS7L