HP48 Clock Variableness

[Christoph Giesselink]

(02-12-2021 07:38 PM)Jonathan Busby Wrote:  AFAIK, the system time ( and TIMER2, which controls the system time ) is directly controlled by the crystal oscillator and not by the PLL. I had thought that the crystal oscillator, from my experience at least, was stable, but I guess I was wrong. Do you have any specific measurements of the crystal's amount of deviation from its nominal frequency?

Regards,

Jonathan

Jonathan is right, the HP48 clock is driven by the 16384Hz TIMER2 directly derived from the 32768Hz crystal. A 32768Hz crystal is a low budget component available in many qualities. Quote: "a typical quartz clock or wristwatch will gain or lose 15 seconds per 30 days" from https://en.wikipedia.org/wiki/Quartz_clock#Accuracy.

I can remember of quartz wristwatches in the 90'ties where the owner replaced the orignal 32768Hz crystal in the hope of better accuray. The existence of the CLKADJ command shows directly, that HP knows about the quality and temperature stabilization of the used crystal.