HP48 Clock Variableness

[Jonathan Busby]

(02-12-2021 06:12 PM)TomC Wrote:  So, as you may have guessed, my concern with the HP48 clock variability is how/why it affects the 'Human Read Clock' [HRC] - I am using this terminology/abbreviation to differentiate it from the '32,768Hz Clock (crystal)' - [32H], or the 'Microprocessor Clock' - [uP] (about 4MHz).

As we all have known, the HRC is not very stable - I believe it is most always slow; losing on the order of 10s of seconds a week. While this may be stable for most of us, I rely on this clock as an alarm, chime and it would be nice to have it a bit better. As we know, the HP48 internally keeps track of 'Ticks' (1/8192 per second), that are likely updated by the uP.

I have in the past measured various machines variance and set an alarm during the night that would apply a 'CLKADJ' command with an appropriately calculated factor that will help this. I have not been able to complete that analysis and plan on redoing it.

But this has gotten me wondering - what are the factors that may affect this lack of accuracy of the HRC?
1. How often the machine is not being used?
2. The clock display active?
3. Port memory? Amount of memory?
4. How often the machine is being used (and how-do memory accesses affect timing)?
5. Is this lack of accuracy constant?
6. Battery Status?
7. Is this level of accuracy related to the difference of the (diagnostic measured) clock speed from 4MHz? (which is always a bit less than 4MHz).

I plan to redo this 'experiment' again shortly - with a few machines, monitoring their accuracy over a week period, calculating the appropriate CLKADJ factor, applying and checking results.

Of course all of these machines are at Standard temperature, Humidity and Pressure, so we can rule that out.

Thank you all again for your thoughts/ideas,
TomC

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