HP48G Clock Variability

[Jonathan Busby]

(02-10-2021 09:58 PM)Jonathan Busby Wrote:  

(02-06-2021 01:43 PM)TomC Wrote:  As we know, the processor clock of the HP48 has a bit of apparent variability. The hardware multiplies the installed 32.768kHz crystal to about 4MHz. (one finds this when doing the <ON><D>A test.

Has anybody studied or understood the source of this variability?

It is either due to:
1. The precision of the installed 32.768 kHz crystal.
2. The code that produces the apparent clock frequency.
3. The accuracy of the PLL that performs the multiplication of the crystal frequency.
4. Other source?

While I suspect it is 3, it would be nice if it is 1, then a replacement of the crystal would solve this variability.

TIA,
TomC

AFAIK, the variability comes from the PLL, although I'm not sure if this is some sort of "spread spectrum" feature or not. For the HP48G/G+/GX, the crystal oscillator is used as an input to the PLL which multiplies the crystal oscillator's 32768Hz frequency by 16, and then by 15 and finally divides the result by 2 to give a CPU frequency of about : 3932160Hz .

Regards,

Jonathan

Forgot to mention that the variability in the CPU clock most likely comes from the VCO ( Voltage Controlled Oscillator ) portion of the PLL, and *not* the crystal, although I may be wrong.

Regards,

Jonathan