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10 pF is in the right ballpark. 15 nF is three orders of magnitude too much! I guess since they're not actually using it in the calculator they didn't double-check that they specified the right value on the schematic.
It is somewhat tricky to determine the optimal value to use because it depends on the capacitance characteristics of the chip, the crystal, and the PCB layout. 32kHz crystal oscillators can be particularly finicky compared to more traditional crystal oscillators (e.g., 4-20 MHz).
Edited: 11 May 2011, 6:29 a.m.
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HP's schematics are wrong.
In my personally-untested "20b hacker's kit" I include 18 pF capacitors, which are probably close enough, but nobody has reported success or failure with them.
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Eric:
I used your kit (with the 18 pF caps) and it seems to work fine on my 30b. I found the little strip of Kapton tape from the crystal package useful for sticking the crystal to the board before soldering it in. Thanks for making the kit available!
Marcus:
Only ein Flieger. :-) Is the ON-X-X a one-way setting (i.e. not a toggle)? That is, once it's selected the calculator uses the external oscillator unless it's reset? Thanks for including that function!
Regards,
Bob
Edited: 17 May 2011, 10:00 p.m.