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Adding a crystal to a 20b or 30b - Printable Version

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Adding a crystal to a 20b or 30b - Marcus von Cube, Germany - 05-11-2011

I'm not a hardware guy so I need advice here.

I want to add a 32768 Hz crystal to my 20b and my 30b. HP's schematics show a value of 15 nF for the capacitors while Atmel says something in the 10 pF range. Who is correct on the matter?

Edited: 11 May 2011, 4:24 a.m.


Re: Adding a crystal to a 20b or 30b - Eric Smith - 05-11-2011

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.


Re: Adding a crystal to a 20b or 30b - Eric Rechlin - 05-11-2011

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.


Re: Adding a crystal to a 20b or 30b - sylvandb - 05-11-2011

Quote:
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.

LOL

Or perhaps "crystal isn't working, we don't need it so leave it off the build and also leave off its capacitor."




Re: Adding a crystal to a 20b or 30b - success! - Marcus von Cube, Germany - 05-11-2011

Eric, so it was your page that led me to the right physical dimensions 0603. :-) Thanks for the info!

I bought 15pF capacitors. They are so tiny that I call them "Fliegenschiss". Soldering required a tiny 12V soldering "iron" (more of a needle), a magnifying glass (originally bought for my aging mother as a reading aid) and two attempts. If you solder the crystal, be careful not to place it outside the marked area. Mine came off when I reassembled the case. The second attempt was successful.

With the most recent wp34s build you can press ON+X (twice) to activate the crystal. If you do it without the mod, your calc needs a reset.


Re: Adding a crystal to a 20b or 30b - success! - Eric Rechlin - 05-12-2011

Quote:
Eric, so it was your page that led me to the right physical dimensions 0603. :-) Thanks for the info!

Glad to hear the information was useful!

Quote:
They are so tiny that I call them "Fliegenschiss".

Oh, I like soldering 0603 capacitors a lot more than 0402 capacitors. My first revision of the 50g serial cable used 0402s, and that was a real pain to solder without a stereoscope.

Quote:
With the most recent wp34s build you can press ON+X (twice) to activate the crystal. If you do it without the mod, your calc needs a reset.

Excellent! Looking forward to that.

Eric


Re: Adding a crystal to a 20b or 30b - BobVA - 05-17-2011

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.


Re: Adding a crystal to a 20b or 30b - Marcus von Cube, Germany - 05-18-2011

Quote:
Is the ON-X-X a one-way setting (i.e. not a toggle)?

Yes! The reason is easy: The chip allows to switch the oscillator exactly once after a reset. No way (and no reason) to go back. Likewise there is no way to check the presence of the crystal. If you select the function without a working oscillator, the device hangs. Even the LCD stops working as it's driven by the same oscillator. There is neither hope that the watchdog may kick in: It uses the same clock source.