wp34S timing crystal

I have succesfuly flashed a HP30b into a WP34s, but I'd like to improve the timing by adding a crystal. I've seen in some earlier posts that it takes a 32768 kHz crystal and 2 capacitors, but since they're SMD devices, I would like to make sure I get the right specs; does anyone have part numbers or specs or even better, a small 'howto' to make the modification?

Peter Mansvelder


Hi Peter,

I purchased the caps and the xtal from Digikey which succesfully added more accurate time function, here are the part details:

300-8340-1-ND .68000 1.36 T
HTSUS: 8541.60.0010 ECCN: EAR99

445-1272-1-ND .10000 .50 T
CAP CER 18PF 50V C0G 5% 0603
HTSUS: 8532.24.0020 ECCN: EAR99

Cheers, Kerem

Edited: 16 Jan 2012, 2:11 p.m.


Ok, thanks, I found the parts at farnell, they are on order.
Now on to another challenge: I got the overlay, but it is for the 3.0 version, and I haven't found an obvious way to get that image from the sourceforge site, I will try with the latest SVN revision...



Let us know how the crystal installation process goes.
A small "How to" would be welcome :-)

Edited: 17 Jan 2012, 3:56 p.m.


I'm afraid you will need to be able to solder grains of salt under a microscope. 0603 is definitely too tiny for the average soldering iron. I've done this trice and none of my attempts is worth a beauty contest price (even if all are working fine.)


You are absolutely correct. Magnifying lenses and good lighting is a must. I have done these four times by now (2x30b,2x20b). What I found works the best and cleanest is to Tin the contact pads with enough solder (using a low power 8W-12W soldering iron with a fine tip) for attaching 0603 SMD components (holding them with a very fine tipped tweezers) without having to add more solder to the tip of the soldering iron. Also it is important to make sure the tip is cleaned with a tip cleaner and has no excess solder or kind of residues.


I live here in Brazil it is impossible to purchase these components can anyone help me?


I can get the components you need and send them to you, if this helps. You can contact me at kkapkin (at) gmail (dot) com


Do you have a brand/model/source suggestion for the soldering iron? (I did search at digikey and amazon, but there are lots of choices.
This one looked most promising, but is 18 Watts and does not look like it has a "needle" tip.)

Edited: 18 Jan 2012, 10:29 a.m.


I'm using a low voltage (12V) soldering 'needle' which has roughly the overall size of a pen. I drive it off my lab power supply.


So far I have come across no such animal in my search.


Go to a Radio Shack or similar! I found mine in a local electronics store.

Maybe this link can help: http://hardforum.com/showthread.php?t=816830.

Edited: 18 Jan 2012, 11:26 a.m.


OK, I just ordered a soldering station, 10 crystals and 20 capacitors*, so I should be able to ruin one of my 30b calculators attempting this in a week or so! (Actually, I think I'll practice on at least one 20b.)

* - ordered 10 crystals because it was something like $10 for 5 or $12 for 10. Ordered 20 capacitors because you need 2 capacitors per crystal and they were dirt cheap.

Edited: 18 Jan 2012, 1:10 p.m.


Hi Kerem,

I have been researching how to do this(not much information out there) and I found this document.

How WP 34S Came into Existence

Page 18 of this pdf shows the crystal and cap(s) c3 and c4 and you say caps. Are 2 caps required for this modification? I haven't opened my 30b yet so I'm not sure what is there and what is not.

Thank you,


Marcus' suggestion of a soldering "needle" would the best, but unfortunately I didn't have one. Instead I used an old low power radio shack soldering iron similar to this.
Also I used a fine tip similar to this one.

If you go to RadioShack or a local electronic store you will find many choices for low power soldering irons (pens) with fine tips.

Make sure your hands are steady, no coffee or redbull before you begin soldering. Try to do it the right the first time, comes out cleaner, longer time you spend gets messier based on my experience. Also you can get blank, cheap boards with copper contacts from the same electronics store or RadioShack to practice first.

Edited: 18 Jan 2012, 1:19 p.m.


Hi Christopher,

You are correct. These are the two (2) capacitors which are placed on C3 and C4 on the left and right side of the Crystal.

In order to open the 30b there are four screws. Two of the screws are under the long lower rubber pad. You don't have to remove the entire lower pad to get access to these screws, just lift the ends of the rubber pad enough to expose the screws. That way, once you are done the pad still retains its double sided glue and sticks very well back into its place.



You don't need a fancy soldering iron. You could do it with a $10 Radio Shack one if you wanted.

That said, I have a Metcal (OKi) PS-900 and I love it.


Edited: 18 Jan 2012, 3:07 p.m.


You can make a soldering needle: take your fine tip and use a file to thin it down some more. You will have to re-tin it. I have done that with one of those cheap Radio Shack irons.

You could probably also literally use a needle - use some wire to wrap it to the existing tip. You need to make sure there is adequate heat conduction to the needle - this might actually be good: you will effectively have a lower wattage tip (and, again, you will have to tin it somehow - you might want to file off whatever coating might be on the needle originally).


Very Nice!! Those cheap RadioShack irons last very long time, they won't fail to justify getting a new iron like Metcal PS-900. That what I should have gotton in the first place. Let's see soldering a few more WP-34s, time module, pil-box,.. may be I need an upgrade.


I did not order anything fancy. I needed a new one anyway, and prefer clicking on my computer to walking into a Radio Shack.


Hello Kerem,
I managed to get the parts and solder them onto the HP30B, it is rather simple, here are my findings:

  • Backup up the calculator using ON + Store + Store
  • Take off the battery cover and take out the batteries
  • Take out the 5 screws: 2 on top, one in the middle, 2 under the 'antislip' strip at the bottom of the backside (outside the battery cover)
  • flip the calculator over and pry off one of the sides by slightly bending the plastic outer housing, the center 'metallic' part has to come loose. First one side, then the top, then the other side. Actually this is so tight that I wonder where the screws are for...
  • Now you have the complete calculator front with the back of the PCB visible.
  • Top right you see a place for 3 components, marked C3, C4 and Y1. Each one has 2 copper-coloured pads.
  • Take the smallest soldering iron you can find (I use a Weller 12W with a pointy tip) and put some solder on them.
  • Take the 2 capacitors (I used 18pF ones in 0603 size, farnell article code 1612182), and solder them into place, using tweezers, a needle and a magnifying glass. It takes some practice, but it is not impossible: this is only my second time, and I haven't fouled up anything yet.
  • I used a SMD crystal too (farnell part number 1712821), but there is enough room for a regular 'tubular' watch type crystal, which is much easier. Same method, be careful not to let any solder flow too wide.
  • If there is any excess solder, clean it with a needle and some tissue (or better, use desoldering tape). I used regular solder, which is much too big for SMD use, so my work looked somewhat sloppy. But no matter, it worked!
  • Put the calculator back in reverse order, and try to set the crystal: ON+C+C. If the crystal is mounted correctly, you get 'OK' and you now have a much improved WP34s! In my example, there used to be almost 10 minutes to the hour discrepancy in time keeping, but now it runs like a clock!
Hope my findings help someone, it is really an easy job anybody with some electronics experience can succceed at.

Peter Mansvelder


Hello Peter,

Very good summary, thanks for sharing.

Actually Tubular crystals also come in SMD package (which I used and I believe most people have been using) with slightly open leads with a proper angle to allow surface mount on SMD pads, as well as straight leads for through-hole.

AFAIK You are the first/only person reported using non-cylindrical SMD packaging for 30b/20b, great information, thanks. I was wondering if it would work or fit on the space allocated for the crystal on the PCB.

Although you need to do ON+C+C once to get the crystal engaged, I sometimes forget to do the ON+C+C after each firmware update. Firmware updates disengages the crystal timing circuit.




Thank you for the report. It gives me some confidence that I will be able to add the crystal. I received the parts two days ago, and the capacitors do look somewhat like grains of sand. Speaking of the capacitors, I purchased 15 pF, I assume that will not be a problem since the crystal calls for 12.5 pF and you used 18 pF. Now all I need is my new soldering iron, and working up some nerve to make the attempt.


18pf load capacitance is a bit high in my experience in using 32.768KHz crystals. Most crystal manufacturers recommend somewhere between 10pf and 12pf. The 18pf will work, you just don't want to load down the oscillator circuit too much.


Some research on getting the best accuracy from a 32.768 KHz ATMEL AT91SAM7L128 ARM Processor timing circuit suggested that I should use 18pF capacitors. The design specifications noted in this document from ATMEL, recommends 18pF capacitors C12 and C13 shown in the schematics. Therefore I selected 18pF.


Firmware updates disengages the crystal timing circuit.

There is no way for the software to tell if a crystal is installed or not. The only way is to just set-up the hardware and see what happens. If no crystal is installed, the system simply hangs and can only be reset manually. :-(

15pF do work. I've modified three devices this way.



There is no way for the software to tell if a crystal is installed or not. The only way is to just set-up the hardware and see what happens. If no crystal is installed, the system simply hangs and can only be reset manually. :-(

Couldn't the calculator write something to flash memory to say the crystal is enabled, and read it back after an update?


Couldn't the calculator write something to flash memory to say the crystal is enabled, and read it back after an update?

That's of course possible but updates are often incompatible with previous backup images (even if the CRC is still valid). Now imagine the calculator tries to restore such an invalid image and inadvertently tries to activate the (non existent) crystal just because the respective bit is left over from an older software image. It will be a dead end.

Not a big deal, I just have to remind myself to ON+C+C after the firmware updates. For those who are just installing their capacitors and the crystal for the timing circuit, alhough it is documented in the user manual, ON+C+C after updates ;)

The only thing (which is a nice to have more than a need, especially given valuable limited memory space for the capabilities), is to have a time/date display like the one on HP-17BII+.

Edited: 23 Jan 2012, 4:25 p.m.


No one said they didn't. People are just quoting their various references for their choice of capacitor. For that matter probably anything from 10pf to 20pf will do. Some crystals even recommend as low as 6pf. Then we haven't even started talking about tolerance, temperature stability and drift (which all have a cumulative effect on the capacitance).

Personally, I'd go with the value recommended by the crystal being used. Chances are the Atmel designer just specified the value for the crytal they were using or had the datasheet for (but remember to calulate the required CL as detailed below).

AT91SAM7L128/64 datasheet, paragraph 35.4.3, table 35-15 only states a maximum of 20pf.

This application note gives more info on the design for the xtal and the internal equivalent CL.

Remeber too that

CLtotal=(CL1*CL2)/(CL1+CL2) + CLi

where CL1 and CL2 are the two load capacitors (C3 & C4 on the 20b/30b board) and CLi is the equivalent internal load capacitance of the uP.

Edited: 24 Jan 2012, 9:46 a.m.

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