A fast-growing baby

Since I found out about the low-power PIC18LF2550, development of my FXO project took off. I sat quite a few hours to correct my schematic and then fix a PCB that I had already semi-prepared. Here, then, is the latest schematic:

You can click on the above image to get a more readable view. You cannot miss the big “UNTESTED” seal on the schematic. This is just to give a warning that the circuit has not yet been tested and thus I do not advise anyone to try it out. And here are the top and the bottom PCB views, both with the same warning:

So, my next steps were to export my BOM and to order the materials (I used Mouser, since I could not find any other store selling Silabs chips on retail). In the next few days I will also place an order for the prototype PCBs and when I get back home from holidays (yes, I am typing this post while on holidays), I will start with assembling my first three prototypes.

So, isn’t Open USB FXO a really fast-growing baby? I just hope it’s not too fast, that is, that I have not made any big mistakes in my haste. But what is prototyping for, if not for finding out exactly this? Reality will show anyway. So, I am leaving this post as it is now and getting back into exporting my Eagle files in Gerber format and ordering my PCBs. Updates may follow later on. Good luck!


8 Responses to “A fast-growing baby”

  1. Ricardo Landim Says:

    Do you thought use the fxs/fxo modules of Rowetel?


    Your board will be generic…. only usb+pic!

    • Angelos Varvitsiotis Says:

      Hi Ricardo,

      Thanks for your comment and for your suggestion. This would be a great idea, however it presents some difficulties.

      I have been inspired by David’s work and have looked extensively into his module boards. For the FXS board, the most-affecting factor is that the Vunreg voltage fed to the DC-DC converter on a FXS module from David is 12V instead of the 5V present in USB. This difference affects a number of components around the converter. In other words, I could have used the design from David’s FXS module, however I would have to populate the module board with different components and assemble it myself. This would not be very convenient, would it? One more factor that kept me off that path was bulk: if I were to design a generic USB+PIC board as a motherboard to plug David’s module daughterboards onto it, I would end up with something bulkier than the “dongle” form I have now.

      You are probably right in that one of David’s FXO modules could be plugged as-is into a “generic” PIC+USB board, since there should not be any voltage differences between the two — only I have not checked that thouroughly.

      Thanks again for the very constructive comment!



  2. Ricardo Landim Says:

    Hi Angelos,

    First, congratulations for the new son (openusbfxo). I am really thinking build this generic board. The FXO/FXS modules are very easy to match and buy. In China you can buy for less than 20 dollars. I never spoke directly with David, but maybe he can help us. (Vunreg and others details).

    Ricardo Landim

    • Angelos Varvitsiotis Says:

      Hi Ricardo,

      Sure, go ahead! In the future, I will try anyway to merge the FXS and FXO PIC firmware into a single piece of code, so this will match perfectly what you are thinking. In any case, please note that you will need to use the LF version of the PIC under 3.3V for your generic board.



  3. Ricardo Landim Says:

    HI Angelos,

    in Brazil LF version of the PIC is not easy to buy. I think that my board will have two voltages references. 5V for PIC and 3.3 for FXS/FXO board. 5V in USB and 3.3V using LM7833 or BAV99 (probaly BAV99, I am reading LM7833 datasheet now and the Vin min for this regulator is 5.8V). Thinking now, I am remembering of the Vunreg (12V DC-DC) for FXS.

    Today I am reading the source code of your device driver(openusbfxs). I’m learning a lot. I hope to help in any way.

    Ricardo Landim

    • Angelos Varvitsiotis Says:

      Hi Ricardo,

      You can always order an LF PIC from an electronics e-shop abroad. I use Mouser, but there are lots of others that should have it too. If you stick with 5V in the PIC, it will output all control signals of the SDI and PCM busses at 5V, and this will not interface with the Si3050 in the FXO module — pretty much the same discussion I had in earlier posts.



  4. Ricardo Landim Says:

    Thanks Angelo, I had not thought about… when you sent your board for manufacture? you now BatchPCB?

  5. Ricardo Landim Says:

    sorry… you know BatchPCB?

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