Google Code project created

Long time no hear! This was because I had nothing new to report. I did nothing new with Open USB FXO and I did not have the slightest idea why my prototype failed to work.

I still do not have the slightest idea. However, in a private conversation with one of the blog readers, I was convinced that letting people take a look at my work could help in spotting the error in my design, whatever this error is (this statement makes the funny assumption that the number of errors my design accounts to just one).

Therefore, I created a google code project for Open USB FXO. It can be found at, and the sources can be browsed at

Please do not take the correctness of any of the uploaded files (schematics, boards and libraries) for granted. They are uploaded just for public scrutiny and review, in the hope that someone else can spot this error that I cannot see myself. Hopefully someone will.


14 Responses to “Google Code project created”

  1. Ricardo Landim Says:

    What kind of problems is happening?

  2. Angelos Varvitsiotis Says:

    Hi Ricardo,

    Yes, MAX88888 is regulated and will deliver a stable voltage. In contrast, the diodes in BAV99 have a voltage drop that depends on the current drawn by the circuit and the temperature. It should deliver a volage close to 3.5V, but this is not guaranteed.


  3. Ricardo Landim Says:

    Maybe you can use some developer kit of PIC. This will ensure that circuit is working fine and you will able to test some code.

  4. Aranga Nanayakkara (@a_chamara) Says:

    What about this project ?

    • Angelos Varvitsiotis Says:

      Hi Aranga,

      I have been inspired by David’s $10 ATA in the first place, when I started my Open USB FXS project (we are in Open USB FXO here). David’s line of thought, if I am not mistaken, is that one can get rid of the specialized-and-expensive telephony driver chips and do all the work in the microcontroller. I am not sure whether he has finished this project, but it is a very interesting project to watch.

      I have followed a more conventional approach: try to provide a USB-based interface using the same telephony driver chipsets as the modules of David’s IP04 and IP08 designs do. I have managed to get FXS working, and I have sort of got stuck with the FXO case.



  5. Telecommunication Blog Says:

    Where did you buy Silicon Lab IC’s? What vendor sites?

    • Angelos Varvitsiotis Says:

      Hi Marcello,

      I have been using mostly Mouser. They tend to have a stock of Silicon Labs chips.



  6. navaismo Says:

    Hi Angelos,

    This is a great great stuff, I was looking at your Job and checking your schematics. In your previous post you said that the PIC18LF2550 never start.

    In the past I played with the 2550 in CDC mode and found that the most of the issues are the capacitors in the XTAL, VUSB and the power supply. In VUSB I recommend to use a 470nF ceramic capacitor. You already are using 22pF in the XTAL so that’s ok.

    About the program, do you have basic routine to blink some LEDs? just to notice that the PIC’s program is up & running. Your change to the LDO chip must help a lot.

    Please continue with your great effort, If I can help you with something please let me know.

    • Angelos Varvitsiotis Says:

      Hi navaismo,

      It took me months to notice there was a comment from you. I apologize deeply. I have seen the same issues as you have with the 2550 in various prototypes of my FXS board. Halas, I have not managed to make the FXO board show even the slightest sign of life.



  7. David Griffith Says:

    Have you been able to attack the problem recently, perhaps using a different programmer?

    • Angelos Varvitsiotis Says:

      Hi David,

      I apologize deeply for taking ages to reply. No, I have not had any progress. It is not the fault of the programmer, this I can tell, because eventually the programmer worked and I had it compare the flash-stored program to that on the local computer file. It is either some weirdness of the PCB (because I had not managed to make the programmer “see” the PIC on *this* PCB) or some other error. Never had the time to look extensively into it though.



Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

%d bloggers like this: