UmTRX aka OpenBTS HW update

Robin Coxe coxe at
Wed Oct 5 13:50:12 UTC 2011

Hi Alexander.  I took a look at the schematic last night.  Protel is hard
for me to read, probably because I'm not used to it.  Overall, the board
looks very nice.  I have a couple of questions/comments:

1) Is there a particular reason why there's an external SRAM on the board?
I notice that there is one on the Ettus N210 board as well, but do guys have
a particular purpose in mind for it?   The Spartan-6 FPGA has ample on-board
Block RAM resources. If it's not strictly necessary, it would remove an ~$10
part from the BOM.

2) Maybe it was just because it was really late, but I couldn't figure out
what was happening to the 1 PPS GPS timing pulse.   How are you planning to
use GPS information to discipline the 26 MHz oscillator?

3) One general recommendation:  add lots of test points, particularly around
the DC power sources, clock signals, and digital signals of interest going
to and from the FPGA.  Probing BGAs is a pain without them.  Also,
strategically placed ground test posts that you can use to clip on a scope
probe ground connection will make debugging much easier.

4) 6.5 V is kind of a weird voltage.  I'd be inclined to go with a 12V
connection to an external power source, which would enable the system to be
powered off a car battery, but this point is a matter of opinion more than
anything else.


On Tue, Oct 4, 2011 at 5:45 PM, Alexander Chemeris <
alexander.chemeris at> wrote:

> Hi all,
> Attached is a new version of our schematics and PCB layout (note, they
> may differ a bit with schematics being more recent).
>  Btw, I found diffpdf tool very useful for schematics comparison:
> It nicely highlights changes, so you don't miss something.
> Our current plan for hardware prototyping is like this:
> 1. Most important parts of PCB layout should be done by the end of this
> week
> 2. Then we have 1 week (maximum!) to do any small changes to layout we
> may want and fix bugs we (hopefully) find.
> 3. Then 1 week for the final layout polishing, adding labels, etc.
> 4. Final ACK for the layout and we push it to the fab for printing.
> Manufacturing and delivery to Moscow will take 2-3 weeks.
> 5. 1 week for assembly
> This is about 5-7 weeks and then we will have a lot of fun with
> hardware and software debug.
> Robin, do you want to get an assembled board to participate in the
> hardware debug session? :)
> Btw, could you get a quote for the FPGA from Xilinx for 100pcs?
> --
> Regards,
> Alexander Chemeris.

Robin Coxe  |  Close-Haul Communications, Inc.  |  Boston, MA
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