Re: [Discuss-gnuradio] USRP1 Inband firmware questions

[Sylvain Munaut]

Hi Eric,


>>  - How can I know the 'current' TX timestamp ? I want to send a ping
>> command, but I don't see a way to know what timestamp I should use.
> 
> It is in the header of every Rx packet.  If you need to send a command
> without knowing the current timestamp, use timestamp = 0, which will
> always execute immediately.  (There are some caveats which can usually
> be ignored)

Mmm, in the code I've read, the tx timestamp counter was different than
the rx timestamp counter and with different reset signals.


>>  - The timestamp seems imprecise: I often get a timestamp difference
>> between two packets of 24191 or 24193  while I should in fact get 24192
>> (I'm at 192 decimation). With such a high decimator, I can fix it up
>> myself, but still, that looks weird.
> 
> Which inband code are you using exactly?  

I'm looking at git://git.ettus.com/ettus/fpga.git which AFAIK it the
most 'official' repo I could find.


> My inband work (which was in
> the ets developer branch) fixed exactly that problem.  The root cause
> was that time-stamping was done after sample buffering, so the stamps
> would vary based on the buffer levels.
> 
> The new code is accurate and reflects the exact value of the 64MHz
> counter for the first sample in the packet.  There is an additional
> offset between rx/tx times at analog due to various signal processing
> stages before the time-stamping (which can vary depending on the FPGA
> configuration).  This value is easily compensated for on the host.

Nice.

In which git is that code ? And more importantly, why isn't it in the
main repo :)

As I'd like the inband function to become mainline in the future UHD
support, I think it's important to make sure the code is pushed.


Cheers,

    Sylvain