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Re: [Discuss-gnuradio] FPGA time

From: Anisha Gorur
Subject: Re: [Discuss-gnuradio] FPGA time
Date: Tue, 28 Aug 2012 18:24:10 -0400

Sorry for the confusion. We are trying to synchronize 3 usrps for collect. The devices seem to be time aligned in that the samples are timestamped with the same metadata, so we believed that synchronization had been achieved. However, when the data collected from the usrps was correlated, samples that should have had 0 time difference of arrival were off by as many as 5 samples, at a sample rate of 6.25MS/S. So even though the timestamps are time aligned, the data does not seem to be. The devices have been synchronized PPS times, not uspr.get_time_now().
Thank you,

On Tue, Aug 28, 2012 at 5:40 PM, Josh Blum <address@hidden> wrote:

On 08/28/2012 01:39 PM, Anisha Gorur wrote:
> Thats actually what we are using, however, when printing out the tick
> count, we randomly, and only sometimes, see a jump of about 30 ticks, and
> if each tick is 10ns, that is a huge 300ns jump. Do you know why this could
> be happening?

I'm very lost. Where are you getting the time_spec from? A receive
packet? usrp.get_time_now()? Or is this a time difference between devices?

get_time_now is completely asynchronous to the device, if thats what you
are calling in a loop, I would expect to see a lot of variation in time


> We are using gpsdo's as both a time and clock reference, though this
> problem seems to occur even if we set the clock and time reference to
> external. However, setting them to gpsdo seems to make it more stable, as
> we have seen the ~30 tick jump fewer times with those references. Another
> question is why would there be a big difference in what the reference is
> set as when either way, supposedly the internal GPSDO is being used.
> Thanks,
> On Tue, Aug 28, 2012 at 4:13 PM, Josh Blum <address@hidden> wrote:
>> On 08/28/2012 12:03 PM, Anisha Gorur wrote:
>>> Hello,
>>> We've been having some trouble with the synchronizing aspects of our
>>> project. What we want is the exact same time as the FPGA tick counter,
>> not
>>> a time_spec_t representation of time. Where can we get access to that?
>>> Thanks!
>>> -Anisha
>> call to_ticks with the clock rate of the device:
>> http://files.ettus.com/uhd_docs/doxygen/html/classuhd_1_1time__spec__t.html#ad75960ba4c788cd30a5d0c8904888da3
>> -josh
>>> _______________________________________________
>>> Discuss-gnuradio mailing list
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Anisha Gorur
Class of 2012
Electrical Engineering

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