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Re: [Discuss-gnuradio] inband pings and clock

From: Brian Padalino
Subject: Re: [Discuss-gnuradio] inband pings and clock
Date: Fri, 3 Aug 2007 13:11:01 -0400

On 8/3/07, George Nychis <address@hidden> wrote:
> Second question.  For an application to properly timestamp outgoing
> packets, the application needs some general idea of the current clock
> value on the USRP.  At first I was thinking "oh well the app can just
> send a ping and read the timestamp off of the response while calculating
> the delay."  Well, the ping doesn't carry the clock back up to the
> application.  RX packets (response-recv-raw-samples) carry the timestamp
> back up to the application, but there is no notion of calculating delay
> here.
> Is the clock stored in a readable register somewhere on the USRP that a
> register read could be used?  The delay could be calculated here between
> the request and response.  I checked our wiki but see no register:
> http://gnuradio.org/trac/wiki/UsrpFPGA

I am not sure how the host side of things wants to handle sending
something out - so this is just complete speculation on my part.

What I thought would happen is that the RX side of things sends in
timestamps with the packets.  During processing, if a correlation or
some special event occurs - the timestamp for that event can be
calculated (based on decimation rates, etc).  Given that reference,
you can calculate a new timestamp based on whatever your MAC requires.

Do you think something like that would work?


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