Re: [Discuss-gnuradio] M-block integration status

[Eric Blossom]

On Fri, Jan 15, 2010 at 06:46:47PM +0000, Ben Gear wrote:
> Hi Eric,
> 
> Great to have an update on how the new features are progressing.
> m-blocks and the replacement features have been mentioned a few times
> on this mailing list in relation to building TDMA MACs.  Having VRT
> metadata available in gr blocks is going to give access to timestamps
> for Rx samples; will we be able to specify tight timings for Tx using
> the VRT headers in the same manner? In the git VRT code I've noticed
> that the VRT header that is passed with the Tx data to the USRP
> contains timing info.  Is this used by the USRP at the moment, or is
> it purely for the Rx side of things?

The VRT frames transmitted to the USRP2 contain either no timestamp
info, or they contain both the integer and fractional seconds
timestamps.  If they do not contain a timestamp, the frame is
transmitted "now" in a FIFO manner.  If a frame contains a timestamp
and the "start of burst" flag is set, the frame is transmitted at that
time.  If a frame contains a timestamp, and the "start of burst"
flag is not set, and we haven't "underrun" in the middle of the burst,
the frame is enqueued into the FIFO.  If a frame is late, it's
dropped.  The timestamps in the packets must be monotonically
increasing.  No in-FPGA rearrangement is done.

Eric