[Discuss-gnuradio] problem with start/end of burst inband FPGA code

[George Nychis]

Hey all,

We're looking for someone to look over our shoulder on an inband FPGA 
problem we've been stuck on for a couple days now.

The problem is that we never had code that throws out additional packets 
in a burst if the initial packet had a timestamp that expired.  When an 
m-block message is fragmented into multiple USB packets, the packets are 
considered a single burst, where the first packet has the start of burst 
flag set and the last packet has the end of burst flag set.  The first 
packet has the timestamp set to the desired timestamp, and each 
additional packet in the burst is given the 'NOW' timestamp.

The problem was that our code would throw away the first packet if the 
timestamp expired, but the additional packets in the burst had a 
timestamp of 'NOW' so they would be sent... but they shouldn't be since 
they are part of an 'expired' burst.

Ok, so on to the code:
http://gnuradio.org/trac/browser/gnuradio/branches/developers/zhuochen/burst/usrp/fpga/inband_lib/chan_fifo_reader.v

The logic needs to be "if the packet with the start of burst flag was 
thrown away, throw away all packets until we see the end of burst flag."

If we see the start of burst flag, we set 'burst' to be 1.
http://gnuradio.org/trac/browser/gnuradio/branches/developers/zhuochen/burst/usrp/fpga/inband_lib/chan_fifo_reader.v#L97

If the timestamp has expired on the packet, set 'trash' to 1, and 'skip' 
set to 1:
http://gnuradio.org/trac/browser/gnuradio/branches/developers/zhuochen/burst/usrp/fpga/inband_lib/chan_fifo_reader.v#L148

Finally, if we get a packet that does not have start of burst set, it 
*must* be an intermediate packet or the last packet in a burst... either 
way it does not matter.  If 'trash' is set to 1, we want to skip it:
http://gnuradio.org/trac/browser/gnuradio/branches/developers/zhuochen/burst/usrp/fpga/inband_lib/chan_fifo_reader.v#L106

If we see the end of burst flag, 'burst' is reset:
http://gnuradio.org/trac/browser/gnuradio/branches/developers/zhuochen/burst/usrp/fpga/inband_lib/chan_fifo_reader.v#L102

If the next packet has a correct timestamp, we set trash back to 0.  We 
could just set trash to 0 when end of burst is read, but it doesn't 
really matter:
http://gnuradio.org/trac/browser/gnuradio/branches/developers/zhuochen/burst/usrp/fpga/inband_lib/chan_fifo_reader.v#L137

So... as you can guess, this doesn't work.  This hasn't been our only 
attempt.  We've trashed the code and re-written it a couple different 
times with no success.

To test this we've been setting all packets to have a timestamp of 0. 
This means we should get no sine wave on an oscope when transmitting, 
regardless of the number of packet sin the burst.  However, we see 
chunks of the wave here and there when the number of packets in the 
burst is greater than 1.  If we set it so that a single m-block message 
only creates 1 USB packet, it works... everything is thrown away.

We'd greatly appreciate any help, thanks!

- George, Leo