[Discuss-gnuradio] Understanding the Data Structure for the USB transfer

[Uwe Bonnes]

Hello,

the last day I am banging my head against the problem how the data is
structured for the USB transfer between the FX2 and the PC. Datasheet,
FX2 reference and the web didn't givve me the right clue.
What I understand:
- the FPGA pumps interleaved samples into the FX2 fifo via GPIF. E.g.
http://www.nd.edu/~jnl/sdr/docs/tutorials/4.html talks about a sequence
I0 Q0 I1 Q1 I2 Q2 I3 Q3 I0 Q0 I1 Q1, etc.
- these samples are somehow grouped into 512 byte blocks and sent over USB
- the PC receives these blocks and must somehow know where a data group
starts

I can only think that always 512 byte blocks are transferred. With a
full number of data groups fitting into the block, the block would always
start with I0. But what happens if the PC gets out of sync, by not reading a
full block with some strange event? Then the next 512 byte blocks read would
always start somewhere in the datagroup. Any way to resync? Are short
packets and the PKTEND signal used? 

Thanks for any hints

-- 
Uwe Bonnes                address@hidden

Institut fuer Kernphysik  Schlossgartenstrasse 9  64289 Darmstadt
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