Philip Balister wrote: > I've been working on getting the usrp working with a USB full speed > interface connected to an OMAP starter kit (OSK) board from TI. I'm > not using GNU Radio, but the OSSIE open source SCA. The OSK only has a > USB 11 controller that sends 64 byte packets, not the 512 byte packets > sent by USB2.0 high speed interfaces. > > The code I have uses the usrp-0.12 package to communicate with the > USRP. This email is mainly of interest to people interested in low > level USRP interfacing and the FPGA to FX2 interface. > > I have a waveform that generates random BPSK that works fine on a PC > with is USB high speed interface. The output is raised cosine pulse > shaped and I monitor it on a Tektronix signal analyzer. When I run the > waveform on a PC with a USB2.0 hidg speed interface I see a good eye > diagram. > > When I compile the code for the OSK, I see two symbols transmitted, > then dead air. Two symbols is 64 bytes (2 symbols x 8 sample/symbol x > 4 bytes/sample), then there are 512 - 64 bytes of no signal. It feels > like the 64 byte packets are read into a buffer of 512 bytes, where > all the un-written bytes are zeros. > > It feels like somewhere in the USB chain, I send the 64 byte packet, > then something reads the 64 bytes and zero pads the packet to 512 > bytes. I suspect the FPGA tries to read 512 bytes from the FX2 chip, > but that is only because this is the area of the code I least > understand :)
Philip, If you go to http://gnuradio.utah.edu/trac/browser/gnuradio/trunk/usrp/fpga/sdr_lib/tx_buffer.v And look at line 94, you will see the test for "end of packet". write_count[8] goes high when we have put 256 elements (512 bytes) into the fifo. You would need to modify this to write_count[5] which will go high when 32 elements (64 bytes) have been put into the fifo. You probably also need to modify some firmware. Matt _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio