[Discuss-gnuradio] USRP and USB full speed (1.1) transmit

[Philip Balister]

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 :)

I see that some people on this list (beyond Matt and Eric) are
interested in the signal flow from the PC to the ADC/DAC's so I'm
hoping someone has some suggestions.

Philip