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

[Matt Ettus]

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