[Discuss-gnuradio] USB speed data point

[ldoolitt]

Hi -

I'm bringing up a board
  http://recycle.lbl.gov/llrf4/
with a hardware and software USB stack based on and (for this purpose)
equivalent to the GNU Radio design, and measured its USB data transfer
capabilities more carefully than I have done before.  There is a
distant possibility someone on this list might make use of the result,
so here it is:

Reading only, on a lightly loaded AMD64 3500+ machine (2.2 GHz,
dual-channel RAM), I can sustain 35.7 MByte/sec without errors.
Attempting 35.8 MByte/sec, packets get dropped left and right.
The host end of the USB is a VT8237 Chipset, seemingly run in
EHCI mode by Linux-2.6.16 (Debian sid 2.6.16-2-amd64-k8).

I think the limitation is on the 8051 end.  One 512-byte packet takes
8.53 microseconds to cross the USB channel, and the 35.7 MByte/sec
sustained rate implies the 8051 sets up the next packet in only 5.81
microseconds.  I don't think there is any pipelining at this level.

   - Larry