USB-2 transfer rate limitation

From: [at]} <Wojtek>
Date: Tue, 27 Nov 2007 15:28:23 -0500

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On Tue, 27 Nov 2007, Maksimiuk Darek wrote:

> Wojtek and all,
> I have some experience with a USB based acquisition system
> (www.ettus.com - the USRP HW) working under Linux (www.gnuradio.org)
> (in fact, this is the GNU radio project). I guess is that 32 MB/s
> from the USB 2.0 is the maximum you can get, and sometimes it depends
> on the underlying USB controller on the PC. I got 32 MB/s
> from the USRP system on my Lenovo ThinkPad 61 machine.
> You can also try this one: http://www.orpal.eu/ang/rejestrator.html

Darek:

  your feedback is very welcome. We may contact you soon with more
questions, as soon as we dig into the driver details. (Eryk, could you
please explore this topic? *)

Some details of our project are available in the following location.
Please note these details may not be final because we are just working
on all this: www.pas.rochester.edu/~skulski 2nd paragraph "Dark matter".

Hardware-wise, we are using EZ-USB-FX2LP from Cypress, part number
CY7C68013A-128AXC. The chip is a 8051-compatible microcontroller
with USB-2 hardware acceleration. The chip has a hardware-controlled
"endpoint FIFO" on dedicated pins. The FPGA writes into the FIFO
and the USB-2 packet goes directly to the wire without microcontroller
being aware of it. The burst rate of the FIFO is 96 MB/s, which is
almost twice the sustained USB-2 rate. Therefore it is safe to say
that hardware is not limiting the data transfer on the transmitter side.

The limitation may lie on the receiving side. I have heard rumors that
some PC chips are not capable of receiving the high-speed USB-2 rate.
If true, it would be very surprising to me. I somewhat suspect these are
just rumors. If the rumors turn out to be true then we will have to find
a PC with a proper USB-2 chip, which will be capable of receiving 480 mbps.

In one of their app notes Cypress claims that their SW driver can achieve
the full 480 mbps (i.e., no limitation from the software driver). However,
the source for that driver is not available. We may be able to use this
driver the same way as Cypress did, that is as a "black box", and still
achieve the same result.

At this point we are using an open source driver (usblib or something, not
sure, but Eryk knows) but the best we achieved so far was about 20 MB/s.
It is not clear whether we are dealing with an inferior PC box with
one of these rumored USB chips, or perhaps the open source driver is not
as good as the Cypress driver.

As soon as we explore both the receiver chips and the USB driver,
we will be ready for the speed tests.

Once again, thank you for the info.

Wojtek

--
*) Eryk is responsible for USB driver details in our group.
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Received on Tue Nov 27 2007 - 21:28:23 UTC