[KLUG Hardware] Re: USB v2.0 Compatibility? -- don't expect > 4x unless you have EHCI+compatible-OS

[Bryan J. Smith]

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On Sat, 2002-12-07 at 11:04, mag00 wrote:
> USB hardware experts...
> If a device says it requires USB v2.0 is that a RECENT spec?
> Would you think that a Gateway E-4200 P2-300MHz has USB v1.0?
> The hardware profile on the PC says Std. USB with TI chipset.
> An external CD-R/RW says it requires USB v2.0???=20
> Anybody know more about this?

USB 1.x and 2.0 are easier to understand if you talk about the two
actual USB "controller" types:

  OHCI:  USB 1.x, upto 12Mbps (1.5MBps)
  EHCI:  USB 2.0, upto 480Mbps (60MBps), USB 1.x compatible

Typically on a 5-6 port USB 2.0 controller, you will get:

  2 x OHCI (3-4 ports), 1 x EHCI (1-2 ports)

And not many OSes support EHCI ports:

  OHCI:  Windows 95-OSR2+, 2000+, Linux, most 97/98+ OSes
  EHCI:  NT 5.1-only w/patch (aka XP) and Linux 2.4

To get USB 2.0's "480Mbps" (60MBps), you _must_ make sure you plug into
the EHCI port.  Finding this out is usually not easy either.  Otherwise
you're limited to only USB 1.x ultra-slow speeds.  If you put even one
USB 1.1 device on a USB 2.0 bus, you slow the whole bus down as well.

Hardly _any_ mainboards on the market are shipping with USB 2.0 yet.  So
your P2-300MHz is going to be OHCI, and capable of only a measly
1.5MBps.

Now USB 2.0 devices _should_ "drop back" to OHCI speeds if you don't
have an EHCI port in your system.  But then, again, it's 40x _slower_
theoretically.  OHCI speed are _only_ good for ~4x CD-R/RW speeds,
forget DVD-R/RW (+R/+RW, -R/RAM).

USB was _never_ designed for block device transfers, only slow,
programmed I/O character driven devices.  It was never supposed to be
released in a faster version, FireWare was considered the bus for that.=20
USB's design is inheritly horrendous, designed by Intel and Microsoft
for "easy of design" on the host/OS side -- putting all the "effort" on
the device/driver end.  Hence why putting more than a couple simplistic
USB devices on the same cable is usually impossbile -- at least on
closed source OSes because different vendor drivers conflict in
assumptions (long story).  But Intel created USB 2.0 out of political
convience with regards to their reniging on Apple FireWire support.

According to various testing out there -- not only from independent
eWeek to Intel itself -- even the 480Mbps EHCI is only capable of
~64-80Mbps (~8-10MBps) maximum, which is about 1/3-1/5th normal burst
per performance of ATA, so I wouldn't use it for hard drives.  But EHCI
_can_ handle 40x+ CD-R/RW speeds, so it's fine for devices that never
break ~50Mbps (~6.25MBps) like most optical devices.


--=20
Bryan J. Smith, E.I. (BSECE)       Contact Info:  http://thebs.org
[ http://thebs.org/files/resume/BryanJonSmith_certifications.pdf ]
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