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Message #73324 of 126777
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Date: 2005-03-01
From: Lee Jones
Subject: Re: [PIC] USB low speed slower than specs?

>>> Consider something that supports at least USB 2.0 (people won't be
>>> cursing your name for something thats low speed USB1.1 slowing down
>>> all there USB toys, think of it that way.)

>>    "At least" USB 2.0?  The USB spec is at version 2.0; there isn't
>>    any revision higher than that.  Besides, the low-speed parts are
>>    compliant with the 2.0 spec anyway.

> I think the issue was having USB 1.1 devices on a USB 2.0 'network'
> using a ridiculous amount of the link time relative to the actual data
> transferred (and taking available bandwidth away from 2.0 devices).

It can't if it follows USB 2.0 (i.e. a USB 2.0 compliant hub).

Assume a USB 2.0 host port and a USB 1.1 peripheral device.  If the
USB 1.1 device is hooked directly to the USB 2.0 host port, no other
device can be impacted.

If the USB 1.1 device is plugged into a USB 2.0 hub, the host to
hub transfers occur at high-speed (480Mbps).  The hub to USB 1.1
device link runs at either full- (12Mbps) or low-speed (1.5Mbps).
See:

Universal Serial Bus Specification 2.0, 27 April 2000,
 
  chapter 4 Architechtural Overview
    section 4.2.1 Electrical
      USB 2.0 host controllers and hubs provide capabilities so that
      full-speed and low-speed data can be transmitted at high-speed
      between the host controller and the hub, but transmitted between
      the hub and device at full-speed or low-speed.  This capability
      minimizes the impact that full-speed and low-speed devices have
      upon the bandwidth available for high-speed devices.

  chapter 11 Hub Specification
    section 11.1.1 Hub Architecture
      The hub must operate at high-speed when its upsteram port is
      connected at high-speed.

      The Transaction Translator takes high-speed split transactions
      and translates then to full-/low-speed transactions when the
      hub is opearting at high-speed and has full-/low-speed devices
      attached [to downstream ports].


The only impact on transfer rate occurs if you have a USB 2.0 host
port, a USB 2.0 device, and connect it up via a USB 1.1 hub. (*)

(*) at least according to the spec.  I personally prefer Firewire.

						Lee Jones

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