Re: usb question

"Twayne" <nobody@xxxxxxxxxxxxxxxxxxx> wrote in message
news:e%23$ano9NJHA.2912@xxxxxxxxxxxxxxxxxxxxxxx
"Twayne" <nobody@xxxxxxxxxxxxxxxxxxx> wrote:

Since the specs are a total of 500 mA,

The USB low power bus mode specifies a maximum of 100 ma per port.

However, if you have two hard drives that want to suck 250 mA...

USB powered drives stay within the 100 ma per port requirement. If
they draw more than the 100 ma limit they supply a two port cable
and suck power from 2 ports.

EVER device has a power requirement of xxx mA at xx
volts. Exceed those specs, and the device will no longer receive
power, along with any others attached to the same controller.

A correctly designed computer will be able to supply 100 ma to each
USB port. A correctly designed USB powered drive will draw no more
than 100 ma from each USB port. No problems no matter how many ports
involved. Not a hard concept to understand.

It's not even close to rocket science,

I would agree.

You boys can have the last word...

OK, here you go, since you're too lazy to look for yourself. It's
simple if you can just bother to READ and comprehend, not just the
parts you like, but ALL of the specs.

...

A USB device specifies its power consumption expressed in 2mA units
in the configuration descriptor which we will examine in detail
later. A device cannot increase its power consumption, greater than
what it specifies during enumeration, even if it looses external
power. There are three classes of USB functions,

a.. *Low-power* bus powered functions aka 100 mA
b.. *High-power* bus powered functions aka 500 mA
c.. *Self-powered* functions
*Low power* bus powered functions draw all its power from the VBUS
and *cannot draw any more than one unit load.* The USB specification
defines *a unit load as 100mA. Low power* bus powered functions must
also be
designed to work down to a VBUS voltage of 4.40V and up to a maximum
voltage of 5.25V measured at the upsteam plug of the device. For
many 3.3V devices, LDO regulators are mandatory.

*High power* bus powered functions will draw *all* its power from
*the bus* and cannot draw more than one unit load *until it has been
configured*, after which *it can then drain 5 unit loads (500mA Max)*
provided it asked for this in its descriptor. High power bus
functions *must* be able to be detected and enumerated at a minimum
4.40V. When operating at a full unit load, a minimum VBUS of 4.75 V
is specified with a maximum of 5.25V. Once again, these measurements
are taken at the upstream plug.

Self power functions may draw up to 1 unit load from the bus and
derive the rest of it's power from an external source. Should this
external source fail, it must have provisions in place to draw no
more than 1 unit load from the bus. Self powered functions are
easier to design to specification as there is not so much of an
issue with power consumption. The 1 unit bus powered load allows the
detection and enumeration of devices without mains/secondary power
applied. No USB device, whether bus powered or self powered can drive
the
VBUS on its upstream facing port. If VBUS is lost, the device has a
lengthy 10 seconds to remove power from the D+/D- pull-up resistors
used for speed identification.

Other VBUS considerations are the Inrush current which must be
limited. This is outlined in the USB specification paragraph 7.2.4.1
and is commonly overlooked. Inrush current is contributed to the
amount of capacitance on your device between VBUS and ground. The
spec therefore specifies that the maximum decoupling capacitance you
can have on your device is 10uF. When you disconnect the device
after current is flowing through the inductive USB cable, a large
flyback voltage can occur on the open end of the cable. To prevent
this, a 1uF minimum VBUS decoupling capacitance is specified.

For the typical bus powered device, *it can not drain any more than
500mA* which is *not* unreasonable. So what is the complication you
ask? Perhaps Suspend Mode?

...

What about laptops? My HP laptop will provide up to at least 1000 mA
from it's own internal power. I did some measurements. Up to 500 mA
the USB hub provides the power according to Device Manager. Go
over 500 mA, and the USB hub current drops to a residual 10 mA, and
the USB port still provides the power, obviously on a switch over to
power directly from the laptop 5V bus.

I have a 2.5" HD in a Vantec external case, which came with an extra
USB power connector on the USB cable. On my laptop, I don't need the
extra connector. If I connect it, it draws no current from that USB
hub. On the other hand, if I connect the USB HD to the front ports
of my Antec case, that are connected to an Asus P5B Dlx MB, with the
single connection, the drive won't power up until I plug in the extra
USB connector. Then it powers up and runs normally, so between the two
case front
USB ports over 500 mA is available.

No doubt you're right, and thanks for the information. Sounds like they
are monitoring what hte controllers are doing and jumping in with an
assist ckt when it's needed. Sounds like a good design, actually, and
I'd imagine it's catching on to more and more computers.
In that light, I should probably make the USB controller "specs" more
prominent. I wonder if that's typical of only front ports and why not
all of them? Mmm, maybe to keep from overtaxing the internal PSU cktry.
Dunno. Maybe I'll take a look at the USB3 specs & see if they address
that.

Thanks!

Twayne


.

Relevant Pages

  • usb_insert_transfer panic with if_zyd
    ... When I use my ZyXEL G-220 Wireless USB Stick with the if_zyd driver, ... <ACPI PCI bus> on pcib0 ... device problem, disabling port 1 ... <Ethernet over FireWire> on firewire0 ...
    (freebsd-current)
  • Re: Problem with memory stick
    ... When I insert it into the USB port, the kernel spits out a large number ... BIOS IRQ 11 for 0.31.INTB is invalid ... <ACPI PCI bus> on pcib0 ...
    (freebsd-stable)
  • Usb keyboard problems with a Dell Precision 380
    ... The machine has an usb keyboard. ... pci0: <ACPI PCI bus> on pcib0 ... ppc0: parallel port not found. ...
    (freebsd-current)
  • USB on 4.11
    ... If I boot the GENERIC kernel, my USB drive is recognized. ... port 1 powered ... scbus0 on sym0 bus 0: ...
    (freebsd-questions)
  • Re: How to get USB / Ethernet (NIC) working on MS Windows CE 5.0 ?
    ... The D- line should NOT be pulled up at insertion for USB full speed ... wait again for the next detection. ... The power LED on the device remains OFF. ... power draw for port 1, ...
    (microsoft.public.windowsce.platbuilder)