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.



.

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