Do USB devices draw current from the computer if a powered hub's current “runs out”?

I have a few heavy duty devices and a powered hub and I wonder what's the flow of current in those things.

What you plan to power over that tiny USB hub ?

If whant to charge or power something outside standard and specification for USB 2.0 then you have high chance to melt USB connectors or to damage circuit inside hub, damage of hub adapter is also possible.

For higher currents use USB 3.0 connectors, its newer standard and specification with new designed connectors which supports higher currents or heavier loads.

Some devices such as car GPS navigations (not all models and manufacturers), detects states when they are charging over adapters and when over USB ports. If they charge battery over PC USB port then they limit current to lower ratings, and when wall mains adapter is used device charge battery on higher current ratings.




:wink:

If the hub is powered, it should have a rating on the wall wart. That could serve as the upper ceiling of possible current draw from your devices combined.

That wasn't the question, I wondered what happens if the hub's connected devices go over the Ampere of the adapter, will it draw current from the computer?

By the way, there are various devices that routinely violate the standard and draw more than 500mA. e.g. most (if not all) keyboards with LEDs on each key. Most of them violate it, and most computers or hubs let them work.

edit: Also the spec allows a tree of unpowered hubs so it's not even a question of violating the standard, the supply may just run out.

For sure device will drain current from PC in that situation.

fateswarm said:

By the way, there are various devices that routinely violate the standard and draw more than 500mA. e.g. most (if not all) keyboards with LEDs on each key. Most of them violate it, and most computers or hubs let them work.

edit: Also the spec allows a tree of unpowered hubs so it's not even a question of violating the standard, the supply may just run out.

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Its not violation, newer motherboards allows more then 500mA on USB 2.0. Older motherboards have 500mA limit on USB ports, and on newer manufactured in 2007 and up you can see 1A, 1,5A on USB 2.0.



On one my older PC which I use for monitoring various values and for CCTV, I have Gigabyte motherboard with USB 2.0 ports which charge car navigation with 800-900mA of current when battery is empty.

They violate it. 2.0 spec allows 500mA only, and it's only higher for chargers. Chargers aren't supposed to be used on computers though. The main reason they exist in the standard is political and market pressure for mobile phones to have the same interface. It can be considered irrelevant, though I guess it will make the case of computers allowing the violation of the standard for non-charger devices more common.

fateswarm said:

They violate it. 2.0 spec allows 500mA only, and it's only higher for chargers. Chargers aren't supposed to be used on computers though. The main reason they exist in the standard is political and market pressure for mobile phones to have the same interface. It can be considered irrelevant though I guess it will make the case of computers allowing the violation of the standard for non-charger devices more common.

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Its not linked only with devices with charging needs, you have external hard disks 2,5" 1,8" designed to drain power only from USB ports. Its ok to use external power source as power injection for USB devices.

I use everyday development board with USB supply with LCD/GLCD plus others parts on dev board, over 500mA for sure.



Check device current needs, check USB hub wall adapter max allowed current.

In case that device need more current then adapter can supply, device should get additional current from USB port.

Referring to the original question

11.5.1.2 Powered-off
(...)
If a hub was configured while the hub was self-powered, and then if external power is lost, the hub must place all ports in the Powered-off state. If the hub is configured while bus powered, then the hub need not change port status if the hub switched to externally applied power. However, if external power is subsequently lost, the hub must place ports in the Powered-off state.

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11.11 Hub Port Power Control
Self-powered hubs may have power switches that control delivery of power downstream facing ports but it is not required. Bus-powered hubs are required to have power switches.

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Consequently, a hub configured in bus-powered state won't be a allowed to impose part of the device power need to the host.

tpetar go read the spec (or just the wikipedia article) because you're arguing against a fact: USB 2.0 allows only 500mA per device unless it's a charger and that's a fact.

That there is a violation of the standard across the board is also more less a fact.

fateswarm said:

tpetar go read the spec (or just the wikipedia article) because you're arguing against a fact: USB 2.0 allows only 500mA per device unless it's a charger and that's a fact.

That there is a violation of the standard across the board is also more less a fact.

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I'm not arguing, I just try to help, sorry for that, I'm out.

FvM said:

Referring to the original question
Consequently, a hub configured in bus-powered state won't be a allowed to impose part of the device power need to the host.

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It says it will be required to have powered switches on that state. I don't know what power switches actually mean in this context. Do they allow only one of the two sources to be used? Do they allow simultaneous powering? (which was the question of this thread)

Also that's just one of the two states. Won't self powered hubs be de facto on 'configured on a self-powered state' since they'll be plugged in to the external source usually when powering on the upstream bus?

The first paragraph is violated here for sure. I unplug the external power from a powered hub and it keeps going with upstream power.

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tpetar said:

I'm not arguing, I just try to help, sorry for that, I'm out.

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You can be out but I'm not arguing either, it's a spec clause.

https://www.gigabyte.com/microsite/239/3x-usb-power.html

"In general, a USB 2.0 hub port has a current limitation of 500mA, but GIGABYTE motherboards equipped with 3x USB Power supply are able to provide 1.5A for each USB 2.0 port. Additionally, USB 3.0 hub ports Moreover the USB3.0 hub port can even provide up to 2.7A of power for USB 3.0 devices.

The 12 USB 2.0 ports on a GIGABYTE motherboard can provide a total of 18A of power and the 2 USB3.0 ports are able to provide a total of 5.4A of power. This means that a GIGABYTE 3x Power designed motherboard are able to deliver a total of 23.4A of power. "

;-)

What are you doing? I specifically said the spec is violated across the board, i.e. very likely by GIGABYTE too. That doesn't mean they don't violate the standard.

As a preliminary remark, most edaboards members will probably look at USB power questions from a PC user rather than an official USB standard viewpoint. Quite obviously tpetar is bringing in this practical view. Nobody is required to "read the spec" as long as he isn't developing compliant equipment.

Personally I'm using a portable harddisk on my notebook without permanently wondering why the host doesn't shut down the overload. i just notice that the host has obviously a large overcurrent margin. Otherwise, the HDD manufacturer suggests me to use the Y-cable with auxilary USB "power" connector, strictly speaking another popular violation of the USB standard.

However, when meeting an USB design problem, I change to my legal USB developer mode. Continuing respectively:

It says it will be required to have powered switches on that state. I don't know what power switches actually mean in this context. Do they allow only one of the two sources to be used? Do they allow simultaneous powering? (which was the question of this thread)

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Power switches means that the hub has means to disconnect the power of individual ports. As far as I see, there's no particularly requirement how both power sources are combined or switched. Diode or-ing isn't banned.

Also that's just one of the two states. Won't self powered hubs be de facto on 'configured on a self-powered state' since they'll be plugged in to the external source usually when powering on the upstream bus?

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I don't think that a self-powered hub (means a hub with self-powering option, in other words an external power connector) will be usually "plugged in to the external source". All USB hubs that I have in my office are optional self-powered but none presently is.

The first paragraph is violated here for sure. I unplug the external power from a powered hub and it keeps going with upstream power.

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Yes, that's also my understanding of the paragraph. Altough the required behaviour seems to be clearly specified in USB 2.0, you might ask if it's 1. reasonable and 2. practical. You possibly want to keep the USB devices in operation during an momentary power loss.

Specification of overcurrent protection is also relative loose. I remember a desktop PC, where shorting the USB power (before any device enumeration) caused a reboot. The USB interface may have used PTC fuses, but apparently the inrush current was above supply current limit.

Please notice that the below specification doesn not contradict a current limit (with a certain transient reserve) near the official 500 mA number or even a preliminary current limit of 100 mA before negotiating higher power mode. But quite obviously nobody seem to need it or want it at all. All those USB spot lights, blinking christmas trees, desktop fans etc. won't work under a strict regime of the USB spec, but they are used everywhere.

7.2.1.2.1 Over-current Protection
The host and all self-powered hubs must implement over-current protection for safety reasons, and the hub must have a way to detect the over-current condition and report it to the USB software. Should the aggregate current drawn by a gang of downstream facing ports exceed a preset value, the over-current protection circuit removes or reduces power from all affected downstream facing ports. The over-current condition is reported through the hub to Host Controller, as described in Section 11.12.5. The preset value cannot exceed 5.0 A and must be sufficiently above the maximum allowable port current such that transient currents (e.g., during power up or dynamic attach or reconfiguration) do not trip the over-current protector. If an over-current condition occurs on any port, subsequent operation of the USB is not guaranteed, and once the condition is removed, it may be necessary to reinitialize the bus as would be done upon power-up. The over-current limiting mechanism must be resettable without user mechanical intervention. Polymeric PTCs and solid-state switches are examples of methods, which can be used for over-current limiting.

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fateswarm said:

I have a few heavy duty devices and a powered hub and I wonder what's the flow of current in those things.

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Look at the schematic for the hub in question!?!?!?