USB mass storage device max. consumption

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Hallo there,

I am close to finish a new product. The product other than act as USB host and accepts USB mass storage devices.

My question is that which is the maximum current than the VBus must support?

I am asking this because I need to choose the right regulator.

Thanks.

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

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Quote:

My question is that which is the maximum current than the VBus must support?

I thought the spec. said this was subject to negotiation? That is, the device would ask first and the host would tell it how much current can be drawn? (this initially being done with minimal current draw)

EDIT:

http://en.wikipedia.org/wiki/Uni...

this seems to suggest that by default one unit (100mA) of power my be drawn but this can be increased to five units (500mA)

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I would like to be more detailed and to tell you that when I am talking about a USB mass storage device I mean a USM flash memory stick.

Also, the host must support any manufacturer's device. So I need to know the maximum needed consuption this type of devices.

If, as you told me, the device tells the host to set the needed current consumption, then which is it's minimum value?

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

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My understanding is that devices must ask the host how much current they can pull and if it says a limited amount (which looks like being 100mA) then the device must be able to operate from that (or presumably not at all?)

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It is a matter of negotiation. But a maximum value of 500 mA is set. If no negotiation is done, it is set to 100 mA.

It is NOT the master that tells the device, it is the device that asks the master for a certain max current. The master then have the option to turn the device off if the current limit is exceeded. And thus if no negotiation is done, you can pull 100mA before the host turns the port off.

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Quote:

It is NOT the master that tells the device, it is the device that asks the master for a certain max current. The master then have the option to turn the device off if the current limit is exceeded. And thus if no negotiation is done, you can pull 100mA before the host turns the port off.

Ah, thanks for clarifying that - this was just a distant memory, I knew there was a low/high current capability then just assumed (dangerous I know) that the two devices would negotiate/agree - it seems a bit fierce to just ditch the device without warning if it draw too much current. Hopefully it knows to turn on the high current parts of its circuit and then wait a while before doing anything that might cause problems if there were a sudden loss of power?

(BTW I cannot imagine flash arrays ever requiring hundreds of milliamps anyway?)

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From memory: nobody can pull more than 0.5A, and anyone can pull max 0.1A until PC enumerates the USB device and the PC gains knowledge how much the USB device actually wants, and then either allows it to draw more or not.

I am not sure if you can request less than 0.1A, but it would be handy, as then you could have 50 devices taking 10mA, instead of 5 device, all taking 10mA, but assumed to take 100mA.

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clawson wrote:

(BTW I cannot imagine flash arrays ever requiring hundreds of milliamps anyway?)

Me neither, but eons ago when USB2 was new technology, I bough my wife a whopping 256MB DataTraveler USB stick.

It was the first USB stick I had ever seen to request 200mA, and thus it did not work in many computers. Previously USB sticks I had seen only requested 100mA.

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If you want to be sure all USB sticks work you'll have to be able to supply the full 500mA. If you do less you'll have one in 100 customers complaining that the stick he is using does not work. There are quite many multifunction devices with mass storage as one of several functions and they might well consume consume the full 500mA, even if temporarily.

It may even be that a stick which needs more than 100mA just requests the 500mA even if it does not need them because most USB hosts can supply 500mA on a port.

You might even want to be able to supply somewhat more than the spec to make sure even most USB hard disk drives work. Many of those are supplied with a special cable to connect it to two USB ports because they exceed the current capability of a single port (or a single port on low-cost PC's).

One alternative is to supply only low power (100mA) and instruct the users to use a hub between your device and the offending high power consuming stick. Hubs, if on their own power supply, can run on low power upstream and supply full power downstream.

Markus

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Many USB drives become very warm. But then 2.5W is quite a lot of heat.