USB Power

Hi,

It seems the specification allows USB devices to draw up to 100mA (for a "low power" port) and 500mA (high power port).

Does anyone have any idea of the incidence of high power ports? Especially on laptops.

For a design with USB control, I am trying to decide if it is worth including an option to take all the power from USB, rather than a wall-wart type adapter. It would be great just to have the one plug, and also allow fully mobile operation. But if a large fraction of laptops only have low power ports, it may be more trouble than it is worth. (It occurs to me that in order to support plugging in an external hub, all laptop ports *ought* to be high power).

Thanks,

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John Devereux
Reply to
John Devereux
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As I understand it, the master port has to be able to supply 500mA, though the slave must not draw more than 100mA at startup or when plugged in, requesting the extra current after the enumeration process.

JS

Reply to
JSprocket

Yes, but is the 500mA capability mandatory? I got the impression that it was optional. But hopefully most built-in ports do support it? That is what I am trying to find out.

Thanks,

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John Devereux
Reply to
John Devereux

My recollection is that the master must supply 100mA at all times and be able to supply 500 mA on demand. I think this is mandatory for a compliant device. That is not to say that all devices are truly compliant...

Reply to
Bob

John

For a PC (non embedded) host, the host has to be able to supply 100mA during enumeration and 500mA during operation. If you have an embedded host then it need only be able to supply 8mA during enumeration and then the maximum current described in their "targetted peripheral list" (TPL). If the current consumption of the devices on the TPL are not known then the embedded host must also be able to supply 500mA.

This is described both in the USB specification and in the addendum "Requirements and recommendations for USB products with embedded hosts and/or multiple receptacles". [These are available from the USB-IF website]

Andrew

Reply to
Andrew Jackson

According to the USB spec a mains powered device must be able to supply 500mA from the root hub. Battery powered devices may be limited to 100mA. Even for mains powered devices things are not straightforward since it is not unheard of for front panel ports to be on a bus-powered hub that itself is only capable of 100mA per port.

Is their any reason why you can't make your device switch modes? Work as a self powered device if connected to an external power supply, or a bus powered device if there is no external supply connected and the USB port is good for the power.

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Andrew Smallshaw
andrews@sdf.lonestar.org
Reply to
Andrew Smallshaw

Yes, that is what I intended to do. (It is not totally trivial since I will need a boost regulator to get the required voltage from the USB supply). So I wanted to make sure that, usually, a laptop would be able to power it up OK. If it turns out that most laptops don't bother with a 500mA capability, then I would not bother with it. But if they are all "supposed to" (and most do) then it is worthwhile.

Thanks,

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John Devereux
Reply to
John Devereux

It's safe to say they're ubiquitous. I've never seen a low-power (only) USB port that's integral to the laptop (as opposed to being on, e.g., a PCMCIA USB card or an unpowered hub), although I'm sure someone can point to an example where this was done. (On the other hand, I've definitely seen a few laptops where they plus another proprietary connector next to a USB socket that provides even *more* power than USB alone can for, e.g., older DVD burners/big hard drivers -- said peripherals then use a "combination" plug that simultaneously contacts the USB socket and the proprietary power socket.)

I'd vote for, "yes, absolutely." If you don't need more than 2.5W, it's almost criminal not to allow power to be drawn from USB alone. :-)

---Joel

Reply to
Joel Koltner

Fantastic, thanks Joel :)

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John Devereux
Reply to
John Devereux

Out ThinkPads run external drives without wall warts, so I'm sure they're "high" power. Don't have a spec that sez so though.

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Keith
Reply to
krw

I don't know of any laptop drive that would actually power up with only 100ma of +5V. Most need more than half a watt just idling, and typically need around 2W while doing actual I/O to the disk.

Reply to
robertwessel2

"John Devereux" a écrit dans le message de news: snipped-for-privacy@cordelia.devereux.me.uk...

During enumeration a device can't use more than 100mA. The host then read the required amount of current in the device's descriptors. A host must always accept a device requiring only 100mA, but can accept or reject a device requiring more depending on the host available power but also on the USB bus architecture. In particular be careful as any device connected behind a bus powered hub is limited to 100mA... Having said that I've never met a host not compatible with 500mA current, except hubs.

Robert

Reply to
Robert Lacoste

There is another, lower current, restriction (though I've forgotten what it is) on power drawn during power saving modes- perhaps more pressing with laptops than desktops. This could be important if you need to maintain state across power down periods- perhaps a battery backup in your thingy.

JS

Reply to
JSprocket

Good point, that could well be an issue.

The "thingy" performs a test sequence that takes a few minutes, so the user will be basically waiting for it to complete. Not much good if it shuts down halfway through. I think if this coincides with his laptop going to sleep, that is OK - it's his own daft fault for letting it do that. And there are reasonably obvious settings for setting the "sleep" delay. What I don't want is for the laptop to "secretly" turn off the port to save power, while otherwise working normally.

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John Devereux
Reply to
John Devereux

This one works fine on my ThinkPad T60/61s (it was $30 cheaper last=20 week).

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--=20 Keith

Reply to
krw

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