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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On Aug 5, 7:29=EF=BF=BDam, John Devereux wrot= e:

This is from memory -- becuase everytime I think "USB", I have to go get the books - which are at the office.

So, I'll just mention this: There are hubs, ports and controllers. I think controllers are all high-power?? This is what would be on the laptop. Hubs would be downstream from this, and could be either - or can also be powered hubs meaning they have a wall wart. Ports are downstream from there and are enumerated as either high or low power, standard, or high speed, etc... Depending on what's available to be delivered to them.

I might have the actual terminology wrong, but the jist is right.

But as to low-power controllers on laptops?? I don't know. I wouldn't think so.???? Someone here will know for sure. Good luck with the project.

-mpm

Reply to
mpm

That is what I am hoping for... I can specify that the device must be plugged directly into the laptop, or perhaps a hub if specifically labelled "high-power". But I don't think a user is going to know if his laptop port is high-power or not.

Thanks!

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John Devereux
Reply to
John Devereux

Key to remember is that the device is meant to _ask_ to have the extra power. Technically it is not allowed just to draw it (though on many machines this will be OK). So your device needs to be able to wake up, and connect to the USB, without drawing more than 100mA. The host, then can allow or refuse the request. One solution to more power, is to use two USB headers. This is commonly done on some older USB powered hard drives, where the 'spin up' current is borderline for the port. You need to be fairly careful, when switching to using the full power, that the inrush current is kept low.

Best Wishes

Reply to
Roger Hamlett

I was looking at using the FT232R, which seems to be able to negotiate this for me. .

No, that's nasty :)

Thanks, this is mentioned in the FT232R datasheet too (and there is a control pin and a nice little circuit to deal with it).

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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

Yes, although as I imagine we're all aware many peripherals don't bother and few USB controllers actively restrict current to less than 500mA... hence some devices cheat and do draw >100mA even before asking permission.

I've *never* seen one of these that supports the full "ask first" protocol on

*both* connectors. :-)

---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

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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