USB charging, part 1: requirements [LWN.net]

USB charging, part 1: requirements

Posted Jul 25, 2016 22:43 UTC (Mon) by mathstuf (subscriber, #69389)
In reply to: USB charging, part 1: requirements by eds
Parent article: USB charging, part 1: requirements

If you don't trust the host computer…don't plug a cable into it? If you want just power, look to the SyncStop[1] (formerly the "USB Condom").

[1]http://syncstop.com/

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USB charging, part 1: requirements

Posted Jul 26, 2016 6:15 UTC (Tue) by eds (guest, #69511) [Link] (6 responses)

Yes, I'm specifically asking what happens when such a device is in use. SyncStop blocks the data pins, which is exactly what I described.

If no data pins means no enumeration, and no enumeration means the host will cut the current back, then it's not a very useful way to recharge.

USB charging, part 1: requirements

Posted Jul 26, 2016 10:48 UTC (Tue) by gioele (subscriber, #61675) [Link] (2 responses)

> Yes, I'm specifically asking what happens when such a device is in use. SyncStop blocks the data pins, which is exactly what I described.
>
> If no data pins means no enumeration, and no enumeration means the host will cut the current back, then it's not a very useful way to recharge.

What SyncStop/USB condom should do is to have a chip connected to the data pins that does the enumeration-dance with the host and, once it is done, it should forward the current to the gadget/phone/camera attached to it. In this way the enumeration is done, but not by the phone/camera itself.

USB charging, part 1: requirements

Posted Jul 26, 2016 14:12 UTC (Tue) by jezuch (subscriber, #52988) [Link] (1 responses)

> What SyncStop/USB condom should do is to have a chip connected to the data pins that does the enumeration-dance with the host and, once it is done, it should forward the current to the gadget/phone/camera attached to it.

How does it know that it doesn't send too much power for the device not to fry?

USB charging, part 1: requirements

Posted Jul 26, 2016 15:40 UTC (Tue) by jem (subscriber, #24231) [Link]

A power supply does not send power to a device, the device draws power from the supply. The voltage is fixed at 5 volts (newer versions can supply 20 volts, when negotiated), the current drawn is up to the device.

USB charging, part 1: requirements

Posted Jul 26, 2016 11:26 UTC (Tue) by mathstuf (subscriber, #69389) [Link] (2 responses)

Yes, it blocks the data pins to your device, but will negotiate on its behalf (and as a "proof" it doesn't put data through to your device, it doesn't hook the data lines up in that side).

USB charging, part 1: requirements

Posted Jul 29, 2016 5:38 UTC (Fri) by eds (guest, #69511) [Link] (1 responses)

I can find no evidence that SyncStop / USB condom has anything at all connected to the data lines. Everything I've seen describes it as a passthrough with data lines cut.

But perhaps I just haven't searched hard enough. Do you have a link? Or know of a competing product that does this?

USB charging, part 1: requirements

Posted Jul 29, 2016 5:41 UTC (Fri) by mathstuf (subscriber, #69389) [Link]

It has to communicate with the host for power. Otherwise it's just be a simple cable which doesn't route any of the data lines at all. It doesn't have data lines to the device side though.