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currently I have a problem with an USB Charging Plug (from china).

There are some designs, where the D+ and D- line is shorted together, in other there are held to 2.7V.

On my plug on the table, there are blank pads and no resistors at all, so the D+ and D+ are floating (to GND).

In my opinion, the charged device has to presume, that it's a normal USB port on the other side and just take up to 500mA.

On the other hand I have an iPhone 7, that is not charging with this plug at all. Is it possible, that the iPhone refuses to charge, if the D+ and D- are floating? Or it's trying to get more than 1A out of the plug?

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    \$\begingroup\$ Maybe someone should consider writing a generic question and answer to this type of sub-question. It comes up quite often. \$\endgroup\$ – Andy aka Jun 13 '17 at 11:36
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    \$\begingroup\$ For reference, this question also has some useful answers on this subject: electronics.stackexchange.com/questions/123172/… \$\endgroup\$ – Bimpelrekkie Jun 13 '17 at 15:02
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How a device interacts (if at all) depends on the device (for example: a phone) and the charger.

Problem is that there is no universally applied standard for USB charging so many manufacturers "do their own thing".

I found here an application note by Maxim. From this the following picture confirms that chargers can be different in how they treat the D+ and D- lines:

enter image description here

This is just the charger's side. How a phone etc. responds to this is another matter.

Apple products are well known for not working with non-Apple / generic accessories so your iPhone 7 will no doubt refuse to charge from anything else but an Apply charger or a charger which is good enough in convincing the phone that it is an Apple charger.

You're right that a device should only draw (not 500 mA but) 100 mA from a USB port when D+ and D- are floating so no communication can take place (the device cannot be enumerated). However, not all devices will follow this rule.

Some just pull 1 A no matter what.

Some might just increase the current until the voltage drops too much.

Others simply refuse as they don't know what is charging them (like your iPhone).

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  • \$\begingroup\$ According to the 2.0 spec a device should only use 100mA when it's not enumerated. \$\endgroup\$ – Jeroen3 Jun 13 '17 at 12:13
  • \$\begingroup\$ @Jeroen3 You're right, I'll change that. \$\endgroup\$ – Bimpelrekkie Jun 13 '17 at 12:17
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    \$\begingroup\$ @Jeroen3 - you're correct, but many devices don't honor that little detail in the spec. \$\endgroup\$ – brhans Jun 13 '17 at 12:18
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    \$\begingroup\$ And my guess is that many (charging) ports / devices just "allow" 500 mA anyway without making a fuss because almost no one follows the rules anyway. Hmm, what a mess :-( \$\endgroup\$ – Bimpelrekkie Jun 13 '17 at 12:21
  • \$\begingroup\$ @Bimpelrekkie Devices often use cheap polyfuses as protection instead of more expensive chips. Power requests can actually be denied by the host! The world has a new chance with USB 3.0 Power Delivery. \$\endgroup\$ – Jeroen3 Jun 13 '17 at 13:01

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