I was reading about USB charging ports and how a host can tell the device what current it may draw: How do USB charging and "smart" charging ports (e.g. Anker's PowerIQ) work?

Do all these "identifier" techniques occupy the data lines all the time? Is there no data transfer possible while the host is identified as a "charger"? Or is there also a solution to tell the device it can draw a higher current only at the beginning of the connection and then the data lines are freed up to transfer data?

  • \$\begingroup\$ Sure, all what you suggest is possible. The question is whether any device (phone, tablet) has actually implemented this. Also, since you want to charge from a "data" device like a PC or a laptop, that PC or laptop has to support this as well. I doubt that any implementations exist as it needs cooperation between the phone and the PC/laptop. \$\endgroup\$ – Bimpelrekkie Feb 6 '18 at 13:38
  • \$\begingroup\$ I agree that the device has to support it. As for the host I want to build something myself, this is why I am asking. But if there is no standard for that, no device will probably support this... \$\endgroup\$ – Reto Feb 6 '18 at 13:53
  • \$\begingroup\$ Some devices have USB-C ports that would allow high current charging together with data transfers. Non-C tyoe chargers usually use the data lines to indicate higher currents - this precludes USB data usage. \$\endgroup\$ – Turbo J Feb 6 '18 at 14:34

USB charging ports are supplementary to basic USB 2.0 specifications. In official (USB "sponsored") Battery Charging Specifications (BC1.2) the advertising of port power capabilities occurs BEFORE any connect event occurs, so the levels of initial BC1.2 handshake are chosen to be BELOW normal initial USB signaling levels, such that the USB part of front-end transceiver won't be confused. After the BC1.2 protocol completes and VBUS capability is communicated to device, the D+/D- lines are free from the "charging signature", and can operate in accord with standard USB protocol. This is how the "CDP", Charging Downstream Port is defined, the advertising handshake happens, and then it is gone.

I am not aware of any definition of "CDP" (host data ports) with proprietary charging methods/signatures, all the methods are typically meant for stand-alone chargers. At least for QC ver.2 this TI document implies that if D+ doesn't hold its special value on device side, the power interface drops down to default 5-V level. So you can't maintain QC and free D+/D- for data communication. How things are with Qc v3.0 and v.4.0 is unclear.

However, if a host port supports USB 3.0+ communication, methods other than BC can be implemented, since USB 2.0 communication is secondary to USB 3.0 connection. Same goes for Type-C connector standard, which is/was quite open to possibility to have proprietary charging signatures even when Type-C has its own means to advertise its basic charging capability (at 5V level).


This depends on what standard you are using. "High current" can mean quite a few different protocols.

For example, the standard 5V output can go above 500mA whilst also allowing data IF the port is a charging downstream port. Most data lines are not configured as such.

USB data pin configurations for charging

There are many newer standards, such as those above 5V.

Power Distribution 2.0 called for a BPSK modulated high-voltage negotiation signal to occur on the power pins (not data). This appears to be depreciated in PD 3.0.

Qualcom quick charge uses the data lines for negotiating the voltage, thus only power is available:

quick charge voltage table

This is obviously not an exhaustive list of every protocol, but I have not worked with every single protocol.

  • \$\begingroup\$ I was referring to 5V with 1A or 1.5A... so USB-C seems to be the only way. \$\endgroup\$ – Reto Feb 6 '18 at 14:39

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