Standard USB cables usually have four wires: Power: VCC (+5VDC) and Ground, and Data: D+ and D-. Using a combination of these wires, there are three basic modes of operation:
When the data lines are in normal use (ie transferring information), a maximum power of up to 500ma (.5 amp) is typically supplied by the USB specification. This is Standard downstream port connection setup (SDP) and is the most common connection when using a smartphone as a connected device.
However, if the data lines are manipulated properly, a handshake protocol occurs and the connected device may require more current (up to 1.5A for USB 3.0). This is Downstream Port Loading Connection Configuration (CDP). This is the most sophisticated connection. Your phone may or may not be able to use this feature based on its software and hardware configuration.
If the two data lines are shorted by the connected device or cable, with no data communication at all, then the current will be set to deliver up to 1.5A, depending on what the available power source can handle. This is mainly used for chargers. This is called Dedicated charging port Connection Configuration (DCP).
This is largely why you may see different charging times and capabilities when connecting to the Ascent infotainment system. If you want to know more, keep reading, if not, you can stop here.
In a USB connection there is usually a “host”, most often a computer, but in this case the Ascent’s infotainment system (which is a computer) and the connecting device. The connecting device can be a smartphone, a USB drive, a USB hard drive, etc.
It is not the main function of the host to charge a connecting device. The main function is data exchange. However, it is also recognized that the host is responsible for powering a connecting device that does not have its own power source, such as a Therefore, the USB specification requires that 2.5ma-500ma (900ma w/USB 3.0) current is nominally available to power a connected device. This is the Standard Downstream Port (SDP) connection configuration. If a device needs more than 500mA, then it is expected to supply its own power. This is why some USB drives have their own external power source and some don’t, depending on whether they can only run on 500ma. A smartphone supplies its own power, but battery life is limited, so it may want to charge using this available host current. You can charge a smartphone battery with only 500mA, but it will probably be significantly slower than you want.
Thanks in part to Apple (who disliked supplying chargers for their docking devices like the iPod), it was later widely understood that the host must also supply power for charging. However, the host can be a battery-powered laptop or with limited internal power capability, or in the case of the Ascent, can only run on the car’s battery at times. So the host won’t just dump more of its potentially limited power on a binding device by default without being asked. Also, simply supplying more current by default can cause problems or even damage some sensitive connecting devices. Therefore, the connecting device must require somewhat more power from the host. Since pre-USB 3.0 USB connectors only had two power wires and two data wires (plus an extra ground), the easiest way to do this was to simply effectively shorten the data wires. Then the host knew that the connecting device was in “charge mode” and not “data mode” and would supply (at its discretion) up to 1.5A for charging purposes. This is the dedicated charging port (DCP) connection setup. In this way, data cannot usually be exchanged. Also, please note that some so-called “charging cables” try to force this mode by internally shorting the data connectors. This may or may not work depending on how the host is designed to react. This also often causes a lot of confusion about USB cables. People think that “charging cables” have thicker wires and thus can draw more current. The ampacity of USB cables has only a small effect on current draw over short lengths. Usually the better designed USB cables can draw enough current to supply 1.5 amps in lengths under a few meters. Only the cheapest cannot. So it’s rarely just a cable problem when you think USB charging is too slow.
Finally, it was later realized that a USB connected device can require power for both data usage AND charging at the same time, as in the case of smartphones, or that a connected device simply needed more power than the default 500ma. So a more sophisticated way was created to make the data lines require more current from the host. In this way, data could still be exchanged while the device was being charged and the required amount of current could be supplied (up to 1.5A). However, this mode requires that both the host and the connecting device be designed and programmed to set up and understand the handshake protocol, and that the host be willing and able to provide more power. This is the configuration of the Charge Downstream Port (CDP) connection.
So the Ascent can have all three modes, but if the connecting device (your smartphone) is not designed to use CDP mode, it will not perform the necessary handshaking and will be stuck in the actual current SDP mode. You can use a charging cable to try to force it into higher current DCP mode, but then you’ll lose the simultaneous ability to transfer data while charging.
So again, it’s more complex than you think, but I hope this clears up USB charging for everyone.
