I have an iPhone, iPad, and a Mac and when I connect my phone or iPad to the Mac, it automatically starts charging. But why? Why not the other way around?
My thinking is that Mac’s have a bigger battery? But still what about that makes the Mac a charging source in the presence of an iPhone/ iPad?
In: Technology
The charging circuit of a laptop is not connected to the USB port in most cases. So the port on a laptop is not designed to take in power. In a USB connection, the laptop is almost always configured as the host device. The host device must provide 5V and and a certain amount of current to the voltage bus line per the USB specification. Any port connected to the laptop becomes a USB slave device and cannot (by the spec) contend with the host to provide power.
If the phone is connected to a peripheral device (not a laptop) then it becomes the host device and the peripheral will become the slave device.
How it does so is that there is a signal line used on the USB for the negotiation between host and slave devices.
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