USB hub with power switch device detection

[Bobo_E]

Hi

Could someone please explain to me how a self-powered usb hub with a power switch detects that a device has been plugged to one of its ports? Is it right that a USB hub, on its own without using a power switch, detects a device by monitoring the voltage on the data lines? The hub has 15k pull-down resistors on data lines whilst the device has 1.5k pull-up on one of the data lines so when the device is connected then the power is first applied to it, therefore, also to the 1.5k pull-up resistor which then creates a voltage divider with the 15k pull-down resistors. That is how the hub detects the device without a power switch.

Getting to the point...Having a power switch (activated by the hub once the device is detected) means that the power is not applied to the device straight away when it is connected, therefore, how is it detected by the hub in the first place so that the hub can activate the switch?

 

[BradtheRad]

When first plugged into usb, a device can only draw a certain amount of milliAmps (about 100 mA). Before it can draw more power it must negotiate with the host. The process is called enumeration.

Does the host detect current flow? Does it detect a voltage drop? Does it detect digital communication? Not sure which.

Here's one discussion from among Google hits.

https://electronics.stackexchange.com/questions/5498/how-to-get-more-than-100ma-from-a-usb-port

 

[KlausST]

Hi,

Document about enumeration.
https://www.ftdichip.com/Documents/...ied Description of USB Device Enumeration.pdf

In short:
USB power is always active. Usually there is only a polyfuse installed to limit the current to the device. (Never seen hubs with power switches or current sensors, but there may be some). So the device can draw a lot of power, but sometimes it is not allowed to.
With the pullup at the data line the device tells not only it's presence, but also it's speed.
The power switch usually is in the devices, not the hub. It is controlled with USB protocol.
The host may send a command to the device to power down (usb suspend), but must not necessarily switch off the usb power.

*****
This is how the USB specification tells, but there are many (often cheap) usb devices that don't keep to the rules.

Klaus