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USB power levels

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osmosis321:
I'm very unclear on how USB provides power to connected devices.

For example, if USB detects a device already has a voltage level, what happens then?  If USB powers a device, will it always drive it with 5v or can it do another voltage..

I ask because I've been scrutinizing the layout of this board I'm working on, and it turns out the USB power is directly connected to the main VDD of the board, and I know the PIC24FJ won't survive that voltage for long.

I got lucky - before I became aware of this potential issue I'd connected the USB with the board already powered at 3.3v, and nothing bad happened.  Presumably the USB hardware detected the voltage and didn't provide any of it's own?  And presumably if I connect the board to USB without it's own power, USB will start providing 5v and my MCU will overheat and die?

alm:
USB can only provide 5(ish)V. Not sure about detecting already powered devices, I wouldn't rely on this behavior, unless it's clearly part of the spec (and even then, low-end manufacturers usually only implement part of the spec). I've never heard of this, it may just be a side effect of some other protection circuit (eg. reverse power protection diode in 3.3V regulator conducting and overcurrent protection of the USB port kicking in). You may be able to disconnect the USB power pin for self-powered operation.

Mechatrommer:
5V and always. if your circuit is made poorly, sure it can kill your mcu. and even your PC USB port/hub.

migsantiago:
Every single USB port must deliver the following power:

Low power port: 5V, 100mA maximum
High power port: 5V, 500mA maximum

Good USB ports disable the 5V supply when the computer is turned off (by software or by hardware jumpers). Cheap USB ports always enable the 5V output, even with your computer turned off.

If you are going to use a PIC24 and a USB port, use a 3.3V regulator to power the PIC.

Make sure that your PIC will not draw more than 100mA.

NEVER SUPPLY ANY VOLTAGE OR CURRENT BACK TO THE VBUS PIN (5V pin).

NEVER CONNECT MORE THAN 10uF OF CAPACITANCE TO THE VBUS PIN (5V pin).

NiHaoMike:

--- Quote from: migsantiago on October 24, 2010, 03:48:02 pm ---Good USB ports disable the 5V supply when the computer is turned off (by software or by hardware jumpers). Cheap USB ports always enable the 5V output, even with your computer turned off.
--- End quote ---
The only USB ports I have seen that could remain on with the computer off were in a Dell laptop, and it was marketed as a feature. Basically, the USB ports turn on whenever the AC adapter is plugged in, to allow users to charge cell phones and other electronics from the USB ports without turning the computer on. That feature can be disabled in BIOS.

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