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I understand a standard USB 2.0 port provides 0.5A and a standard USB 3.0 port provides ~0.9A. If a USB 2.0 device is connected to a USB 3.0 port and requires more than 0.5A, will the USB 3.0 provide current all the way up to 0.9A, or will it be limited to 0.5A since it's still a USB 2.0 device that's connected?

(The USB 2.0 device in question is a 32' repeater extension cable with 2 USB ports at its output, which draws a certain amount of current to amplify the data signal over the 32' length. So a standard USB 2.0 port's 0.5A might not be enough for both the amplification and the connected devices, but a standard USB 3.0 port's 0.9A might.)

Thanks so much.

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You cannot force a device to take on more power than it needs, nor will a device try to draw more power than it can use.

A USB 2 device by definition requires at most 0.5A. It will draw as much as it needs from the port, even if the port is capable of supplying more than that. The amount depends on the device, but by definition this will be 0.5A or less.

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  • Thanks. The USB device I'm talking about is a 2-port USB hub with 32' extension cable and repeater, so because it has 2 ports at its output and also has a repeater amplifying the data signal, it will draw more current than a regular USB device. It actually will accept an optional AC adapter to power it, but in some cases might get by with just the current from the USB port. – Jyang Feb 1 '20 at 4:40
  • The device will negotiate a USB2 protocol with the port so cannot draw more, by definition. – harrymc Feb 1 '20 at 6:34
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To the question of whether a USB 3.0 port would provide 0.9A (USB 3.0 standard) or 0.5A (USB 2.0 standard) to a connected USB 2.0 device, according to this link, the answer is "it would provide only 0.5A (the USB 2.0 standard)":

https://electronics.stackexchange.com/questions/178678/usb-3-0-power-levels-on-usb-2-0-device

Q: USB 3.0 power levels on USB 2.0 device

Is it possible to negotiate a USB 3 power level, such as 900 mA, if the device is connected to a USB 3 port, without being a device capable of the higher speeds? I'd like to use this higher level if available for charging.

A: The answer to a related question suggests this will not be possible.

When operating at USB 2 (or below) the unit of power is 2mA. At USB 3.0 the unit is 8mA, allowing a higher maximum power.

Therefore unless you can negotiate as a USB 3.0 device you will be limited to the same ~500mA maximum (an unsigned 8 bit integer multiplied by 2mA).

The below link doesn't answer the question directly, but provides related info that supports the above link as correct: that the current provided is dependent on and limited by the spec that is negotiated. I won't provide a block quote for this:

Will a mobile phone charge faster if plugged into a USB 3.0 port

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  • This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review – Burgi Jan 31 '20 at 14:35
  • I'm the author of both the question and this answer, and this does provide an answer to my own question. The question is "Does USB 3.0 port provide 0.9A (USB 3.0 standard) or 0.5A (USB 2.0 standard) to a connected USB 2.0 device?", and the answer is "A USB 3.0 port provides 0.5A (USB 2.0 standard), and not 0.9A (USB 3.0 standard), to a connected USB 2.0 device." Because my answer was in response both to harrymc and to my orig question, I posted it as an answer directly below my orig question. Thanks. – Jyang Jan 31 '20 at 16:15
  • Perhaps you could quote the relevant sections from the links you included? I merely reviewed your answer after someone else flagged it as low quality. I'll be honest I found it borderline on quality. The original comment is automatically generated and lacks the subtleties of what I was asked to review. If you click the "review" link you will see the context of the review. – Burgi Jan 31 '20 at 16:21
  • Added quote of text from first link and separated response to harrymc to separate comment. – Jyang Feb 1 '20 at 4:53
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A classic USB 3.0 host port must provide "at least" 900 mA of current. See Section 11.4.1 of USB 3.1 Specifications. Since the VBUS is common for both USB3 and USB2 data connections, a USB device (2.0 or 3.x) can practically draw "at least" 900 mA.

In general, classic Type-A USB ports don't have means to monitor the actual current consumed by downstream devices, nor discriminate between USB2 and USB3 devices. There is no architectural definitions for that. In many cases the VBUS on mainboards is "ganged" together to a +5V power rail, with resettable polyfuses between the ports. So in practice one can draw several amps out of a USB port, up to what the USB connector can carry without melting and smoking.

With USB Type-C ports the situation is a but different, up to 5 A might be available per port, with mandatory individual power switches and their built-in hardware cut-offs. The Type-C CC-signaling schema doesn't differentiate between USB2 or USB3 data connection.

Yet the situation is more different in host systems that implement USB Power Delivery. Then the port must supply only what has been negotiated in power contract, and the host is obligated to limit the supply current to a capability advertised by device. Some new devices with pretense to use PD are trying to pull out the old USB2 cheat and advertise only minimal current during PD contract; these devices usually fail on Type-C ports that implement PD, but will work on ports without PD.

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