Undervolt on powered USB 2 hub?


#1

Both of my plugable hubs ‘USB2-HUB7BC’ are acting weird. First, they only deliver 4.88 Volts output, even if there is no load whatsoever. An original Apple 12W adapter delivers 5.12 Volts. The 4.88 Volts are enough to trigger an undervolt warning on a Raspberry Pi.

Second, a ‘smart’ measurement device (YZXStudio USB 3.0 Power Monitor) detects all ports as regular 0.5 amp USB devices (D+ and D- being at 0 Volts). The same original Apple 12W charger is detected as ‘Apple 2.4A’. D+ and D- showing 2.77 Volts.

Are both devices broken, or is my measurement device off?


#2

Hello,

Sorry to hear about these issues.

The USB-IF specification for USB 3.0 voltage is between 4.45 to 5.25V. They lowered the acceptable minimum voltage from 4.75 which was the standard for USB 1.x and 2.0.

My test unit with no load sits right at 5V (5.02). With an Apple iPad Air 2 charging I do get a small voltage drop to 4.71V (6.6%).

It’s possible that your voltage measurement device is running a bit low, or that mine is running a bit high, it’s hard to say. Do you have a multimeter available to check the voltage straight out of the power adapter?

Thanks!

!](https://d2r1vs3d9006ap.cloudfront.net/s3_images/1287431/IMG_20151004_094553~2_inline.jpg?1443977759)](https://d2r1vs3d9006ap.cloudfront.net/s3_images/1287431/IMG_20151004_094553~2.jpg?1443977759) !](https://d2r1vs3d9006ap.cloudfront.net/s3_images/1287432/IMG_20151004_094759~2_inline.jpg?1443977772)](https://d2r1vs3d9006ap.cloudfront.net/s3_images/1287432/IMG_20151004_094759~2.jpg?1443977772)

For the hub being detected as a regular .5A port, your smart device may not be BC 1.2 compliant to show the hub operating in 1.5A CDP mode.

If you disconnect the hub from the host computer, the ports should show up as Apple 2A for example as it then tries to emulate proprietary charging signals.


#3

Hello, I did some additional measurements, plus I upgraded the firmware of my USB meter. I believe the meter did not show a proper result if a BC 1.2 device was attached.

This is how it looks empty, 4.71 Volts, and a regular USB Device (no special signaling):

!](https://d2r1vs3d9006ap.cloudfront.net/s3_images/1290686/2015-10-10%2317-03-53-4037_inline.jpg?1444491055)](https://d2r1vs3d9006ap.cloudfront.net/s3_images/1290686/2015-10-10%2317-03-53-4037.jpg?1444491055)

Then I connect an iPad. The iPad seems to negotiate with the hub and agree on a charging mode. The voltage drops to 4.6V and the iPad charges with 0.94A:

!](https://d2r1vs3d9006ap.cloudfront.net/s3_images/1290688/2015-10-10%2317-06-31-4041_inline.jpg?1444491226)](https://d2r1vs3d9006ap.cloudfront.net/s3_images/1290688/2015-10-10%2317-06-31-4041.jpg?1444491226)

When I disconnect the iPad, the hub does a strange thing. It no longer shows up as a regular 0.5A USB port, but now it looks different. Ignore the attached cable: IT is empty on the other side, and it looked the same without the cable:

!](https://d2r1vs3d9006ap.cloudfront.net/s3_images/1290689/2015-10-10%2317-14-57-4044_inline.jpg?1444491355)](https://d2r1vs3d9006ap.cloudfront.net/s3_images/1290689/2015-10-10%2317-14-57-4044.jpg?1444491355)

In comparison, an Apple 12W charger:

!](https://d2r1vs3d9006ap.cloudfront.net/s3_images/1290692/2015-10-10%2317-18-09-4046_inline.jpg?1444491390)](https://d2r1vs3d9006ap.cloudfront.net/s3_images/1290692/2015-10-10%2317-18-09-4046.jpg?1444491390)

The iPad charges with almost 2A, plus it has a bit more Volts. So it will charge in less than half the time compared to the charging hub:

!](https://d2r1vs3d9006ap.cloudfront.net/s3_images/1290695/2015-10-10%2317-18-40-4047_inline.jpg?1444491535)](https://d2r1vs3d9006ap.cloudfront.net/s3_images/1290695/2015-10-10%2317-18-40-4047.jpg?1444491535)

All this is without a computer connected.

Does this look normal? How can I get up to 2A when an iPad charges?

Thanks! Martin


#4

Actually I think the firmware of the YZXStudio gets confused as soon as the iPad is connected. Somehow it misinterprets the voltage that the iPad applies to D+ and D- (probably to check if the data lines are in fact shorted, as they should be on a BC 1.2 DCP). It seems to think that these are voltages to signal QC2.0 probing, which they are clearly not.

So in the end the hub is a BC 1.2 device, as advertised. For some reason the iPad (and the iPhone) limit the current they draw from a BC 1.2 device to 1A, even if the standard seems to allow up to 5A.

What remains is the voltage, which should really not drop to 4.6V when the current is only 1A. IMO, it should stay at least over 4.75V, preferably even over 5V to allow some voltage drop on the USB cable.

You wrote: “If you disconnect the hub from the host computer, the ports should show up as Apple 2A for example as it then tries to emulate proprietary charging signals.”
-> This does never happen. The hub always shows as a BC1.2 device.


#5

“You wrote: “If you disconnect the hub from the host computer, the ports should show up as Apple 2A for example as it then tries to emulate proprietary charging signals.”
-> This does never happen. The hub always shows as a BC1.2 device.”

Oh my! I realized you have the 2.0 version and I was testing and talking about the 3.0 version!

Thanks for doing the testing, I need to go back on Monday and test our 2.0 hub and see what’s going on. Sorry for any confusion.


#6