Correctly terminating unused USB data lines
Alex M, Sat Jan 25 2014, 02:11AMHi,
Is there a correct way to terminate unused data + and - lines on a USB powered device? I am only using the +5v and GND to power a homemade microphone amplifier but am unsure as to what the correct procedure is for devices like this.
Should they be pulled down/up with resistors or can they just be left flapping? The USB cable is about 3 metres long so they probably make for good antennas ...
Any advice would be greatly appreciated!
Thanks.
Re: Correctly terminating unused USB data lines
PSCG, Sat Jan 25 2014, 08:23AM
Judging from the USB specifications, data lines (D+ and D-) are pulled down (grounded) at the host controller via 15 K resistors, indicating a disconnected state. Since they are grounded and the host does not expect a connection from the other end, i assume that you can use the power lines freely (but always inside the maximum operating power standards - 150 mA / 0.75 W for USB 1.0 and 500 mA / 2.5 W for USB 2.0).
You could probably bypass the data lines with 10 - 20 pF capacitors to the ground cable to be sure that no RF noise can disturb the host controller.
PSCG, Sat Jan 25 2014, 08:23AM
Judging from the USB specifications, data lines (D+ and D-) are pulled down (grounded) at the host controller via 15 K resistors, indicating a disconnected state. Since they are grounded and the host does not expect a connection from the other end, i assume that you can use the power lines freely (but always inside the maximum operating power standards - 150 mA / 0.75 W for USB 1.0 and 500 mA / 2.5 W for USB 2.0).
You could probably bypass the data lines with 10 - 20 pF capacitors to the ground cable to be sure that no RF noise can disturb the host controller.
Re: Correctly terminating unused USB data lines
Alex M, Sat Jan 25 2014, 01:38PM
@PSCG Thanks. I added the capacitors like you said and so far so good. The power consumption of my circuit is well within limits, in fact the power LED consumes more power than the amplifier!
I also connected a 4.7nF cap in parallel with a 1M resistor between cable shield and circuit ground as noted in a number of USB app note documents for good measure.
Alex M, Sat Jan 25 2014, 01:38PM
@PSCG Thanks. I added the capacitors like you said and so far so good. The power consumption of my circuit is well within limits, in fact the power LED consumes more power than the amplifier!
I also connected a 4.7nF cap in parallel with a 1M resistor between cable shield and circuit ground as noted in a number of USB app note documents for good measure.
Re: Correctly terminating unused USB data lines
PSCG, Sun Jan 26 2014, 11:30AM
I'm glad to hear that everything went OK! I will keep a note about the capacitor / resistor network for future reference.
PSCG, Sun Jan 26 2014, 11:30AM
I'm glad to hear that everything went OK! I will keep a note about the capacitor / resistor network for future reference.
Re: Correctly terminating unused USB data lines
Alex M, Sun Jan 26 2014, 10:58PM
@PSCG Thanks again
I went on a bit of a USB standards crusade and came across this rather handy document
Toward the end of page 6 and carrying onto page 7 is the part regarding the USB cables shield conductor.
Alex M, Sun Jan 26 2014, 10:58PM
@PSCG Thanks again
I went on a bit of a USB standards crusade and came across this rather handy document
Toward the end of page 6 and carrying onto page 7 is the part regarding the USB cables shield conductor.
Re: Correctly terminating unused USB data lines
PSCG, Mon Jan 27 2014, 09:42AM
Thank you for the PDF. There is lots of good information inside.
PSCG, Mon Jan 27 2014, 09:42AM
Thank you for the PDF. There is lots of good information inside.
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