[SI-LIST] Re: USB device cable shield connection
- From: earl albin <earlalbin@xxxxxxxxx>
- To: Colin D Bennett <colin@xxxxxxxxxxx>
- Date: Mon, 29 Aug 2011 17:56:52 -0700
Colin:
I read the TI, Intel, and Cypress app notes. The Cypress app note is
certainly insufficient as a guide. For one, the 4700pF cap as shown and as
intended, RF mitigation, is incorrect in that it's really an LRC. Proposed
network couples DC (1-MOhm) and depending on the cap, likely up to about
10-20 MHz AC between the shield and signal grounds. The 2-meter cable is not
an efficient radiator at those frequencies. Again though this all depends on
the cap and the layout. The ferrite bead accomplishes this effect sort of in
the opposite direction
I thought both TI and Intel were generally in agreement, perhaps not exactly
on the values but on the approach. Intel did mention that this was what they
found to work. Likely the first attempt didn't then they made
adjustments/changes till they got it to pass.
So at least you could start with either Intell or TI, better than guessing.
On Mon, Aug 29, 2011 at 11:42 AM, Colin D Bennett <colin@xxxxxxxxxxx> wrote:
> I have been trying to understand how to ideally handle the cable shield
> on a USB device. (Full Speed USB, in this particular instance.)
>
> As seems to be the case with many signal integrity issues,
> contradictory recommendations abound, each with its own unsupported
> claims. Even authoritative-sounding sources such as Texas Instruments,
> Intel, FTDI, and Cypress Semiconductor seem to disagree on the correct
> way to handle the cable shield on USB devices.
>
> Contrary to my initial supposition, the purpose of the USB cable shield
> is not to protect the USB data lines from outside interference, but
> rather to prevent the USB device from radiating EMI.
>
> Here are some of the options that have been recommended.
> Note that (2)â??series capacitor to pass high frequencies onlyâ??seems to
> directly contradict (3)â??series ferrite bead to block high frequencies
> only.
>
> (1) Connect shield directly to signal ground.
>
> - â??Full speed devices use a shielded cable which requires that the
> connector shell be tied to the ground plane.â??
> Intel. EMI Design Guidelines for USB Components. Sec 5.4 (p. 9).
> <http://www.ti.com/sc/docs/apps/msp/intrface/usb/emitest.pdf>.
>
> (2) Connect shield to signal ground through a capacitor.
> (Possibly with high-value parallel resistor approximately 1 Mohm.)
>
> - Connect shield to signal ground with 0.01 µF to 0.47 µF capacitor.
> FTDI. Debugging FT232BM and FT245BM Designs. Section 3.2 (p. 11).
> <http://www.ftdichip.com/Documents/AppNotes/AN232B-06_11.pdf>.
>
> - Cypress recommends a 1 Mohm resistor in parallel with a 4.7 nF
> capacitor.
> Steve Kolokowsky & Trevor Davis (Cypress Semiconductor).
> Common USB Development Mistakes â?? You Donâ??t Have To Make Them All
> Yourself! Figure 7 (p. 7).
> <http://uk.farnell.com/images/en/ede/pdf/usb_dev_mistake.pdf>.
>
> - â??Tying the shield directly to ground would create a direct path
> from the ground plane to the shield, turning the USB cable into
> an antenna. To limit the USB cable antenna effect, it is
> recommended to connect the shield and ground through an RC
> filter. Typically, R = 1MΩ and C = 4.7nF in Figure 3-5.â??
> Atmel AVR1017: XMEGA - USB Hardware Design Recommendations.
> Section 3.3.3 (p. 8).
> <
> http://atmel.com/dyn/resources/prod_documents/doc8388.pdf?doc_id448&family_id`7
> >.
>
> (3) Connect shield to signal ground through a ferrite bead.
>
> - â??Place a ferrite in series with the cable shield pins near the
> USB connector socket to keep EMI from getting onto the cable
> shield.â??
> Texas Instruments Application Report.
> USB 2.0 Board Design and Layout Guidelines. Sec 2.2.4 (p. 3).
> SPRAAR7 â?? December 2007.
> <
> http://focus.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=spraar7&fileType=pdf
> >.
>
>
> Whether or not the device has a metal chassis, and the handling
> of chassis ground and signal grounds, (as well as how the USB cable
> ground is connected to either one) is certainly important as well, but
> this isn't clearly discussed in most of the writings on USB cable shield
> grounding.
>
> The device I'm developing is a bus-powered device which will likely be
> in an unshielded plastic enclosure.
>
> Thanks in advance for any bits of wisdom on this topic full of
> contradictory information.
>
> Regards,
> Colin
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