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Observer ami@
Observer
2,054 Views
Registered: ‎11-16-2017

Series capacitors on SATA,USB3

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Hi

 

At my design I use XCZU3EG-1SFVC784IES.

The GTR block implement  SATA,USB3.

By ug583, the TX/RX channels need incorporate 0.1uF caps.

At ZED EVB I saw caps on TX/RX channels of SATA, and only at TX channel of USB3.

I assume they should be near the transmitter.

Pls. advise.

 

Ami

 

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Xilinx Employee
Xilinx Employee
2,225 Views
Registered: ‎08-07-2007

Re: Series capacitors on SATA,USB3

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hi ami@

 

i don't think zed board is wrong.

Looking at sata spec, it shows AC coupling on both TX and RX side.

for USB, please put AC coupling caps on TX side.

 

you can also refer to Xilinx eval board ZCU102 schematics.

https://www.xilinx.com/member/forms/download/design-license.html?cid=473359&filename=zcu102-schematic-xtp454.zip

 

Thanks,

Boris

 

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sata.png
9 Replies
Scholar drjohnsmith
Scholar
2,043 Views
Registered: ‎07-09-2009

Re: Series capacitors on SATA,USB3

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can you give us a link to the zed evb you refer to please

 

for instance, the boards I have just looked through, have no capacitors on the USB, 

 

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2,013 Views
Registered: ‎01-08-2012

Re: Series capacitors on SATA,USB3

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From the USB 3.0 specification, section 3.2.1

"Each differential link is AC-coupled with capacitors located on the transmitter side of the differential link."

 

EDIT: PCI Express specifies something similar.

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Scholar drjohnsmith
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1,997 Views
Registered: ‎07-09-2009

Re: Series capacitors on SATA,USB3

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Id still like to see the Zed board links,

 

But

 

to put a "what could have happened " touch on this,

 

Boards such as the ZED and other eval boards are designed to be 'general purpose', 

    thats why there is a big get out on most eval board schematics saying word to the effect of

         "do not use this as a reference.design, refer to data sheets"

 

Some boards are good, some 'marginal', 

    but all of them contain stuff that is either just in case or there for testing,

 

something like extra capacitors on the Tx and Rx, I could see happening

    The design would still work. If both ends of the link have capacitors on, it 'just' gives a lower capacitance,

          and in practice , the design would work,

 

Well spotted , I'd suggest you follow the specification , and report the design to who ever made your eval board.

     there should be a forum on the ZED site for that .

 

 

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Scholar drjohnsmith
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1,994 Views
Registered: ‎07-09-2009

Re: Series capacitors on SATA,USB3

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Regarding position and series capacitor values,

 

By convention, if AC coupled only on one end of the link, they are placed at the Tx end as you say,

    Technically it does not matter, its all about near end and far end cross talk / impedance matching,

 

Its is important not to come up from an inner layer to / from the capacitor,

     but bottom / top , else you get a stub in the via, ( unless you have blind vias or back drill, in which case you know all this anyway )

 

You will also need anti pads under the capacitor, and preferably use a 0402 size or smaller.

 

As to the value,

    I have suspicion that its 100nf on USB3 and 10 nf on SATA 3,

 

but please please as I said before, check the spec , 

 

 

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Observer ami@
Observer
1,953 Views
Registered: ‎11-16-2017

Re: Series capacitors on SATA,USB3

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Scholar drjohnsmith
Scholar
1,948 Views
Registered: ‎07-09-2009

Re: Series capacitors on SATA,USB3

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ami@

 

Well spotted,

 

The SATA on that schematic def has 100 nf series capacitors on the TX and RX , ( C26 to C28 ) on connector  J3

 

I'd suggest you bring it up with the avnet people who designed and sell that board.

 

http://www.zedboard.org/contact

 

or

 

http://www.zedboard.org/forums/zed-english-forum

 

 

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Observer ami@
Observer
1,868 Views
Registered: ‎11-16-2017

Re: Series capacitors on SATA,USB3

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Hi

 

Can I have Xilinx recommendation for the caps placement?

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Xilinx Employee
Xilinx Employee
2,226 Views
Registered: ‎08-07-2007

Re: Series capacitors on SATA,USB3

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hi ami@

 

i don't think zed board is wrong.

Looking at sata spec, it shows AC coupling on both TX and RX side.

for USB, please put AC coupling caps on TX side.

 

you can also refer to Xilinx eval board ZCU102 schematics.

https://www.xilinx.com/member/forms/download/design-license.html?cid=473359&filename=zcu102-schematic-xtp454.zip

 

Thanks,

Boris

 

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Observer ami@
Observer
1,786 Views
Registered: ‎11-16-2017

Re: Series capacitors on SATA,USB3

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Thanks Boris!

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