PCIe AC coupling capacitors values

xabiven — Thu, 02 Nov 2017 09:12:42 GMT

Hello all,

I have a question on the value of the required AC coupling capacitor for PCIe links.

Indeed, the QorIQ data-sheets (Txxx and LSxxx) specify the AC coupling capacitors depending on the targeted protocol. So, it recommends:

* PCIe GEN2 (2.5 GT/s): CTX = 75 to 200 nF

* PCIe GEN2 (5.0 GT/s): CTX = 75 to 200 nF

* PCIe GEN3 (8.0 GT/s): CTX = 176 to 265 nF

I observed that the reference boards always have 100 nF capacitor, like on traditional GEN2 designs.

=> Does GEN3 is implemented on demo-boards ?

=> Should I use capacitor in the range 176-200 nF if I design an interface which can run wether at GEN2 or GEN3 rates ?

=> Or, should I use a classic 220nF capacitor for a PCIe GEN3 capable interface, considering it will be able to negotiate the link down to GEN2 speed ? Will it provide a reliable GEN 2 link ?

Best regards,

Re: PCIe AC coupling capacitors values

ufedor — Fri, 03 Nov 2017 03:28:56 GMT

New designs have to have 220 nF AC coupling capacitors which are fine for PCIe Gen 3 as well as Gen 2/Gen 1.

Re: PCIe AC coupling capacitors values

xabiven — Mon, 06 Nov 2017 08:15:05 GMT

Hi Ufedor,

It's noted, thanks a lot for you quick feedback

Regards,

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PCIe AC coupling capacitors values

Re: PCIe AC coupling capacitors values

Re: PCIe AC coupling capacitors values