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 ?