Idea for PCIe design

[thomasvl]

Hello,

I am designing a board with a switch PCIe. There is an option on it to have either one x8 or two x4.
I have two connectors PCIe on my board but I don’t know how to design it so that with first option, one connector has x8 lanes and if I use second option, first connector has x4 lanes and second connector has also x4 lanes ?
I was thinking about a relay but won’t it be too “bad” for high speed signals ?

If you have any idea, you are more than welcome ^^

[marekr]

You need to use high speed muxes that are intended for those speeds.

Such as (example):
https://www.diodes.com/products/connectivity-and-timing/switches-mux/protocol-switches/pci-express-signal-switch/

[GyrosGeier]

So you want the same board, but different connectors, and whichever set of connectors you populate should work?

The sanest approach, honestly: use a switch that gives you 12 (more likely 16) lanes, route 8 to the x8 connector spot and 4 to the x4 connector spot. Anything but a straight impedance controlled connection pair will not work at that kind of speed, and the engineering effort to use a switch with 8 downstream lanes instead of one with 16 vastly exceeds the price difference in any production run that’s less than a million units.