Zak has managed to bridge the Raspberry Pi 4’s PCI-Express bus to a USB 3.0 port. This uses a PCIe bridge “chip” (tiny printed circuit board) which replaces the VL805 USB 3.0 controller chip on the Pi, giving access to the PCI-Express bus on a USB 3.0 port.
The bridge “chip” is a 0.8mm thick PCB from OSHPark with copper pads in the same locations as a real VL805 QFN68 IC package, then traces connecting the PCIe pads to the USB pads that connect to the upper USB 3.0 port. RESET, WAKE and a few other signals were also connected to the lower USB 3.0 port.
A quick test using 4 USB 3.0 flash drives plugged into the VL805 expansion card resulted with a total read throughput of 3 Gbps out of a maximum theoretical throughput of 4 Gbps over a 5 Gbps PCIe 2.0 link.
See this blog post for the connection details. The bridge PCBs are available on his Tindie store.
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