AMD confirmed that the older motherboards based on the X470, X370, B350and A320 Chipsets will not support the new PCIe 4.0 technology.
The next generation Ryzen 3000 CPUs coming next month will support PCIe 4.0, which is offering double the bandwidth and a range of other optimizations compared to the PCIe 3.0.
However, the only motherboards that will support PCIe 4.0 in the future will be the 500-series chipset, which is currently only the X570 range, although the B550 line-up is expected to come later this year. These chipsets are qualified for PCIe 4.0, but older chipsets will not be.
Robert Hallock, one of the Ryzen product managers at AMD, wrote in a post on Reddit:
"Pre-X570 boards will not support PCIe Gen 4. There's no guarantee that older motherboards can reliably run the more stringent signaling requirements of Gen4, and we simply cannot have a mix of "yes, no, maybe" in the market for all the older motherboards. The potential for confusion is too high. When final BIOSes are released for 3rd Gen Ryzen (AGESA 1000+), Gen4 will not be an option anymore. We wish we could've enabled this backwards, but the risk is too great."
This means that if a user wants full access to PCIe 4.0, they will have to purchase an X570 motherboard. All Ryzen 3000 CPUs are likely to still run in X470 and X370 motherboards with a BIOS update, but because PCIe is backwards compatible, these CPUs will run in PCIe 3.0 mode.