Update 2/14/2018, 9:45 AM: AMD has posted a support page with steps users can take to resolve this specific issue should they build an unbootable system with a Ryzen APU and an older motherboard. The company suggests updating motherboard firmware using a compatible CPU, requesting assistance from a retailer with a compatible CPU, requesting an RMA exchange for a compatible motherboard, or using an AMD-provided boot kit to update the firmware. The original article continues below.
As builders shop AMD's Ryzen desktop processors with Radeon Vega graphics onboard, they can rest somewhat assured that a Ryzen 3 2200G or Ryzen 5 2400G will physically fit into and light up the various ports and slots of Socket AM4 motherboards on the market today. Especially eager gerbils without an older Ryzen CPU or Bristol Ridge APU on hand may want to give e-tailers time to refresh their motherboard inventories, however, as mobos with older firmwares installed might not boot up with a Ryzen APU in their sockets.
We can't speak to the capabilities or behavior of every manufacturer's Socket AM4 boards, but we tried dropping a Ryzen APU into a budget-friendly Gigabyte AB350-Gaming 3 that we've left un-updated for some time to see whether an older firmware version would at least POST and enter the BIOS. Sadly, that proved not to be the case. The board hung at some early stage of POSTing and never reached its settings screen. We ultimately had to bootstrap the board into Ryzen APU compatibility using another one of the first-generation Ryzen CPUs in the TR labs.
Fancier motherboards from Gigabyte, ASRock, MSI, and Asus might be able to get around this issue thanks to their ability to update firmware without a CPU or memory installed. If your ASRock or Asus board touts BIOS Flashback support, your Gigabyte board offers the company's Q-Flash Plus capability, or your MSI board includes the BIOS Flashback+ feature, you can likely download your board's latest firmware and install it without the help of an older Ryzen chip. That said, we doubt many value-minded builders are considering the very-highest-end Socket AM4 boards to go with their slice of affordable gaming power.
For its part, AMD will be implementing a labeling program for boards that are ready to go with Ryzen APUs. Look for the "AMD Ryzen Desktop 2000 Ready" logo from the box insert above on any motherboard you might be considering for worry-free Raven Ridge compatibility. Unless your mobo of choice features that label, it might be wise to tread carefully until the latest runs of AM4 boards reach retail shelves.