Will a 4 lane PCIe device work if plugged into a PCIe socket the only 1 lane electrically connected

Question

I am designing a motherboard that uses a SMARC format plugin card for the CPU. This card is based on an Intel Atom x6413E CPU. The SMARC CPU card has various interfaces available, including USB2.0, USB3.0, SDIO, PCIex4 (Gen 3), SATA (Gen 3), UART. We would like two M.2 sockets (X and Y) on our motherboard, one used for an SSD and the other used for WiFi/Bluetooth. I am not sure of the best key arrangement to use for each M.2 socket or what interfaces need connecting to each socket.

I think there are a few possible arrangements:

1. NVMe SSD in M.2 socket X (M.2X), using all four PCIe lanes (M-key card). WiFi/BT in M.2 socket Y (M.2Y E or A-key card). I am not sure if this would even work if the WiFi/BT card also uses the PCIe interface, since the SSD is using all of the available lanes. Maybe I could use the USB or SDIO interface for the WiFi/BT card, but this may restrict the choice of available WiFi/BT cards considerably.

2. SATA SSD in M.2X (B or M-key card) and WiFi/BT in M.2Y (E or A-key card). The SSD uses the SATA interface and the WiFi/BT uses PCIe. The SATA interface will allow 6Gbit/s which may be fast enough for our needs, but this format of SSD may not be available for much longer, as NVMe seems to be becoming standard.

3. Do we need to route all four PCIe lanes to both M.2 sockets? Maybe we could just bring two PCIe lanes to M.2X and two PCIe lanes to M.2Y, or three PCIe lanes to M.2X and one PCIe lane to M.2Y? Would the NVMe SSD work with only a two lane or one lane PCIe interface?

4. I have a very limited understanding of the PCIe interface and the M.2 format, so I have a few questions outlined below. It doesn't help that the PCIe M.2 specification isn't freely available. It seems that you need to pay PCI-SIG $2000 for a copy of the specification. Maybe this wouldn't help though.

5. Can anyone point me to some good sources of information on using and designing PCIe interfaces, along with the M.2 format?

Please can someone help answer my questions.

Answer 1

As long as the socket connects it's PCIe lane to lane 0 of your device, all PCIe devices are required by the specification to support forming a x1 link.

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Upon closer reading of your point 3, no this will likely not work. Generally speaking you can't split up the lanes of a PCIe port. It is required that lane 0 on both the device and upstream port are connect - as there is only one lane 0 on the upstream port, you couldn't say, pass three lanes to the SSD and one to the Wi-Fi, one of the two devices would never be detected.

The only way that would work is if the CPU supports bifurcation of its port. In bifurcation mode, the CPU reconfigures its PCIe port into two or more discrete ports, each of which can operate independently. However you would have to check if this is supported as it is by no means universally possible (its unlikely it does).

The option 1. to use USB for Wi-Fi/BT seems like a solid choice. There are a lot of Wi-Fi cards which are designed to be USB-only. Likely you won't get 11ac data rates over USB (maybe with USB 3.x), but there will probably be quite a few cards out there that support that option.

Option 2. is also not unreasonable, SATA is likely to be around for some time. Whether it remains simple to get SATA drives in M.2 format I can't comment on.

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A quick check of the spec sheet and it seems you are in luck. The Atom X6413E does indeed support bifurcation of its x4 Port 0 in the following modes:

• Single x4 Link
• Dual x2 Link
• Single x2 Link + Dual x1 Link
• Quad x1 link.

In fact even without bifurcation, it seems to have multiple PCIe ports:

• Port 0 is the x4 link (bifurcatable)
• Port 1-3 are each x1 or x2 links.

So in that case, connect Port 0 up to your M.2. connector for SSD, and Port 1 up to another connector for your Wi-Fi.