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I'm designing a card adaptor which converts from M.2 Key E type to M.2 Key M type (to insert an NVMe SSD into a port designed originally for Wifi cards).

I found some reference design for the M.2 KeyM socket:

enter image description here

MY questions:

  1. If I only wish to use this converter for NVMe SSD (PCIe), can I leave open PEDET (pin 69), or pullup is required?
  2. If my design will only connect pins from keyE to this keyM socket, are C[317..343] needed or can be left out?

Reason for these questions: I earlier used a design made by CRImier (thank you):

enter image description here

And it doesn't use any single pullup-resistor or capacitor (while it still works as intended).

Moreover, the repo's history contains a commit:

nvme_ae_to_m: v2, fixed tx/rx pair swaps, removed caps

So author used capacitors, but eventually he decided to remove those for some unknown reason.

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  • \$\begingroup\$ What would your adapter do better than the one you already tried? The Key E does not support SATA, and it is an outpuy from SSD to put M2 socket from SATA to PCIE mode, it is rather irrelevant. And the bypass caps seem necessary as bypass caps. But there is no info which caps were removed, PCIe lane caps or bypass caps or what. \$\endgroup\$
    – Justme
    May 7, 2023 at 18:26
  • \$\begingroup\$ I need some more parts on it (microcontrollers, etc), it's not part of the question to discuss, I wish to focus mainly for the topic described. I know keyE doesn't support SATA. Thx for /PEDET explain, I thought it's for signalling something to SSD and from SSD. Removed caps were between +3.3V and GND. \$\endgroup\$
    – Daniel
    May 7, 2023 at 18:49
  • \$\begingroup\$ Shouldn't bypass caps provided by manufacturer of SSD on SSD's PCB itself? What's the reason behind putting bypass-caps onto a socket? \$\endgroup\$
    – Daniel
    May 7, 2023 at 18:57

1 Answer 1

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  1. The PEDET should be irrelevant. It is pulled up by host and it selects between SATA and PCIe mode in Key M based on insertion of SATA or PCIe card. Since your board is Key M host you could pull it up, but if you don't do anything with that info, you might as well skip it.

But technically your board allows attaching a SATA device to Key M socket and Key E socket has no support for SATA, so SATA won't work.

  1. The capacitors may be important. Especially if you have devices onboard the adaptor module. It would be best to not leave them out. Surely the SSD would have it's own bypass caps, and the motherboard, but generally those two are directly connected together, and if your board does not have bypass caps, then there are longer wiring through two connectors and the PCB, so there is more inductance. Now will it matter is another question, but it is difficult to say if they are mandatory for operation, or what problems you might encounter if they are not there. Their purpose is to provide current to SSD when it needs it, and provide it quickly when it needs it. Any resistance will cause voltage drop and any inductance will cause initial voltage drop as the current is not available quickly.
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  • \$\begingroup\$ I agree with all. I'm planning to add bypass-caps to all other components on the adapter board, but these caps are best to place as-close-as-possible to the active component (i.e. to microcontroller, or SSD on its own board), IMHO. \$\endgroup\$
    – Daniel
    May 7, 2023 at 19:30

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