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I am designing a device that shall use PE on the PC via M.2 key M socket, and I cannot find info about how to connect it properly. In different sources I see different info regarding M.2 config pins (1, 21, 69 and 75). I found some docs over internet, and according to these docs, these pins are:

  • Pin 1. Config 3
  • Pin 21. Config 0
  • Pin 69. Config 1
  • Pin 75. Config 2

As I understand, the purpose of these pins is to configure each lane to either PE or SATA mode, i. e. grounded pin means that appropriate lane shall be in SATA mode, and not connected pin means the corresponding lane is PE. Is it correct? As I don't use sata and want to use only one (first) PE lane, do I need to leave all these pins not connected? Also, as most mobos I've seen have just one SATA connected to the M.2, do I understand correctly, that only pin 21 is relevant?

Or, to ask more general question: What is the purpose of these pins and shall I leave these pins unconnected in order to use the socket lanes as PE, not SATA?

Giacomo1968
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BUKTOP
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1 Answers1

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The Config_X pins are set by the device in order to signal to the host what type of device it is. The full table is located here.

This applies to M-Key and B-Key style sockets.

For SSD SATA/PCIe devices, the Config_X pins would be:

Config_0 Config_1 Config_2 Config_3 Type
pin 21 pin 69 pin 75 pin 1
0 0 0 0 SSD-SATA
0 1 0 0 SSD-PCIe

According to the M.2 Spec, the host is responsible for providing pull-up resistors on these pins. The device either grounds the pin to signal 0 (low) or leaves it not connected to signal 1 (high).

It appears that the host can choose what voltage to connect the pull-ups to since it seems the intent is to generally hardwire the configuration.

bryscus
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