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I am trying to build a board to bridge the MXM slot in my laptop with a PCIe riser. Have just started working out which pins from the MXM slot should connect to which pin on a PCIe slot. I have put my initial planning work on github - https://github.com/gautamz/mxm3-pcie-rise-connector.

I need help with the following MXM pins,

  1. PWR_GOOD & PEX_RST# - PWR_GOOD indicates to the board that the internal power state of the endpoint is fine. On PCIe this is indicated by PERST#. So I am connecting these two. But then what do I connect the PEX_RST# pin on the MXM slot to? Can I just leave this unconnected? Setting PWR_GOOD is important to ensure I get PCIe v3 speeds, or else the system will default to PCIe v1.
  2. PEX_STD_SW# - This determines the swing for the MXM GPU. However I saw nothing for this in PCIe specifications. There the swing appears to be managed by the REFCLK signals. In the MXM specs if I connect this to ground I am indicating standard (desktop) swing. Should I connect this to ground, or leave it unconnected?
  3. 3V3 - Do I need to connect the PCIe 3.3V pins to the 3V3 pins on the MXM slot?
  4. I am planning to only connect to the following MXM pins - WAKE#, PWR_GOOD, SMB_DAT, SMB_CLK, PEXRXn & PEXTXn (n = 0 to 15), PEX_REFCLK, PEX_CLK_REQ#, PEX_RST#(?), PEX_STD_SW#(?). This should be enough for the PCIe card since I will power it with an external PSU. Any other MXM connections are mandatory?

In the github repo I have a spread sheet where I am working out the needed pin connections before I begin working on the PCB layout. If you have any other advice for this, it would be appreciated. I will also put up the PCB layouts on the github repo.

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  • \$\begingroup\$ Have you looked at schematics for laptops that implement MXM slots? That could possibly offer a little more insight. I know I have one for an older laptop of mine, a clevo P150HM1. \$\endgroup\$ Commented Jan 29, 2018 at 4:21
  • \$\begingroup\$ I checked two I found online. In one PWR_GOOD was connected to a 3.3V power source through a 10K_04 resistor. In another to the 3.3V source and another sensor in parallel. I am going to go with connecting PWR_GOOD to 3.3V source. That means I can connect PERST and PEX_RST. \$\endgroup\$ Commented Jan 29, 2018 at 18:57
  • \$\begingroup\$ Also the PEX_STD_SW was connected to Ground in both. So that's definitely solved. No on to making the PCB and hoping this works. \$\endgroup\$ Commented Jan 29, 2018 at 18:59
  • \$\begingroup\$ since you were able to answer your own question, you should provide an answer so so that other looking for the same information can find it and see that it was an answered question. \$\endgroup\$ Commented Mar 23, 2018 at 13:13
  • \$\begingroup\$ Sure. Have posted it as an answer. Thanks for the heads up. \$\endgroup\$ Commented Mar 31, 2018 at 16:45

1 Answer 1

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I found the following myself, leaving as a note for anyone researching this -
1) PWR_GOOD to connect to 3.3V source through a 10K_04 resistor.
2) PEX_STD_SW to connect to ground.
3) PEX_RST on MXM to connect to PERST on PCIe.
4) Connecting the PCIe 3.3V connections to the MXM 3V3 connections is fine. Mind the polarity.

My project has stalled since events in my life have gained priority. I've also had to sell my laptop, reducing the incentive for this. I may however still pick this up once I get a new laptop as I intend to get one where I can upgrade most components.

In case you progress on this, please drop a note here. It would be great to know how it worked out.

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