I have a small linux SBC and I used 7304 and 7354 modems previously without any issues, but now I am upgrading them to use mc7455s and I noticed that they would not work at all. No USB enumeration, nothing, like the modules are not inserted in the mini-pcie slots.
Then comparing the 7304,7455 and the mini-pcie standard pinout, it seems that the 7455 might be breaking the standard.
pin 11,13 on the modem VREF_1.8 output and USIM2_PWR (3V) output. But pcie pinout shows refclk+/- there.
pin 23,25 on the modem is USB3TX+/- but the std has PERN/P.
pin 31,33 on the modem is USB3RX+/- but that is PETN/P on pcie.
These collisions were enough to stop the modem from working at all. As a test I have stuck small stripes of scotch tape to these pins and voila the modem is back.
Did I miss some sentence in the 7455’s technical spec that says: careful, might not be compatible with mini-pcie?
Note that this document leaves it up to the platform to select USB 3 or PCIe interface. Older platforms will of course not know anything about this, so they will use those pins for PCIe if connected. And you end up with a rather confused PCIe system.
Tape is the correct solution. I did the same to use an MC7455 in my oldish laptop. But you won’t get USB 3 support that way, of course. Note that you probably only need to cover the USB3 TX/RX pairs, and that it doesn’t matter if you cover the ground pins between them. So you can just put a single piece of tape over pins 23, 25, 27, 29, 31, and 33.
I believe the reasoning behind this design choice is that the Mini Card form factor modules are meant for new embedded designs, where the Mini Card slot is wired up specifically for those modules. The m.2 modules are more appropriate for the laptop market.
I’ve followed as instructed (tapped #23 through #33) on mc7455 and instsll into x220t but no luck.Prior to that I dont have sny issues with mc7304. Yes, I know wwan + ssd device supports only USB2.0 … and mc7304 is USB 2.0 whilst mc7455 configured to USB 3.0 but has backward configurayion to USB 2.0. Someone did mentioned mc7455 working fine on x301 by tapping #23 through #33 but no luck foe me on x220t. Any other valuable inputs how to resolve this problem ? …
Just to inform where current BIOS release 1.42 was modified in order to remove BIOS white-lists. As I had mentioned earlier, by using this method MC7304 was able to be installed and running fine and able to access internet with full 4G LTE. I would like to install MC7455 as well and had followed all steps as mentioned earlier but still no luck.
About all related drivers are downloaded from “sierra source” website and don’t have any ideas how to enable MC7455 be recognized on X220T.