EC-25 Sim Detection issue, CME error 10, USIM VDD 0V


We have around 100 terminals that use EC-25 Mini PCIe module for comms.

  1. Some of them works flawlessly
  2. Some will drop offline here and there that can resume connection randomly or by rebooting or by re-inserting sim card
  3. A few that never connects

The issue is pretty much same as described in Quectel EC25 Mini PCIe USIM detection problem (USIM_VDD=0V)

AT log will show CME error 10, basically can’t find sim card
The Usim VDD is 0V on faulty ones, 1.8v on good ones

On a closer look on the PCB, the 15K pull-up resistor between USIM-VDD and USIM-DATA is not soldered on. There’s no sim detection circuit or code, it’s just 6pin sim connector.

However, comparing the sim circuit from another terminal manufacture who also uses EC-25 with 8pin sim tray and sim detection, the 2nd manufature also left out the 15K pull up.

The resistor in question shown below

My question is that would this explain the cause of some terminals not detecting sim cards? I can’t find any documents substainciating the backend logics of this 15K resistor apart it’s a pull up

On a side note, I have been dealing with this “dropout” issue for over a year now, started thinking its issue with the SIM cards or the tray connector, but increasingly confident that this issue has nothing to do with loose connections, but rather circuit charactoristics of indivisual PCB & its components.

I have for many times seen the connection resumes with

  • commandline reboot - not touching anything no chance of virbation
  • swapping EC-25 and terminal combination

1.SIM card circuit, USIM_VDD=0 reason is that the SIM card could not be detected. The module search SIM card sequence is 1.8V SIM card -> 3.0V SIM card, if 3.0V SIM card is not be detected, than the USIM_VDD output low(0V).

2.If some of your device works flawlessly, some of them never connect or work discontinuously, I suggest that you had better mount the 15K pull-up resistor, this resistor can anti the interference from outside.

3.Check your TVS diodes whether its parastic capacity is over 15pF or not, if it is over 15pF, than consider to replace it with not over 15pF’s.

4.Check your design whether follow the criteria as below: