The RM520N-GL is not detecting my SIM card. Pins 36 (USIM1_VDD) and 48 (USIM2_VDD) are both at GND (measured 0 V). I expected these to be 1.8V out, as the hardware manual says they are PO. There appears to be no voltage on the detect line. Consequently, it never sees a SIM card plugged in.
I have double checked with 2 modems, and believe the socket is working correctly and that my pin counting is correct. AT+QSIMDET shows no SIM no matter what I do.
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Can you provide a schematic diagram of the SIM card? USIM.VDD does not have voltage output, which is likely due to abnormal communication between the module and the SIM card. The module then shuts off the power output.
You can try testing the voltage of each signal pin on the SIM card and use an oscilloscope to capture the voltage levels of all signal pins during the module startup process. During the startup process, there will definitely be output. If any signal is found to be missing, then focus on checking this pin.