Unable to detect SIM on custom PCB and SIM VCC never goes high

RE: Unable to detect SIM on custom PCB and SIM VCC never goes high

MJJ — Fri, 10 Oct 2025 23:18:24 GMT

Hi Jennifer, Greg,

Great ideas and thanks for getting to the root cause of this! That was it -- the primary issue on our end is that the SIM card holder we used in this run of boards didn't have an internal switch that connects C5 to GND when the SIM is installed, so C5 and therefore our SIM was never getting properly grounded.

After grounding C5, we're able to correctly communicate with the SIM and see activity on the IO line with responses for all of the commands I was previously issuing. We're in the same place now with our boards as the DK.

Thank you all for your help debugging this issue! We'll move forward from here and will reach out if there's anything else we run into along the way.

-Andreea

RE: Unable to detect SIM on custom PCB and SIM VCC never goes high

Deval — Tue, 07 Oct 2025 18:48:05 GMT

Hello,

We were able to get probing of the lines after we run AT+CFUN=1

From our HW team:

We also tried testing with and without a SIM card populated and the results are still the same.

When issuing a command like AT+CFUN=1 to enable the modem, the SIM_1V8 goes high, the CLK line begins to PWM, and the IO line goes high but there is no other data on that line.
Partway through the AT+CFUN=1 response, there's a brief moment where clocking stops, IO goes low, and SIM_1V8 drops to approximately 1.3V with this "blip".
When issuing a command like AT%XSIM? there is no movement on any three of these lines, including nothing on SIM_1V8. Everything stays at 0 even though we receive a %XSIM: 0,9 response on the terminal.

RE: Unable to detect SIM on custom PCB and SIM VCC never goes high

Michal — Wed, 01 Oct 2025 13:58:46 GMT

Are you able to measure the lines connected to the SIM card reader with a logic analyzer or an oscilloscope?

Best regards,

Michal

RE: Unable to detect SIM on custom PCB and SIM VCC never goes high

Who_Knew — Wed, 24 Sep 2025 17:57:47 GMT

I wonder if under the hood, the SIM_1V8 pin is simply a GPIO in structure. Perhaps it's possible to remove the SIM from the SIM holder and experimentally drive this pin high by our firmware and confirm that the pin is capable of delivering 1.8v.

RE: Unable to detect SIM on custom PCB and SIM VCC never goes high

Deval — Mon, 22 Sep 2025 22:42:05 GMT

Yes I have confirmed the SIM works with the DK

RE: Unable to detect SIM on custom PCB and SIM VCC never goes high

Michal — Mon, 22 Sep 2025 21:59:47 GMT

Hello,

Are you able to use this SIM card with a DK?

Best regards,

Michal

RE: Unable to detect SIM on custom PCB and SIM VCC never goes high

Deval — Fri, 19 Sep 2025 20:18:29 GMT

Additional information

AT+CEREG=5

OK
AT+CFUN=1

OK

+CEREG: 90
AT+CFUN?

+CFUN: 1

OK

RE: Unable to detect SIM on custom PCB and SIM VCC never goes high

Deval — Fri, 19 Sep 2025 15:46:33 GMT

Here is the pinout for my device for further context