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

[Deval]

I am using the SLM project with pppd and cmux for Linux on my custom board. I am able to create CMUX and communicate with the nrf 9151 using the CMUX AT port using my custom overlay (which only changes the UART pins). But I am unable to create the pppd because the SIM Card is not detected. 

When I probe the SIM VCC line I always see 0 V. On the nrf9151 DK with the same project I can see that same pin go high and the example project works. 

This is the output I see with the AT commands. 

AT+CFUN=1

OK
AT%XSIM?

%XSIM: 0,9

OK
AT+CEREG?

+CEREG: 0,90

More information. 

AT%SHORTSWVER

%SHORTSWVER: nrf91x1_2.0.2

Do I need to do any additional configuration for the SIM pins? What should I be seeing on all the SIM pins?

[Michal]

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

Best regards,

Michal

[Deval]

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. 
  
  

[MJJ]

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