The SIM card can't work，and the measurement IO(sim_1.8v) port has no output.

[Jerry zheng]

We customized our own development board based on Nordic's Thingy91 development board. Now it's welded, the circuit works, and we've flash demo MQTT_simple and Asset_cracker_v2, including some LTE related programs, but the SIM card doesn't work. The 1.8V output voltage of the chip was not detected, so we want to ask if there is any deficiency in the circuit design. Or what other software configuration is required? Please point the way.

Attached is our schematic diagram.

[Kazi Afroza Sultana]

Hello,

When the sim1 is selected you can remove the R11 and can make the value of R10 is zero. Because at a time one sim can operate. 

Also Vpp should be floated. In our design it looks like the following way.

I can see your VPP and VCC both are connected to 1.8V. 

Could you please check your connection first from this point of view?

Thanks.

Best Regards,

Kazi Afroza Sultana

[Jerry zheng]

Thank.now I have set VPP to floating empty state, but when I set LTM to open,SIM1.8V always has a sudden voltage drop, about 5-7s drop time, I can not find the specific reason, we are not connected to the filter CM1402, but we think the impact is not big, how to explain this drop voltage? SIM's CLK and DATA also have two high level pulses. anxiously, we are looking for your help.

[Kazi Afroza Sultana]

Hello,

What do you mean by ''when I set LTM to open'' ?

Best Regards,

Kazi Afroza Sultana

[Jerry zheng]

Turn on the network, or connect to the SIM

[Kazi Afroza Sultana]

then LTM is LTE right? you have mistyped?