Usim Hot-Plug; Usim Design - Fibocom L850-GL Series Hardware Manual

When the SIM card is inserted, it connects the short circuit between CD and SW pins, and drives the
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SIM_DETECT pin high.

3.5.3 USIM Hot-Plug

The L850 module supports the SIM card hot-plug function, which determines whether the SIM card is
inserted or detached by detecting the SIM_DETECT pin state of the SIM card slot.
The SIM card hot-plug function can be configured by "AT+MSMPD" command, and the description for AT
command is shown in the following table:
AT Command Hot-plug Detection Function Description
AT+MSMPD=1 Enable
AT+MSMPD=0 Disable
After the SIM card hot-plug detection function is enabled, the module detects that the SIM card is inserted
when the SIM_DETECT pin is high, then executes the initialization program and finish the network
registration after reading the SIM card information. When the SIM_DETECT pin is low, the module
determines that the SIM card is detached and does not read the SIM card.
Note:
SIM_DETECT is active high. It can be swapped to active low by AT CMD.

3.5.4 USIM Design

The SIM card circuit design should meet the EMC standards and ESD requirements with the improved
capability to resist interference, to ensure that the SIM card can work stably. The following guidelines
should be noted in design:
The SIM card slot should be placed as close as possible to the module, and away from the RF antenna,
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DC/DC power supply, clock signal lines, and other strong interference sources.
The SIM card slot with a metal shielding housing can improve the anti-interference ability.

The trace length between the SIM card slot and the module should not exceed 100mm, or it could
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reduce the signal quality.
The UIM_CLK and UIM_DATA signal lines should be isolated by GND to avoid crosstalk interference.

If it is difficult for the layout, the whole SIM signal lines should be wrapped with GND as a group at
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FIBOCOM L850-GL Series Hardware Guide
Default value, the SIM card hot-plug detection function is
enabled.
The module can detect whether the SIM card is inserted or not
through the SIM_DETECT pin state.
The SIM card hot-plug detect function is disabled.
The module reads the SIM card when starting up, and the
SIM_DETECT status will not be detected.
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