YUGE CLM920 TD3 Usage Manual page 37

CLM920 TD3 LTE Module Hardware Usage Guide
If you need to use a 2-wire serial port, you can refer to the following serial port design.
The serial port of the module is TTL 1.8V level. If the serial port needs to be connected
to the MCU of 3.3V level, it is necessary to add a level conversion chip externally to achieve
level matching. The chip connection method can refer to the following circuit：
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Figure 3-12 Full-featured serial port design
Figure 3-13 UART serial port design
Figure 3-14 Level conversion chip circuit
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