How to match the UART level between MCU and module?

  1. If the UART level of MCU is 3.3V, the level shift circuit is shown as below.

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  1. If the UART level of MCU is 3V, just change the resistor value of 5.6K in the above Figure to 15K.

  2. If the UART level of MCU is 5V, the level shift circuit is shown as below.

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If VOH and VIH is within the windows then no need to worry.