HTCC-AB01 unintentional power over RX pin

#1

I noticed that when using external CP2102 to connect serial pins (RX/TX) of HTCC-AB01 dev board to my computer, and without the power applied to the board in any way (neither via USB nor via pins) the board boots up and sends logging data (Serial.print()) via the serial connection (clearly, there’s no enough current for LoRa, but the MCU is working).

Is there a way to “isolate” RX/TX from that “parasitic” power and not power the board that way?

#2

The HTCC Tx pin separation is not needed because the Rx pin on the external CP2102 should have 0V when not driven by HTCC.
One of the cheapest and simplest way is to separate the Rx pin of HTCC-AB01 with a diode and add pull-up resistor to provide high idle state.
Common 1N4148 diode should be enough. Eventually you can use Shottky diode for lower voltage drop or decrease the pull-up resistor value.

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