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I have a TJC HMI display similar to Nextion. It communicates with the ESP32 through UART. The display operates at 5V, but the specification table indicates that the Tx and RX pin voltages are 3.3V, which is acceptable for the ESP32.

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Before connecting the display to the ESP32, I checked the voltage using a digital multimeter (DMM). The yellow wire corresponds to the RX pin, which is currently showing a 3.28 voltage reading. Which seems to be Okay

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But when I connect the Tx pin of TJC with DMM, it shows 4.58V, which is alarming for ESP32

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Am I testing the right way, or am I doing it wrong? Your suggestions matter.I want to protect my ESP32

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  • \$\begingroup\$ Power supply sequencing isn't mentioned. You should ensure that the +5V supply for display is only active when ESP32's +3.3V Vdd is active. \$\endgroup\$ Commented Jul 28 at 15:58

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Well,

If you measured a 5V output at the TX pin of your HMI display, then it is indeed 5V, not matter what the datasheet says.

It's easy to just play on the safe side here. If you don't plan to have insanely high baud rate on the UART, you may just use a simple voltage divider to reduce that 5V to 3.3V. Two resistors are enough to do that.

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