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If your ESP32 restarts as soon as you open a serial monitor, the port may be toggling DTR or RTS control lines that are connected to the board’s reset and boot circuitry. This is expected on some board-and-terminal combinations, but it is not universal. The right fix depends on how your board wires those signals.
Why does my ESP32 reset when I open the serial monitor?
Opening a serial port can change the states of its DTR and RTS lines. On many ESP32 development boards, a two-transistor auto-reset circuit connects those lines to EN (also called CHIP_PU) and GPIO0. That circuit lets software reset the chip and select firmware-download mode. Some operating systems or terminal programs activate the lines when opening a port; Espressif notes that this can pull them low together and hold the ESP in reset. DTR and RTS are active-low signals. Espressif’s boot-mode documentation describes this behavior.
The board circuit matters. Espressif’s DevKitC schematic shows a two-transistor design that prevents reset when both lines are asserted together. A simpler board or adapter may wire RTS directly to EN, producing different behavior. Some hardware has no DTR/RTS connections at all, so automatic reset is unavailable. Check the schematic or documentation for your exact board and revision rather than assuming all ESP32 boards behave alike.
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How to diagnose the reset
1. Confirm that opening the port triggers a fresh boot
Set the monitor to the baud rate configured by your firmware. The ESP32 ROM boot log uses 115200 bps. If a new boot message appears immediately after opening the monitor, the chip has likely reset; the second ROM line includes a rst: reason and boot: mode. If there is no fresh boot message, the monitor may simply have started displaying output from a board that was already running. See Espressif’s boot-mode documentation and troubleshooting guide.
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2. Identify the serial bridge and control-line wiring
Find out whether your board has an onboard USB-to-UART bridge or uses a separate adapter. FTDI, CP210x, and CH340x chips are examples of bridges used in these setups, but the chip name alone does not establish how a particular board routes DTR and RTS. Look for the board’s schematic or its manufacturer’s instructions.
3. Match the monitor setting to the wiring
If RTS is wired directly to EN, Espressif advises disabling RTS/CTS hardware flow control in the serial program. This is a targeted fix for that wiring, not a universal ESP32 setting. If your board uses the two-transistor auto-reset circuit, do not assume direct RTS-to-EN wiring explains the reset; check the monitor’s DTR/RTS behavior and the board schematic instead.
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4. Check the Linux-specific HUPCL case
Espressif documents a Linux case in which a serial port may assert RTS by default when nothing is attached, potentially keeping an ESP32 in a reset loop. Disabling HUPCL can help with that documented behavior. Treat this as a Linux-specific diagnostic for applicable wiring, not as a general Windows or macOS fix.
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5. If it keeps resetting, investigate power and firmware
A serial-port setting will not fix an unstable supply, a wiring problem, or firmware that crashes during startup. Espressif recommends checking stable 3.3 V power, pin voltages, devices connected to GPIO pins, and early startup logs. Its troubleshooting guidance says ESP chip supply demand can reach up to 70 mA continuous and 200–300 mA peak, possibly slightly higher; those are vendor figures, not a measurement of your particular board. Espressif also warns that some FTDI FT232R or Arduino 3.3 V outputs may not provide enough current to power an ESP chip reliably. Read Espressif’s troubleshooting guidance.
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A multimeter can help check EN and GPIO0 levels or supply behavior if you are comfortable probing the board. Do not add an EN capacitor as a default fix: Espressif discusses a 1–10 µF EN-to-GND range for its described reset circuit, but board designs differ, and the actual circuit should be checked before making a hardware change.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to enter download mode if auto-reset does not work
Manual bootloader entry is a fallback for flashing firmware, not a way to prevent an ordinary reset when opening the monitor. On an Espressif development board, hold Boot (GPIO0) and press EN. GPIO0 is sampled at reset to determine boot mode. For other hardware, follow its board-specific instructions; Espressif also describes pulling GPIO0 low. See Espressif’s boot-mode documentation.
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