ESP32 Strapping Pins: Complete List for All Chip Variants

Key Observations

Common Strapping Features Across ESP32 Versions

Strapping pins control four main hardware configurations across ESP32 variants. Here's a detailed look at each function:

Boot Mode Selection

Determines whether the ESP32 boots into normal operation mode (to run your program) or download mode (to receive new firmware via USB/UART).

Why it's important: This is the most critical strapping pin function. Without proper boot mode control, you cannot upload firmware to your ESP32. Every time you flash new code, the chip must enter download mode first, then reset to normal mode to run the code.

Voltage Selection (VDD_SPI / VDD_SDIO)

Configures the operating voltage (1.8V or 3.3V) for the SPI flash memory chip that stores your program code and data. Some ESP32 modules support both voltage levels.

Why it's important: Using the wrong voltage can damage your flash memory chip or cause communication failures. Most ESP32 modules use 3.3V flash (most common), but some specialized boards use 1.8V flash for lower power consumption. The voltage must match your hardware.

VDD_SPI Configuration:

JTAG Signal Source Control

Determines whether JTAG debugging signals come from USB (built-in USB-JTAG controller) or external debug pins (traditional JTAG pads). JTAG allows real-time debugging, breakpoints, and memory inspection.

Why it's important: Proper JTAG configuration is essential for advanced debugging with tools like OpenOCD or ESP-IDF's debugging features. It determines whether you debug via USB cable or external JTAG hardware (like J-Link or FT2232H).

GPIO15 Configuration (ESP32-S2/S3):

Note: Works in conjunction with eFuse settings (EFUSE_DIS_PAD_JTAG, EFUSE_DIS_USB_JTAG) which can permanently disable certain JTAG sources for security.

JTAG Pin Name Aliases (MTDI, MTDO, MTCK, MTMS)

ESP32 datasheets and pinout diagrams use legacy JTAG signal names alongside GPIO numbers. These aliases appear frequently in IDE warnings and community discussions:

ROM Boot Messages

Controls whether the ESP32's ROM bootloader prints diagnostic messages during the boot process. These messages show boot mode, flash configuration, and any boot errors.

Why it's important: ROM messages are extremely useful for debugging boot failures, but you may want to disable them in production devices to reduce power consumption, boot time, or prevent information leakage.

How GPIO8/GPIO9 Control Messages:

Output destinations: Messages can be routed to UART0 (default serial port) or USB Serial/JTAG Controller depending on configuration and eFuse settings.

Using Strapping Pins as Regular GPIO

While strapping pins primarily serve as configuration parameters during the boot process, this doesn't mean the pins must be left alone after the initial configuration.

Once the ESP32 microcontroller has successfully initialized, these pins transform into regular GPIO (General Purpose Input/Output) pins, available for user-defined functions. However, you must be careful when manipulating these pins during and immediately after the boot phase.

Can you use strapping pins as regular GPIOs? Yes, but with important caveats:

Design considerations:

• Use series resistors (1kΩ-10kΩ) when connecting peripherals to strapping pins. This allows the internal pull-ups/downs to dominate during boot.
• Avoid strong pull-ups/downs that could override the ESP32's boot configuration.
• Test your design through multiple power cycles and resets to ensure consistent booting.
• Prioritize non-strapping pins for critical functions when possible.

Can I Use GPIO0, GPIO2, or GPIO12 as Output After Boot?

Yes - all strapping pins become fully general-purpose GPIOs once the ROM bootloader finishes sampling them (within ~100 ms of reset). Here's the practical guidance for each commonly questioned pin:

Arduino IDE and ESPHome Strapping Pin Warnings

You may see warnings like these when using strapping pins in your sketches or YAML config:

These warnings come from the ESP-IDF GPIO driver and are informational - they do not mean the pin doesn't work. The warning fires whenever you configure a strapping pin as GPIO. What it means:

ESP32 GPIO Pins to Avoid: Safe Pin Selection Guide

To revise once more, let's take a look at the strapping pins on different ESP32 versions. Strapping pins are used to set configurations such as ESP32 programming mode or specific boot behaviors.

It's important to identify ESP32 reserved pins and avoid using them as GPIO pins, as they are required during boot-up and certain operational modes. The table below provides an overview of ESP32 programming pins, ESP32 pins to avoid, and their default configurations.

ESP32 Strapping Pins Cheatsheet

All chip variants - boot modes, GPIO levels, download mode

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ESP8266 Strapping Pins

While this guide focuses on the ESP32 family, the ESP8266 uses a similar strapping pin concept at boot. If you're working with an ESP8266 (e.g., NodeMCU, Wemos D1 Mini), here are its three boot-critical pins:

Additional Resources

Conclusion

In conclusion, strapping pins play an important role in the initialization and configuration of various ESP32 microcontroller versions, including ESP32-S2, ESP32-S3, ESP32-C3, and ESP32-C6. These pins serve as gatekeepers, influencing critical parameters during startup or reset, such as boot mode, voltage selection, JTAG signal source, and more.

Across different ESP32 versions, certain common strapping pins persist, each with its unique functions and default configurations. GPIO0, for instance, is the most commonly used strapping pin, which is used to select the boot mode.