Boot Mode Selection

This guide explains how to select the boot mode correctly and describes the boot log messages of ESP32-S2.


The ESP32-S2 has a 45k ohm internal pull-up/pull-down resistor at GPIO0 (and other pins). If you want to connect a switch button to enter the boot mode, this has to be a strong pull-down. For example a 10k resistor to GND.

Information about ESP32-S2 strapping pins can also be found in the ESP32-S2 Datasheet, section “Strapping Pins”.

On many development boards with built-in USB/Serial, can automatically reset the board into bootloader mode. For other configurations or custom hardware, you will need to check the orientation of some “strapping pins” to get the correct boot mode:

Select Bootloader Mode


The ESP32-S2 will enter the serial bootloader when GPIO0 is held low on reset. Otherwise it will run the program in flash.

GPIO0 Input



ROM serial bootloader for esptool


Normal execution mode

GPIO0 has an internal pullup resistor, so if it is left unconnected then it will pull high.

Many boards use a button marked “Flash” (or “BOOT” on some Espressif development boards) that pulls GPIO0 low when pressed.


GPIO46 must also be either left unconnected/floating, or driven Low, in order to enter the serial bootloader.

In normal boot mode (GPIO0 high), GPIO46 is ignored.

Other Pins

As well as the above mentioned pins, other ones influence the serial bootloader, please consult the ESP32-S2 Datasheet, section “Strapping Pins”.

Automatic Bootloader resets ESP32-S2 automatically by asserting DTR and RTS control lines of the USB to serial converter chip, i.e., FTDI, CP210x, or CH340x. The DTR and RTS control lines are in turn connected to GPIO0 and EN (CHIP_PU) pins of ESP32-S2, thus changes in the voltage levels of DTR and RTS will boot the ESP32-S2 into Firmware Download mode.


When developing, keep in mind DTR and RTS are active low signals, i.e., True = pin @ 0V, False = pin @ VCC.

As an example of auto-reset curcuitry implementation, check the schematic of the ESP32 DevKitC development board:

  • The Micro USB 5V & USB-UART section shows the DTR and RTS control lines of the USB to serial converter chip connected to GPIO0 and EN pins of the ESP module.

  • Some OS and/or drivers may activate RTS and or DTR automatically when opening the serial port (true only for some serial terminal programs, not, pulling them low together and holding the ESP in reset. If RTS is wired directly to EN then RTS/CTS “hardware flow control” needs to be disabled in the serial program to avoid this. An additional circuitry is implemented in order to avoid this problem - if both RTS and DTR are asserted together, this doesn’t reset the chip. The schematic shows this specific circuit with two transistors and its truth table.

  • If this circuitry is implemented (all Espressif boards have it), adding a capacitor between the EN pin and GND (in the 1uF-10uF range) is necessary for the reset circuitry to work reliably. This is shown in the ESP32 Module section of the schematic.

  • The Switch Button section shows buttons needed for manually switching to bootloader.

Make the following connections for esptool to automatically enter the bootloader of an ESP32-S2 chip:


Serial Pin





In Linux serial ports by default will assert RTS when nothing is attached to them. This can hold the ESP32-S2 in a reset loop which may cause some serial adapters to subsequently reset loop. This functionality can be disabled by disabling HUPCL (ie sudo stty -F /dev/ttyUSB0 -hupcl).

(Some third party ESP32-S2 development boards use an automatic reset circuit for EN & GPIO0 pins, but don’t add a capacitor on the EN pin. This results in unreliable automatic reset, especially on Windows. Adding a 1uF (or higher) value capacitor between EN pin and GND may make automatic reset more reliable.)

In general, you should have no problems with the official Espressif development boards. However, is not able to reset your hardware automatically in the following cases:

  • Your hardware does not have the DTR and RTS lines connected to GPIO0 and EN (CHIP_PU)

  • The DTR and RTS lines are configured differently

  • There are no such serial control lines at all

Manual Bootloader

Depending on the kind of hardware you have, it may also be possible to manually put your ESP32-S2 board into Firmware Download mode (reset).

  • For development boards produced by Espressif, this information can be found in the respective getting started guides or user guides. For example, to manually reset a development board, hold down the Boot button (GPIO0) and press the EN button (EN (CHIP_PU)).

  • For other types of hardware, try pulling GPIO0 down.

Boot Log

Boot Mode Message

After reset, the second line printed by the ESP32-S2 ROM (at 115200bps) is a reset & boot mode message:

ets Jun  8 2016 00:22:57

rst:0xNN (REASON) is an enumerated value (and description) of the reason for the reset. A mapping between the hex value and each reason can be found in the ESP-IDF source under RESET_REASON enum. The value can be read in ESP32-S2 code via the get_reset_reason() ROM function.

boot:0xNN (DESCRIPTION) is the hex value of the strapping pins, as represented in the GPIO_STRAP register.

The individual bit values are as follows:

  • 0x04 - GPIO46

  • 0x08 - GPIO0

If the pin was high on reset, the bit value will be set. If it was low on reset, the bit will be cleared.

A number of boot mode strings can be shown depending on which bits are set:

  • DOWNLOAD_BOOT(UART0/UART1/SDIO_REI_REO_V2) or DOWNLOAD(USB/UART0) - ESP32-S2 is in download flashing mode (suitable for esptool)

  • SPI_FAST_FLASH_BOOT - This is the normal SPI flash boot mode.

  • Other modes (including SPI_FLASH_BOOT, SDIO_REI_FEO_V1_BOOT, ATE_BOOT) may be shown here. This indicates an unsupported boot mode has been selected. Consult the strapping pins shown above (in most cases, one of these modes is selected if GPIO46 has been pulled high when GPIO0 is low).

Later Boot Messages

Later output from the ROM bootloader depends on the strapping pins and the boot mode. Some common output includes:

Early Flash Read Error

flash read err, 1000

This fatal error indicates that the bootloader tried to read the software bootloader header at address 0x1000 but failed to read valid data. Possible reasons for this include:

  • There isn’t actually a bootloader at offset 0x1000 (maybe the bootloader was flashed to the wrong offset by mistake, or the flash has been erased and no bootloader has been flashed yet.)

  • Physical problem with the connection to the flash chip, or flash chip power.

  • Flash encryption is enabled but the bootloader is plaintext. Alternatively, flash encryption is disabled but the bootloader is encrypted ciphertext.

Software Bootloader Header Info

mode:DIO, clock div:1

This is normal boot output based on a combination of eFuse values and information read from the bootloader header at flash offset 0x1000:

  • SPIWP:0xNN indicates a custom WP pin value, which is stored in the bootloader header. This pin value is only used if SPI flash pins have been remapped via eFuse (as shown in the configsip value). All custom pin values but WP are encoded in the configsip byte loaded from eFuse, and WP is supplied in the bootloader header.

  • mode: AAA, clock div: N. SPI flash access mode. Read from the bootloader header, correspond to the --flash_mode and --flash_freq arguments supplied to write_flash or elf2image.

  • mode can be DIO, DOUT, QIO, or QOUT. QIO and QOUT are not supported here, to boot in a Quad I/O mode the ROM bootloader should load the software bootloader in a Dual I/O mode and then the ESP-IDF software bootloader enables Quad I/O based on the detected flash chip mode.

  • clock div: N is the SPI flash clock frequency divider. This is an integer clock divider value from an 80MHz APB clock, based on the supplied --flash_freq argument (ie 80MHz=1, 40MHz=2, etc). The ROM bootloader actually loads the software bootloader at a lower frequency than the flash_freq value. The initial APB clock frequency is equal to the crystal frequency, so with a 40MHz crystal the SPI clock used to load the software bootloader will be half the configured value (40MHz/2=20MHz). When the software bootloader starts it sets the APB clock to 80MHz causing the SPI clock frequency to match the value set when flashing.

Software Bootloader Load Segments

entry 0x40080034

These entries are printed as the ROM bootloader loads each segment in the software bootloader image. The load address and length of each segment is printed.

You can compare these values to the software bootloader image by running --chip esp32s2 image_info /path/to/bootloader.bin to dump image info including a summary of each segment. Corresponding details will also be found in the bootloader ELF file headers.

If there is a problem with the SPI flash chip addressing mode, the values printed by the bootloader here may be corrupted.

The final line shows the entry point address of the software bootloader, where the ROM bootloader will call as it hands over control.