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ESP32 Boot Mode Selection

Angus Gratton edited this page Mar 27, 2019 · 7 revisions

ESP32 Boot Mode Selection

Guide to selecting boot mode correctly on ESP32.

Information about ESP32 strapping pins can also be found in the ESP32 Datasheet, section 2.4 "Strapping Pins".

On many development boards with built-in USB/Serial, this is all done for you and 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 will enter the serial bootloader when GPIO0 is held low on reset. Otherwise it will run the program in flash.

GPIO0 Input Mode
Low/GND ROM serial bootloader for
High/VCC 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.


GPIO2 must also be either left unconnected/floating, or driven Low, in order to enter the serial bootloader.

In normal boot mode (GPIO0 high), GPIO2 is ignored.

Other Pins

As well as GPIO0 and GPIO2, the following pins influence the serial bootloader mode:

GPIO Meaning
12 (MTDI) If driven High, flash voltage (VDD_SDIO) is 1.8V not default 3.3V. Has internal pull-down, so unconnected = Low = 3.3V. May prevent flashing and/or booting if 3.3V flash is used and this pin is pulled high, causing the flash to brownout. See the ESP32 datasheet for more details.
15 (MTDO) If driven Low, silences boot messages printed by the ROM bootloader. Has an internal pull-up, so unconnected = High = normal output.

Automatic bootloader can automatically enter the bootloader on many boards by using the RTS and DTR modem status lines to toggle GPIO0 and EN automatically.

Make the following connections for to automatically enter the bootloader:

ESP32 Pin Serial Pin

Note that some serial terminal programs (not will assert both RTS and DTR when opening the serial port, pulling them low together and holding the ESP32 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.

Development boards (including all Espressif boards) usually use additional circuitry to avoid this problem - if both RTS and DTR are both asserted together, this doesn't reset the chip. Consult Espressif development board schematics for the specific details.

(Some third party ESP32 development boards use an automatic reset circuit for EN & GPIO pins, but don't add a capacitor on the EN pin. This results in unreliable automatic reset, especially on Windows. Adding a 100nF (or higher) value capacitor between EN pin and GND may make automatic reset more reliable..)

Boot Mode Message

After reset, the second line printed by the ESP32 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. The value can be read in ESP32 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:

  • 0x01 - GPIO5
  • 0x02 - MTDO (GPIO15)
  • 0x04 - GPIO4
  • 0x08 - GPIO2
  • 0x10 - GPIO0
  • 0x20 - MTDI (GPIO12)

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) - ESP32 is in download flashing mode (suitable for
  • SPI_FAST_FLASH_BOOT - This is the normal SPI flash boot mode.
  • Other modes (including HSPI_FLASH_BOOT, 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 GPIO2 has been pulled high when GPIO0 is low).

Note: GPIO_STRAP register includes GPIO 4 but this pin is not used by any supported boot mode and be set either high or low for all supported boot modes.

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.
  • Boot mode accidentally set to HSPI_FLASH_BOOT, which uses different SPI flash pins. Check GPIO2 (see above).
  • VDDSDIO has been enabled at 1.8V (due to MTDI/GPIO12, see above), but this flash chip requires 3.3V so it's browning out.
  • Flash encryption is enabled but the bootloader is plaintext. Alternatively, flash encryption is disabled but the bootloader is encrypted ciphertext.

Software bootloader header info

configsip: 0, SPIWP:0x00
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:

  • configsip: N indicates SPI flash config:
    • 0 for default SPI flash
    • 1 if booting from the HSPI bus (due to EFUSE configuration)
    • Any other value indicates that SPI flash pins have been remapped via efuse (the value is the value read from efuse, consult espefuse docs to get an easier to read representation of these pin mappings).
  • 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.
  • clk_drv:0x00,q_drv:0x00,d_drv:0x00,cs0_drv:0x00,hd_drv:0x00,wp_drv:0x00 Custom GPIO drive strength values for SPI flash pins. These are read from the bootloader header in flash. Not currently supported.
  • 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 esp32 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.

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