January 20, 2020

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ESP8266 and ESP32 serial bootloader utility

repo name espressif/esptool
repo link https://github.com/espressif/esptool
language Python
size (curr.) 8075 kB
stars (curr.) 2994
created 2014-09-06
license GNU General Public License v2.0


A Python-based, open source, platform independent, utility to communicate with the ROM bootloader in Espressif ESP8266 & ESP32 chips.

esptool.py was started by Fredrik Ahlberg (@themadinventor) as an unofficial community project. It is now also supported by Espressif. Current primary maintainer is Angus Gratton (@projectgus).

esptool.py is Free Software under a GPLv2 license.

Build Status

Installation / dependencies

Easy Installation

You will need either Python 2.7 or Python 3.4 or newer installed on your system.

The latest stable esptool.py release can be installed from pypi via pip:

$ pip install esptool

With some Python installations this may not work and you’ll receive an error, try python -m pip install esptool or pip2 install esptool.

After installing, you will have esptool.py installed into the default Python executables directory and you should be able to run it with the command esptool.py.

Manual Installation

Manual installation allows you to run the latest development version from this repository.

esptool.py depends on pySerial version 3.0 or newer for serial communication with the target device.

If you choose to install esptool.py system-wide by running python setup.py install, then this will be taken care of automatically.

If not using setup.py, then you’ll have to install pySerial manually by running something like pip install pyserial, easy_install pyserial or apt-get install python-serial, depending on your platform. (The official pySerial installation instructions are here).

esptool.py also bundles the pyaes & ecdsa Python modules as “vendored” libraries. These modules are required when using the ESP32-only espsecure.py and espefuse.py tools. If you install esptool.py via pip or setup.py as shown above, then versions of these libraries will be installed from pypi. If you run esptool.py from the repository directory directly, it will use the “vendored” versions.


Use esptool.py -h to see a summary of all available commands and command line options.

To see all options for a particular command, append -h to the command name. ie esptool.py write_flash -h.

Common Options

Serial Port

  • The serial port is selected using the -p option, like -p /dev/ttyUSB0 (Linux and macOS) or -p COM1 (Windows).
  • A default serial port can be specified by setting the ESPTOOL_PORT environment variable.
  • If no -p option or ESPTOOL_PORT value is specified, esptool.py will enumerate all connected serial ports and try each one until it finds an Espressif device connected (new behaviour in v2.4.0).

Note: Windows and macOS may require drivers to be installed for a particular USB/serial adapter, before a serial port is available. Consult the documentation for your particular device. On macOS, you can also consult System Information’s list of USB devices to identify the manufacturer or device ID when the adapter is plugged in. On Windows, you can use Windows Update or Device Manager to find a driver.

If using Cygwin or WSL on Windows, you have to convert the Windows-style name into an Unix-style path (COM1 -> /dev/ttyS0, and so on). (This is not necessary if using esp-idf for ESP32 with the supplied Windows MSYS2 environment, this environment uses a native Windows Python which accepts COM ports as-is.)

In Linux, the current user may not have access to serial ports and a “Permission Denied” error will appear. On most Linux distributions, the solution is to add the user to the dialout group with a command like sudo usermod -a -G dialout <USERNAME>. Check your Linux distribution’s documentation for more information.

Baud rate

The default esptool.py baud rate is 115200bps. Different rates may be set using -b 921600 (or another baudrate of your choice). A default baud rate can also be specified using the ESPTOOL_BAUD environment variable. This can speed up write_flash and read_flash operations.

The baud rate is limited to 115200 when esptool.py establishes the initial connection, higher speeds are only used for data transfers.

Most hardware configurations will work with -b 230400, some with -b 460800, -b 921600 and/or -b 1500000 or higher.

If you have connectivity problems then you can also set baud rates below 115200. You can also choose 74880, which is the usual baud rate used by the ESP8266 to output boot log information.


Write binary data to flash: write_flash

Binary data can be written to the ESP’s flash chip via the serial write_flash command:

esptool.py --port COM4 write_flash 0x1000 my_app-0x01000.bin

Multiple flash addresses and file names can be given on the same command line:

esptool.py --port COM4 write_flash 0x00000 my_app.elf-0x00000.bin 0x40000 my_app.elf-0x40000.bin

The --chip argument is optional when writing to flash, esptool will detect the type of chip when it connects to the serial port.

The --port argument is documented under Serial Port.

The next arguments to write_flash are one or more pairs of offset (address) and file name. When generating ESP8266 “version 1” images, the file names created by elf2image include the flash offsets as part of the file name. For other types of images, consult your SDK documentation to determine the files to flash at which offsets.

Numeric values passed to write_flash (and other commands) can be specified either in hex (ie 0x1000), or in decimal (ie 4096).

See the Troubleshooting section if the write_flash command is failing, or the flashed module fails to boot.

Setting flash mode and size

You may also need to specify arguments for flash mode and flash size, if you wish to override the defaults. For example:

esptool.py --port /dev/ttyUSB0 write_flash --flash_mode qio --flash_size 32m 0x0 bootloader.bin 0x1000 my_app.bin

Since esptool v2.0, these options are not often needed as the default is to keep the flash mode and size from the .bin image file, and to detect the flash size. See the Flash Modes section for more details.


By default, the serial transfer data is compressed for better performance. The -u/--no-compress option disables this behaviour.

Read Flash Contents: read_flash

The read_flash command allows reading back the contents of flash. The arguments to the command are an address, a size, and a filename to dump the output to. For example, to read a full 2MB of attached flash:

./esptool.py -p PORT -b 460800 read_flash 0 0x200000 flash_contents.bin

(Note that if write_flash updated the boot image’s flash mode and flash size during flashing then these bytes may be different when read back.)

Erase Flash: erase_flash & erase region

To erase the entire flash chip (all data replaced with 0xFF bytes):

esptool.py erase_flash

To erase a region of the flash, starting at address 0x20000 with length 0x4000 bytes (16KB):

esptool.py erase_region 0x20000 0x4000

The address and length must both be multiples of the SPI flash erase sector size. This is 0x1000 (4096) bytes for supported flash chips.

Read built-in MAC address: read_mac

esptool.py read_mac

Read SPI flash id: flash_id

esptool.py flash_id

Example output:

Manufacturer: e0
Device: 4016
Detected flash size: 4MB

Refer to flashrom source code for flash chip manufacturer name and part number.

Convert ELF to Binary: elf2image

The elf2image command converts an ELF file (from compiler/linker output) into the binary executable images which can be flashed and then booted into:

esptool.py --chip esp8266 elf2image my_app.elf

This command does not require a serial connection.

elf2image also accepts the Flash Modes arguments --flash_freq and --flash_mode, which can be used to set the default values in the image header. This is important when generating any image which will be booted directly by the chip. These values can also be overwritten via the write_flash command, see the write_flash command for details.

elf2image for ESP8266

The default command output is two binary files: my_app.elf-0x00000.bin and my_app.elf-0x40000.bin. You can alter the firmware file name prefix using the --output/-o option.

elf2image can also produce a “version 2” image file suitable for use with a software bootloader stub such as rboot or the Espressif bootloader program. You can’t flash a “version 2” image without also flashing a suitable bootloader.

esptool.py --chip esp8266 elf2image --version=2 -o my_app-ota.bin my_app.elf

elf2image for ESP32

For esp32, elf2image produces a single output binary “image file”. By default this has the same name as the .elf file, with a .bin extension. ie:

esptool.py --chip esp32 elf2image my_esp32_app.elf

In the above example, the output image file would be called my_esp32_app.bin.

Output .bin image details: image_info

The image_info command outputs some information (load addresses, sizes, etc) about a .bin file created by elf2image.

esptool.py --chip esp32 image_info my_esp32_app.bin

Note that --chip esp32 is required when reading ESP32 images. Otherwise the default is --chip esp8266 and the image will be interpreted as an invalid ESP8266 image.

Advanced Commands

The following commands are less commonly used, or only of interest to advanced users. They are documented on the wiki:

Additional ESP32 Tools

The following tools for ESP32, bundled with esptool.py, are documented on the wiki:

Serial Connections

The ESP8266 & ESP32 ROM serial bootloader uses a 3.3V UART serial connection. Many development boards make the serial connections for you onboard.

However, if you are wiring the chip yourself to a USB/Serial adapter or similar then the following connections must be made:

ESP32/ESP8266 Pin Serial Port Pin
TX (aka GPIO1) RX (receive)
RX (aka GPIO3) TX (transmit)
Ground Ground

Note that TX (transmit) on the ESP8266 is connected to RX (receive) on the serial port connection, and vice versa.

Do not connect the chip to 5V TTL serial adapters, and especially not to “standard” RS-232 adapters! 3.3V serial only!

Entering the Bootloader

Both ESP8266 and ESP32 have to be reset in a certain way in order to launch the serial bootloader.

On some development boards (including NodeMCU, WeMOS, HUZZAH Feather, Core Board, ESP32-WROVER-KIT), esptool.py can automatically trigger a reset into the serial bootloader - in which case you don’t need to read this section.

For everyone else, three things must happen to enter the serial bootloader - a reset, required pins set correctly, and GPIO0 pulled low:

Boot Mode

Both ESP8266 and ESP32 chooses the boot mode each time it resets. A reset event can happen in one of several ways:

  • Power applied to chip.
  • The nRESET pin was low and is pulled high (on ESP8266 only).
  • The CH_PD/EN pin (“enable”) pin was low and is pulled high.

On ESP8266, both the nRESET and CH_PD pins must be pulled high for the chip to start operating.

For more details on selecting the boot mode, see the following Wiki pages:

Flash Modes

write_flash and some other comands accept command line arguments to set bootloader flash mode, flash size and flash clock frequency. The chip needs correct mode, frequency and size settings in order to run correctly - although there is some flexibility. A header at the beginning of a bootable image contains these values.

To override these values, the options --flash_mode, --flash_size and/or --flash_freq must appear after write_flash on the command line, for example:

esptool.py --port /dev/ttyUSB1 write_flash --flash_mode dio --flash_size 4MB 0x0 bootloader.bin

These options are only consulted when flashing a bootable image to an ESP8266 at offset 0x0, or an ESP32 at offset 0x1000. These are addresses used by the ROM bootloader to load from flash. When flashing at all other offsets, these arguments are not used.

Flash Mode (–flash_mode, -fm)

These set Quad Flash I/O or Dual Flash I/O modes. Valid values are keep, qio, qout, dio, dout. The default is keep, which keeps whatever value is already in the image file. This parameter can also be specified using the environment variable ESPTOOL_FM.

Most boards use qio mode. Some ESP8266 modules, including the ESP-12E modules on some (not all) NodeMCU boards, are dual I/O and the firmware will only boot when flashed with --flash_mode dio. Most ESP32 modules are also dual I/O.

In qio mode, two additional GPIOs (9 and 10) are used for SPI flash communications. If flash mode is set to dio then these pins are available for other purposes.

For a full explanation of these modes, see the SPI Flash Modes wiki page.

Flash Frequency (–flash_freq, -ff)

Clock frequency for SPI flash interactions. Valid values are keep, 40m, 26m, 20m, 80m (MHz). The default is keep, which keeps whatever value is already in the image file. This parameter can also be specified using the environment variable ESPTOOL_FF.

The flash chip connected to most chips works with 40MHz clock speeds, but you can try lower values if the device won’t boot. The highest 80MHz flash clock speed will give best performance, but may cause crashing if the flash or board designis not capable of this speed.

Flash Size (–flash_size, -fs)

Size of the SPI flash, given in megabytes. Valid values vary by chip type:

Chip flash_size values
ESP32 detect, 1MB, 2MB, 4MB, 8MB, 16MB
ESP8266 detect, 256KB, 512KB, 1MB, 2MB, 4MB, 2MB-c1, 4MB-c1, 8MB, 16MB

For ESP8266, some additional sizes & layouts for OTA “firmware slots” are available.

The default --flash_size parameter is detect, which tries to autodetect size based on SPI flash ID. If detection fails, a warning is printed and a default value of of 4MB (4 megabytes) is used.

If flash size is not successfully detected, you can find the flash size by using the flash_id command and then looking up the ID from the output (see Read SPI flash id). Alternatively, read off the silkscreen labelling of the flash chip and search for its datasheet.

The default flash_size parameter can also be overriden using the environment variable ESPTOOL_FS.

ESP8266 and Flash Size

The ESP8266 SDK stores WiFi configuration at the “end” of flash, and it finds the end using this size. However there is no downside to specifying a smaller flash size than you really have, as long as you don’t need to write an image larger than this size.

ESP-12, ESP-12E and ESP-12F modules (and boards that use them such as NodeMCU, HUZZAH, etc.) usually have at least 4 megabyte / 4MB (sometimes labelled 32 megabit) flash.

If using OTA, some additional sizes & layouts for OTA “firmware slots” are available. If not using OTA updates then you can ignore these extra sizes:

flash_size arg Number of OTA slots OTA Slot Size Non-OTA Space
256KB 1 (no OTA) 256KB N/A
512KB 1 (no OTA) 512KB N/A
1MB 2 512KB 0KB
2MB 2 512KB 1024KB
4MB 2 512KB 3072KB
2MB-c1 2 1024KB 0KB
4MB-c1 2 1024KB 2048KB
8MB [^] 2 1024KB 6144KB
16MB [^] 2 1024KB 14336KB
  • [^] Support for 8MB & 16MB flash size is not present in all ESP8266 SDKs. If your SDK doesn’t support these flash sizes, use --flash_size 4MB.

ESP32 and Flash Size

The ESP32 esp-idf flashes a partition table to the flash at offset 0x8000. All of the partitions in this table must fit inside the configured flash size, otherwise the ESP32 will not work correctly.

Advanced Options

See the Advanced Options wiki page for some of the more unusual esptool.py command line options.

Remote Serial Ports

It is possible to connect to any networked remote serial port that supports RFC2217 (Telnet) protocol, or a plain TCP socket. See the Remote Serial Ports wiki page for details.


Flashing problems can be fiddly to troubleshoot. Try the suggestions here if you’re having problems:

Bootloader won’t respond

If you see errors like “Failed to connect” then your chip is probably not entering the bootloader properly:

  • Check you are passing the correct serial port on the command line.
  • Check you have permissions to access the serial port, and other software (such as modem-manager on Linux) is not trying to interact with it. A common pitfall is leaving a serial terminal accessing this port open in another window and forgetting about it.
  • Check the chip is receiving 3.3V from a stable power source (see Insufficient Power for more details.)
  • Check that all pins are connected as described in Entering the bootloader. Check the voltages at each pin with a multimeter, “high” pins should be close to 3.3V and “low” pins should be close to 0V.
  • If you have connected other devices to GPIO pins mentioned above section, try removing them and see if esptool.py starts working.
  • Try using a slower baud rate (-b 9600 is a very slow value that you can use to verify it’s not a baud rate problem.)

write_flash operation fails part way through

If flashing fails with random errors part way through, retry with a lower baud rate.

Power stability problems may also cause this (see Insufficient Power.)

write_flash succeeds but program doesn’t run

If esptool.py can flash your module with write_flash but your program doesn’t run, try the following:

Wrong Flash Mode

Some devices only support the dio flash mode. Writing to flash with qio mode will succeed but the chip can’t read the flash back to run - so nothing happens on boot. Try passing the -fm dio option to write_flash.

See the SPI Flash Modes wiki page for a full description of the flash modes and how to determine which ones are supported on your device.

Insufficient Power

The 3.3V power supply for the ESP8266 and ESP32 has to supply large amounts of current (up to 70mA continuous, 200-300mA peak, slightly higher for ESP32). You also need sufficient capacitance on the power circuit to meet large spikes of power demand.

Insufficient Capacitance

If you’re using a pre-made development board or module then the built-in power regulator & capacitors are usually good enough, provided the input power supply is adequate.

This is not true for some very simple pin breakout modules - similar to this. These breakouts do not integrate enough capacitance to work reliably without additional components.. Surface mount OEM modules like ESP-WROOM02 and ESP-WROOM32 require an external bulk capacitor on the PCB to be reliable, consult the module datasheet.

Power Supply Rating

It is possible to have a power supply that supplies enough current for the serial bootloader stage with esptool.py, but not enough for normal firmware operation. You may see the 3.3V VCC voltage droop down if you measure it with a multimeter, but you can have problems even if this isn’t happening.

Try swapping in a 3.3V supply with a higher current rating, add capacitors to the power line, and/or shorten any 3.3V power wires.

The 3.3V output from FTDI FT232R chips/adapters or Arduino boards do not supply sufficient current to power an ESP8266 or ESP32 (it may seem to work sometimes, but it won’t work reliably). Other USB TTL/serial adapters may also be marginal.

Missing bootloader

Recent ESP8266 SDKs and the ESP32 esp-idf both use a small firmware bootloader program. The hardware bootloader in ROM loads this firmware bootloader from flash, and then it runs the program. On ESP8266. firmware bootloader image (with a filename like boot_v1.x.bin) has to be flashed at offset 0. If the firmware bootloader is missing then the ESP8266 will not boot. On ESP32, the bootloader image should be flashed by esp-idf at offset 0x1000.

Refer to SDK or esp-idf documentation for details regarding which binaries need to be flashed at which offsets.

SPI Pins which must be disconnected

Compared to the ROM bootloader that esptool.py talks to, a running firmware uses more of the chip’s pins to access the SPI flash.

If you set “Quad I/O” mode (-fm qio, the esptool.py default) then GPIOs 7, 8, 9 & 10 are used for reading the SPI flash and must be otherwise disconnected.

If you set “Dual I/O” mode (-fm dio) then GPIOs 7 & 8 are used for reading the SPI flash and must be otherwise disconnected.

Try disconnecting anything from those pins (and/or swap to Dual I/O mode if you were previously using Quad I/O mode but want to attach things to GPIOs 9 & 10). Note that if GPIOs 9 & 10 are also connected to input pins on the SPI flash chip, they may still be unsuitable for use as general purpose I/O.

In addition to these pins, GPIOs 6 & 11 are also used to access the SPI flash (in all modes). However flashing will usually fail completely if these pins are connected incorrectly.

Early stage crash

Use a serial terminal program to view the boot log. (ESP8266 baud rate is 74880bps, ESP32 is 115200bps). See if the program is crashing during early startup or outputting an error message.

Serial Terminal Programs

There are many serial terminal programs suitable for debugging & serial interaction. The pyserial module (which is required for esptool.py) includes one such command line terminal program - miniterm.py. For more details see this page or run miniterm -h.

Note that not every serial program supports the unusual ESP8266 74880bps “boot log” baud rate. Support is especially sparse on Linux. miniterm.py supports this baud rate on all platforms. ESP32 uses the more common 115200bps.

Tracing esptool.py interactions

Running esptool.py --trace will dump all serial interactions to the standard output (this is a lot of output). This can be helpful when debugging issues with the serial connection, or when providing information for bug reports.

Using esptool.py from Python

esptool.py can easily be integrated into Python applications or called from other Python scripts.

While it currently does have a poor Python API, something which #208 will address, it allows for passing CLI arguments to esptool.main(). This workaround makes integration very straightforward as you can pass exactly the same arguments as you would on the CLI.

command = ['--baud', '460800', 'read_flash', '0', '0x200000', 'flash_contents.bin']
print('Using command %s' % ' '.join(command))

Internal Technical Documentation

The repository wiki contains some technical documentation regarding the serial protocol and file format used by the ROM bootloader. This may be useful if you’re developing esptool.py or hacking system internals:


esptool.py was initially created by Fredrik Ahlberg (@themadinventor, @kongo), and is currently maintained by Angus Gratton (@projectgus). It has also received improvements from many members of the ESP8266 community - including @rojer, @jimparis, @jms19, @pfalcon, @tommie, @0ff, @george-hopkins and others.

This document and the attached source code are released under GNU General Public License Version 2. See the accompanying file LICENSE for a copy.

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