Basic Commands

Write Binary Data to Flash: write_flash

Binary data can be written to the ESP’s flash chip via the serial write_flash command: --port COM4 write_flash 0x1000 my_app-0x01000.bin

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

Erasing Flash Before Write

To successfully write data into flash, all 4096-byte memory sectors (the smallest erasable unit) affected by the operation have to be erased first. As a result, when the flashing offset address or the data are not 4096-byte aligned, more memory is erased than actually needed. Esptool will display information about which flash memory sectors will be erased.

Use the -e/--erase-all option to erase all flash sectors (not just the write areas) before programming.

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: -p PORT -b 460800 read_flash 0 0x200000 flash_contents.bin

It is also possible to autodetect flash size by using ALL as size. The above example with autodetection would look like this: -p PORT -b 460800 read_flash 0 ALL flash_contents.bin


When using the read_flash command in combination with the --no-stub argument, it may be necessary to also set the --flash_size argument to ensure proper reading of the flash contents by the ROM.


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): erase_flash

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

Read SPI Flash ID: flash_id 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: --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. Overwriting these values via the write_flash command will produce an image with a recalculated SHA256 digest, otherwise, the image SHA256 digest would be invalidated by rewriting the image header. There is an option to skip appending a SHA256 digest after the image with --dont-append-digest argument of the elf2image command.

By default, elf2image uses the sections in the ELF file to generate each segment in the binary executable. To use segments (PHDRs) instead, pass the --use_segments option.

The default command output for ESP8266 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. --chip ESP8266 elf2image --version=2 -o my_app-ota.bin my_app.elf

Output .bin Image Details: image_info

The image_info command outputs some information (load addresses, sizes, etc) about a .bin file created by elf2image. Command also supports .hex file created by merge_bin command from supported .bin files.

To view more information about the image, such as set flash size, frequency and mode, or extended header information, use the --version 2 option. This extended output will become the default in a future major release.

This information corresponds to the headers described in Firmware Image Format. image_info --version 2 my_esp_app.bin

Merge Binaries for Flashing: merge_bin

The merge_bin command will merge multiple binary files (of any kind) into a single file that can be flashed to a device later. Any gaps between the input files are padded based on the selected output format.

For example: --chip ESP8266 merge_bin -o merged-flash.bin --flash_mode dio --flash_size 4MB 0x1000 bootloader.bin 0x8000 partition-table.bin 0x10000 app.bin

Will create a file merged-flash.bin with the contents of the other 3 files. This file can be later written to flash with write_flash 0x0 merged-flash.bin.

Common options:

  • The merge_bin command supports the same --flash_mode, --flash_size and --flash_freq options as the write_flash command to override the bootloader flash header (see above for details). These options are applied to the output file contents in the same way as when writing to flash. Make sure to pass the --chip parameter if using these options, as the supported values and the bootloader offset both depend on the chip.

  • The --format option will change the format of the output file. For more information about formats see formats description below.

  • The input files can be in either bin or hex format and they will be automatically converted to type selected by --format argument.

  • It is possible to append options from a text file with @filename (see the advanced options page Specifying Arguments via File section for details). As an example, this can be conveniently used with the ESP-IDF build system, which produces a flash_args file in the build directory of a project:

cd build    # The build directory of an ESP-IDF project --chip ESP8266 merge_bin -o merged-flash.bin @flash_args

HEX Output Format

The output of the command will be in Intel Hex format. The gaps between the files won’t be padded.

Intel Hex format offers distinct advantages when compared to the binary format, primarily in the following areas:

  • Transport: Intel Hex files are represented in ASCII text format, significantly increasing the likelihood of flawless transfers across various mediums.

  • Size: Data is carefully allocated to specific memory addresses eliminating the need for unnecessary padding. Binary images often lack detailed addressing information, leading to the inclusion of data for all memory locations from the file’s initial address to its end.

  • Validity Checks: Each line in an Intel Hex file has a checksum to help find errors and make sure data stays unchanged. --chip ESP8266 merge_bin --format hex -o merged-flash.hex --flash_mode dio --flash_size 4MB 0x1000 bootloader.bin 0x8000 partition-table.bin 0x10000 app.bin

RAW Output Format

The output of the command will be in raw format and gaps between individual files will be filled with 0xFF bytes (same as unwritten flash contents).


Because gaps between the input files are padded with 0xFF bytes, when the merged binary is written then any flash sectors between the individual files will be erased. To avoid this, write the files individually.

RAW options:

  • The --fill-flash-size SIZE option will pad the merged binary with 0xFF bytes to the full flash specified size, for example --fill-flash-size 4MB will create a 4MB binary file.

  • The --target-offset 0xNNN option will create a merged binary that should be flashed at the specified offset, instead of at offset 0x0.

UF2 Output Format

This command will generate a UF2 (USB Flashing Format) binary. This UF2 file can be copied to a USB mass storage device exposed by another ESP running the ESP USB Bridge project. The bridge MCU will use it to flash the target MCU. This is as simple copying (or “drag-and-dropping”) the file to the exposed disk accessed by a file explorer in your machine.

Gaps between the files will be filled with 0x00 bytes.

UF2 options:

  • The --chunk-size option will set what portion of 512 byte block will be used for data. A common value is 256 bytes. By default, the largest possible value will be used.

  • The --md5-disable option will disable MD5 checksums at the end of each block. This can be useful for integration with e.g. tinyuf2. --chip ESP8266 merge_bin --format uf2 -o merged-flash.uf2 --flash_mode dio --flash_size 4MB 0x1000 bootloader.bin 0x8000 partition-table.bin 0x10000 app.bin

Advanced Commands

The following commands are less commonly used, or only of interest to advanced users. They are documented in the Advanced Commands section: