is a tool for communicating with an Espressif chip and reading/writing (“burning”) the one-time-programmable efuses values.


Because efuse is one-time-programmable, it is possible to permanently damage or “brick” your ESP32-S2 using this tool. Use it with great care.

For more details about Espressif chips efuse features, see the Technical Reference Manual. is installed alongside, so if (v2.0 or newer) is available on the PATH then should be as well.

Display Efuse Summary --port /dev/ttyUSB1 summary

The options --port and --before can be supplied, and are identical to the equivalent esptool options.

Output from the summary command will look like this: v2.6-beta1
EFUSE_NAME             Description = [Meaningful Value] [Readable/Writeable] (Hex Value)
Security fuses:
FLASH_CRYPT_CNT        Flash encryption mode counter                     = 0 R/W (0x0)
FLASH_CRYPT_CONFIG     Flash encryption config (key tweak bits)          = 0 R/W (0x0)
CONSOLE_DEBUG_DISABLE  Disable ROM BASIC interpreter fallback            = 1 R/W (0x1)
ABS_DONE_0             secure boot enabled for bootloader                = 0 R/W (0x0)
ABS_DONE_1             secure boot abstract 1 locked                     = 0 R/W (0x0)
JTAG_DISABLE           Disable JTAG                                      = 0 R/W (0x0)
DISABLE_DL_ENCRYPT     Disable flash encryption in UART bootloader       = 0 R/W (0x0)
DISABLE_DL_DECRYPT     Disable flash decryption in UART bootloader       = 0 R/W (0x0)
DISABLE_DL_CACHE       Disable flash cache in UART bootloader            = 0 R/W (0x0)
BLK1                   Flash encryption key
  = 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 R/W
BLK2                   Secure boot key
  = 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 R/W
BLK3                   Variable Block 3
  = 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 R/W

Efuse fuses:
WR_DIS                 Efuse write disable mask                          = 0 R/W (0x0)
RD_DIS                 Efuse read disablemask                            = 0 R/W (0x0)
CODING_SCHEME          Efuse variable block length scheme                = 0 R/W (0x0)
KEY_STATUS             Usage of efuse block 3 (reserved)                 = 0 R/W (0x0)

Config fuses:
XPD_SDIO_FORCE         Ignore MTDI pin (GPIO12) for VDD_SDIO on reset    = 0 R/W (0x0)
XPD_SDIO_REG           If XPD_SDIO_FORCE, enable VDD_SDIO reg on reset   = 0 R/W (0x0)
XPD_SDIO_TIEH          If XPD_SDIO_FORCE & XPD_SDIO_REG, 1=3.3V 0=1.8V   = 0 R/W (0x0)
SPI_PAD_CONFIG_CLK     Override SD_CLK pad (GPIO6/SPICLK)                = 0 R/W (0x0)
SPI_PAD_CONFIG_Q       Override SD_DATA_0 pad (GPIO7/SPIQ)               = 0 R/W (0x0)
SPI_PAD_CONFIG_D       Override SD_DATA_1 pad (GPIO8/SPID)               = 0 R/W (0x0)
SPI_PAD_CONFIG_HD      Override SD_DATA_2 pad (GPIO9/SPIHD)              = 0 R/W (0x0)
SPI_PAD_CONFIG_CS0     Override SD_CMD pad (GPIO11/SPICS0)               = 0 R/W (0x0)
DISABLE_SDIO_HOST      Disable SDIO host                                 = 0 R/W (0x0)

Identity fuses:
MAC                    MAC Address
  = 30:ae:a4:c3:86:94 (CRC 99 OK) R/W
CHIP_VER_REV1          Silicon Revision 1                                = 1 R/W (0x1)
CHIP_VERSION           Reserved for future chip versions                 = 2 R/W (0x2)
CHIP_PACKAGE           Chip package identifier                           = 1 R/W (0x1)

Calibration fuses:
BLK3_PART_RESERVE      BLOCK3 partially served for ADC calibration data  = 0 R/W (0x0)
ADC_VREF               Voltage reference calibration                     = 1093 R/W (0x11)

Flash voltage (VDD_SDIO) determined by GPIO12 on reset (High for 1.8V, Low/NC for 3.3V).

On relatively new chip, most efuses are unburned (value 0).

In v2.6 and newer, read-protected efuse values are displayed as question marks (??). On earlier versions, they are displayed as zeroes.

For details on the meaning of each efuse value, refer to the Technical Reference Manual.

Dump Raw Efuse Registers

To display raw efuse register values, use the dump subcommand: --port /dev/ttyUSB1 dump v2.0-dev
EFUSE block 0:
00000000 c40042xx xxxxxxxx 00000000 00000033 00000000 00000000
EFUSE block 1:
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
EFUSE block 2:
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
EFUSE block 3:
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000

Output corresponds directly to efuse register values in the register space.

Burning an Efuse


This command can brick your ESP32-S2 if used incorrectly!

To burn an efuse to a new value, use the burn_efuse command: --port /dev/ttyUSB0 burn_efuse JTAG_DISABLE 1

The arguments to burn_efuse are the name of the efuse (as shown in summary output) and the new value.

New values can be a numeric value in decimal or hex (with 0x prefix). Efuse bits can only be burned from to 0 to 1, attempting to set any back to 0 will have no effect. Most efuses have a limited bit width (many are only 1-bit flags).

Longer efuses (MAC addresses, keys) cannot be set via this command.

By default, will ask you to type BURN before it permanently sets an efuse. The --do-not-confirm option allows you to bypass this.

Setting Flash Voltage (VDD_SDIO)

After reset, the default ESP32-S2 behaviour is to enable and configure the flash voltage regulator (VDD_SDIO) based on the level of the MTDI pin (GPIO12).

The default behaviour on reset is:


VDD_SDIO Internal Regulator

Low or unconnected

Enabled at 3.3V


Enabled at 1.8V

Consult ESP32-S2 Technical Reference Manual for details.

A combination of 3 efuses (XPD_SDIO_FORCE, XPD_SDIO_REG, XPD_SDIO_TIEH) can be burned in order to override this behaviour and disable VDD_SDIO regulator, or set it to a fixed voltage. These efuses can be burned with individual burn_efuse commands, but the set_flash_voltage command makes it easier:

Disable VDD_SDIO Regulator set_flash_voltage OFF

Once set:

  • VDD_SDIO regulator always disabled.

  • MTDI pin (GPIO12) is ignored.

  • Flash must be powered externally and voltage supplied to VDD_SDIO pin of ESP32-S2.

  • Efuse XPD_SDIO_FORCE is burned.

Fixed 1.8V VDD_SDIO set_flash_voltage 1.8V

Once set:

  • VDD_SDIO regulator always enables at 1.8V.

  • MTDI pin (GPIO12) is ignored.

  • External voltage should not be supplied to VDD_SDIO.

  • Efuses XPD_SDIO_FORCE and XPD_SDIO_REG are burned.

Fixed 3.3V VDD_SDIO set_flash_voltage 3.3V

Once set:

  • VDD_SDIO regulator always enables at 3.3V.

  • MTDI pin (GPIO12) is ignored.

  • External voltage should not be supplied to VDD_SDIO.


Subsequent Changes

Once an efuse is burned it cannot be un-burned. However, changes can be made by burning additional efuses:

  • set_flash_voltage OFF can be changed to 1.8V or 3.3V

  • set_flash_voltage 1.8V can be changed to 3.3V

Burning a Key


This command can brick your ESP32-S2 if used incorrectly!

The efuse key blocks BLK1, BLK2 and BLK3 can all hold encryption keys. The burn_key subcommand loads a key (stored as a raw binary file) and burns it to a key block.

flash_encryption can be used as an alias for BLK1, and secure_boot can be used as an alias for BLK2.

  • The burn_key command should only be used for hardware flash encryption or secure boot keys. See Burning non-key data for a command that works for data read by software.

Unprotected Keys

By default, when an encryption key block is burned it is also read and write protected. The --no-protect-key option will disable this behaviour (you can separately read- or write-protect the key later.)


Leaving a key unprotected may compromise its use as a security feature. --port /dev/ttyUSB0 burn_key secure_boot keyfile.bin

Note that the hardware flash encryption and secure boot features require the key to be written to the efuse block in reversed byte order, compared to the order used by the AES algorithm on the host. burn_key automatically reverses the bytes when writing. For this reason, an unprotected key will read back in the reverse order to the keyfile.bin on the host.

Force Writing a Key

Normally, a key will only be burned if the efuse block has not been previously written to. The --force-write-always option can be used to ignore this and try to burn the key anyhow. Note that this option is still limited by the efuse hardware - hardware doesn’t allow any efuse bits to be cleared 1->0, and can’t write anything to write protected efuse blocks.


The --do-not-confirm option can be used with burn_key, otherwise a manual confirmation step is required.


The burn_key command is only suitable for flash encryption and secure boot keys:

  • Complete block is always written. (This means 256-bits for “None” coding Scheme or 192-bits for 3/4 Coding Scheme).

  • The data is written in reverse byte order for compatibility with encryption hardware (see above).

  • By default, the data is read- and write-protected so it can only be used by hardware.

Burning non-Key Data

The burn_block_data command allows writing arbitrary data from a file into an efuse block, for software use.

This command is available in v2.6 and newer.

Example: Write to Efuse BLK3 from binary file device_id.bin, starting at efuse byte offset 6: -p PORT burn_block_data --offset 6 BLK3 device_id.bin
  • Data is written to the Efuse block in normal byte order (treating the efuse block as if it was an array of bytes). It can be read back in firmware from the efuse read registers, but these reads must be always be complete register words (4-byte aligned).

  • Part of the Efuse block can be written at a time. The --offset argument allows writing to a byte offset inside the Efuse block itself.

  • This command is not suitable for writing key data which will be used by flash encryption or secure boot hardware. Use burn_key for this.


For “None” Coding Scheme, there are no restrictions on the range of bytes which can be written in the Efuse block (but any bit in efuse can only be set 0->1, never cleared 1->0).

For “3/4” Coding Scheme, the length of the data file and the offset must both be a multiple of 6 bytes. Each 6 byte span can only be written one time.

SPI Flash Pins

The following efuses configure the SPI flash pins which are used to boot:

SPI_PAD_CONFIG_CLK     Override SD_CLK pad (GPIO6/SPICLK)                = 0 R/W (0x0)
SPI_PAD_CONFIG_Q       Override SD_DATA_0 pad (GPIO7/SPIQ)               = 0 R/W (0x0)
SPI_PAD_CONFIG_D       Override SD_DATA_1 pad (GPIO8/SPID)               = 0 R/W (0x0)
SPI_PAD_CONFIG_HD      Override SD_DATA_2 pad (GPIO9/SPIHD)              = 0 R/W (0x0)
SPI_PAD_CONFIG_CS0     Override SD_CMD pad (GPIO11/SPICS0)               = 0 R/W (0x0)

On ESP32-S2 chips without integrated SPI flash, these efuses are set to zero in the factory. This causes the default GPIO pins (shown in the summary output above) to be used for the SPI flash.

On ESP32-S2 chips with integrated internal SPI flash, these efuses are burned in the factory to the GPIO numbers where the flash is connected. These values override the defaults on boot.

In order to change the SPI flash pin configuration, these efuses can be burned to the GPIO numbers where the flash is connected. If at least one of these efuses is burned, all of of them must be set to the correct values.

If these efuses are burned, GPIO1 (U0TXD pin) is no longer consulted to set the boot mode from SPI to HSPI flash on reset.

These pins can be set to any GPIO number in the range 0-29, 32 or 33. Values 30 and 31 cannot be set. The “raw” hex value for pins 32, 33 is 30, 31 (this is visible in the summary output if these pins are configured for any SPI I/Os.)

For example:

SPI_PAD_CONFIG_CS0     Override SD_CMD pad (GPIO11/SPICS0)               = 32 R/W (0x1e)

If using the burn_efuse command to configure these pins, always specify the actual GPIO number you wish to set.

Read- and Write- Protecting Efuses


This command can severely limit your ESP32-S2 options.

Some efuses can be read- or write-protected, preventing further changes. burn_key subcommand read and write protects new keys by default, but other efuses can be protected iwth the read_protect_efuse and write_protect_efuse commands.

The R/W output in the summary display will change to indicate protected efuses:

  • -/W indicates read protected (value will always show all-zeroes, even though hardware may use the correct value)

  • R/- shows write protected (no further bits can be set)

  • -/- means read and write protected

Example: --port /dev/SOMEPORT read_protect_efuse KEY_STATUS

The --do-not-confirm option can be used with burn_key, otherwise a manual confirmation step is required.


Efuses are often read/write protected as a group, so protecting one will cause some related efuses to become protected. will confirm the full list of efuses that will become protected.

The following efuses can be read protected:




  • BLK1

  • BLK2

  • BLK3

The following efuses can be write protected:



  • MAC













  • ABS_DONE_0

  • ABS_DONE_1






  • BLK1

  • BLK2

  • BLK3

Execute Efuse Python Script execute_scripts ...

This command allows burning all needed efuses at one time based on your own python script and control issues during the burn process if so it will abort the burn process. This command has a few arguments:

  • scripts is a list of scripts. The special format of python scripts can be executed inside

  • --index integer index (it means the number of chip in the batch in the range 1 - the max number of chips in the batch). It allows to retrieve unique data per chip from configfiles and then burn them (ex. CUSTOM_MAC, UNIQUE_ID).

  • --configfiles List of configfiles with data.

Below you can see some examples of the script. This script file is run from as exec(open( it means that some functions and imported libs are available for using like os. Please use only provided functions. If you want to use other libs in the script you can add them manually.

Inside this script, you can call all commands which are available in CLI, see --help. To run a efuse command you need to call espefuse(esp, efuses, args, 'burn_efuse DISABLE_DL_DECRYPT 1'). This command will not burn eFuses immediately, the burn occurs at the end of all scripts. If necessary, you can call efuses.burn_all() which prompts Type 'BURN' (all capitals) to continue.. To skip this check and go without confirmation just add the --do-not-confirm flag to the execute_scripts command.

This command supports nesting. This means that one script can be called from another script (see the test case test_execute_scripts_nesting in esptool/test/ execute_scripts --do-not-confirm

Additionally, you can implement some checks based on the value of efuses. To get value of an efuse use efuses['FLASH_CRYPT_CNT'].get(). Some eFuses have a dictionary to convert from a value to a human-readable as it looks in the table is printed by the summary command. See how it is done for CODING_SCHEME when get_meaning() is called:

  • 0: “NONE (BLK1-3 len=256 bits)”

  • 1: “3/4 (BLK1-3 len=192 bits)”

  • 2: “REPEAT (BLK1-3 len=128 bits) not supported”

  • 3: “NONE (BLK1-3 len=256 bits)”

print("connected chip: %s, coding scheme %s" % (esp.get_chip_description(), efuses["CODING_SCHEME"].get_meaning()))
if os.path.exists("flash_encryption_key.bin"):
    espefuse(esp, efuses, args, "burn_key flash_encryption flash_encryption_key.bin")
    raise esptool.FatalError("The 'flash_encryption_key.bin' file is missing in the project directory")

espefuse(esp, efuses, args, 'burn_efuse FLASH_CRYPT_CNT 0x7')

current_flash_crypt_cnt = efuses['FLASH_CRYPT_CNT'].get()
if current_flash_crypt_cnt in [0, 3]:
    espefuse(esp, efuses, args, 'burn_efuse FLASH_CRYPT_CNT')

espefuse(esp, efuses, args, 'burn_efuse DISABLE_DL_ENCRYPT 1')

espefuse(esp, efuses, args, 'burn_efuse DISABLE_DL_DECRYPT 1')

espefuse(esp, efuses, args, 'burn_efuse DISABLE_DL_CACHE 1')

espefuse(esp, efuses, args, 'burn_efuse JTAG_DISABLE 1')

After efuses.burn_all(), all needed efuses will be burnt to chip in order BLK_MAX to BLK_0. This order prevents cases when protection is set before the value goes to a block. Please note this while developing your scripts. Upon completion, the new eFuses will be read back, and will be done some checks of written eFuses by In production, you might need to check that all written efuses are set properly, see the example below.

The script burns some efuses and checks them after reading back. To check read and write protection, is_readable() and is_writeable() are called.

Burn Unique Data Per Chip

In case you are running the execute_scripts command from your production script, you may need to pass index to get the unique data for each chip from the configfiles (* .txt, * .json, etc.). The espefuse command will be like this, where {index} means the number of chip in the batch, you increment it by your own script in the range 1 - the max number of chips in the batch: execute_scripts --do-not-confirm --index {index} --configfiles mac_addresses.json  unique_id.json

The example of a script to burn custom_mac address and unique_id getting them from configfiles.


mac_addresses = json.load(args.configfiles[0])
unique_id = json.load(args.configfiles[1])

mac_val = mac_addresses[str(args.index)]
cmd = 'burn_custom_mac {}'.format(mac_val)
espefuse(esp, efuses, args, cmd)

unique_id_val = unique_id[str(args.index)]
cmd = 'burn_efuse UNIQUE_ID {}'.format(unique_id_val)
espefuse(esp, efuses, args, cmd)

The example of a script to burn custom_mac address that generated right in the script.


step = 4
base_mac = '0xAABBCCDD0000'
mac = ''
for index in range(100):
    mac = "{:012X}".format(int(base_mac, 16) + (args.index - 1) * step)
    mac = ':'.join(mac[k] + mac [k + 1] for k in range(0, len(mac), 2))

cmd = 'burn_custom_mac mac'
espefuse(esp, efuses, args, cmd)

Perform Multiple Operations In A Single Espefuse Run

Some eFuse blocks have an encoding scheme (Reed-Solomon or 3/4) that requires encoded data, making these blocks only writable once. If you need to write multiple keys/eFuses to one block using different commands, you can use this feature - multiple commands. This feature burns given data once at the end of all commands. All commands supported by version v3.2 or later are supported to be chained together.

The example below shows how to use the two commands burn_key_digest and burn_key to write the Secure Boot key and Flash Encryption key into one BLOCK3 for the ESP32-C2 chip. Using these commands individually will result in only one key being written correctly. -c esp32c2  \
                        burn_key_digest secure_images/ecdsa256_secure_boot_signing_key_v2.pem \
                        burn_key BLOCK_KEY0 images/efuse/128bit_key XTS_AES_128_KEY_DERIVED_FROM_128_EFUSE_BITS