The eFuse Manager library is designed to structure access to eFuse bits and make using these easy. This library operates eFuse bits by a structure name which is assigned in eFuse table. This sections introduces some concepts used by eFuse Manager.
The {IDF_TARGET_NAME} has a number of eFuses which can store system and user parameters. Each eFuse is a one-bit field which can be programmed to 1 after which it cannot be reverted back to 0.
Some of system parameters are using these eFuse bits directly by hardware modules and have special place (for example EFUSE_BLK0).
For more details, see *{IDF_TARGET_NAME} Technical Reference Manual* > *eFuse Controller (eFuse)* [`PDF <{IDF_TARGET_TRM_EN_URL}#efuse>`__]. Some eFuse bits are available for user applications.
The component has API functions for reading and writing fields. Access to the fields is carried out through the structures that describe the location of the eFuse bits in the blocks. The component provides the ability to form fields of any length and from any number of individual bits. The description of the fields is made in a CSV file in a table form. To generate from a tabular form (CSV file) in the C-source uses the tool `efuse_table_gen.py`. The tool checks the CSV file for uniqueness of field names and bit intersection, in case of using a `custom` file from the user's project directory, the utility will check with the `common` CSV file.
* common (`esp_efuse_table.csv`) - contains eFuse fields which are used inside the IDF. C-source generation should be done manually when changing this file (run command ``idf.py efuse-common-table``). Note that changes in this file can lead to incorrect operation.
* custom - (optional and can be enabled by :ref:`CONFIG_EFUSE_CUSTOM_TABLE`) contains eFuse fields that are used by the user in their application. C-source generation should be done manually when changing this file and running ``idf.py efuse-custom-table``.
Name of field. The prefix `ESP_EFUSE_` will be added to the name, and this field name will be available in the code. This name will be used to access the fields. The name must be unique for all fields. If the line has an empty name, then this line is combined with the previous field. This allows you to set an arbitrary order of bits in the field, and expand the field as well (see ``MAC_FACTORY`` field in the common table). The field_name supports structured format using `.` to show that the field belongs to another field (see ``WR_DIS`` and ``RD_DIS`` in the common table).
Start bit number (0..255). The bit_start field can be omitted. In this case, it will be set to bit_start + bit_count from the previous record, if it has the same efuse_block. Otherwise (if efuse_block is different, or this is the first entry), an error will be generated.
The number of bits to use in this field (1..-). This parameter can not be omitted. This field also may be ``MAX_BLK_LEN`` in this case, the field length will have the maximum block length, taking into account the coding scheme (applicable for ``ESP_EFUSE_SECURE_BOOT_KEY`` and ``ESP_EFUSE_ENCRYPT_FLASH_KEY`` fields). The value ``MAX_BLK_LEN`` depends on :ref:`CONFIG_EFUSE_CODE_SCHEME_SELECTOR`, will be replaced with "None" - 256, "3/4" - 192, "REPEAT" - 128.
..only:: not esp32
bit_count
The number of bits to use in this field (1..-). This parameter can not be omitted. This field also may be ``MAX_BLK_LEN`` in this case, the field length will have the maximum block length.
If a non-sequential bit order is required to describe a field, then the field description in the following lines should be continued without specifying a name, this will indicate that it belongs to one field. For example two fields MAC_FACTORY and MAC_FACTORY_CRC:
WR_DIS.RD_DIS, EFUSE_BLK0, 0, 1, Write protection for RD_DIS
WR_DIS.FIELD_1, EFUSE_BLK0, 1, 1, Write protection for FIELD_1
WR_DIS.FIELD_2, EFUSE_BLK0, 2, 4, Write protection for FIELD_2 (includes B1 and B2)
WR_DIS.FIELD_2.B1, EFUSE_BLK0, 2, 2, Write protection for FIELD_2.B1
WR_DIS.FIELD_2.B2, EFUSE_BLK0, 4, 2, Write protection for FIELD_2.B2
WR_DIS.FIELD_3, EFUSE_BLK0, 5, 1, Write protection for FIELD_3
WR_DIS.FIELD_3.ALIAS, EFUSE_BLK0, 5, 1, Write protection for FIELD_3 (just a alias for WR_DIS.FIELD_3)
WR_DIS.FIELD_4, EFUSE_BLK0, 7, 1, Write protection for FIELD_4
The structured eFuse field looks like ``WR_DIS.RD_DIS`` where the dot points that this field belongs to the parent field - ``WR_DIS`` and can not be out of the parent's range.
It is possible to use some levels of structured fields as WR_DIS.FIELD_2.B1 and B2. These fields should not be crossed each other and should be in the range of two fields: ``WR_DIS`` and ``WR_DIS.FIELD_2``.
It is possible to create aliases for fields with the same range, see ``WR_DIS.FIELD_3`` and ``WR_DIS.FIELD_3.ALIAS``.
The IDF names for structured efuse fields should be unique. The ``efuse_table_gen`` tool will generate the final names where the dot will be replaced by ``_``. The names for using in IDF are ESP_EFUSE_WR_DIS, ESP_EFUSE_WR_DIS_RD_DIS, ESP_EFUSE_WR_DIS_FIELD_2_B1, etc.
The tool is designed to generate C-source files from CSV file and validate fields. First of all, the check is carried out on the uniqueness of the names and overlaps of the field bits. If an additional `custom` file is used, it will be checked with the existing `common` file (esp_efuse_table.csv). In case of errors, a message will be displayed and the string that caused the error. C-source files contain structures of type `esp_efuse_desc_t`.
The coding scheme affects only EFUSE_BLK1, EFUSE_BLK2 and EFUSE_BLK3 blocks. EUSE_BLK0 block always has a coding scheme ``None``.
Coding changes the number of bits that can be written into a block, the block length is constant 256, some of these bits are used for encoding and not avaliable for the user.
eFuse tables must always comply with the coding scheme in the chip. There is an :ref:`CONFIG_EFUSE_CODE_SCHEME_SELECTOR` option to select the coding type for tables in a Kconfig. When generating source files, if your tables do not follow the coding scheme, an error message will be displayed. Adjust the length or offset fields.
If your program was compiled with ``None`` encoding and ``3/4`` is used in the chip, then the ``ESP_ERR_CODING`` error may occur when calling the eFuse API (the field is outside the block boundaries). If the field matches the new block boundaries, then the API will work without errors.
Also, 3/4 coding scheme imposes restrictions on writing bits belonging to one coding unit. The whole block with a length of 256 bits is divided into 4 coding units, and in each coding unit there are 6 bytes of useful data and 2 service bytes. These 2 service bytes contain the checksum of the previous 6 data bytes.
It turns out that only one field can be written into one coding unit. Repeated rewriting in one coding unit is prohibited. But if the record was made in advance or through a :cpp:func:`esp_efuse_write_block` function, then reading the fields belonging to one coding unit is possible.
In case ``3/4`` coding scheme, the writing process is divided into the coding units and we can not use the usual mode of writing some fields. We can prepare all the data for writing and burn it in one time. You can also use this mode for ``None`` coding scheme but it is not necessary. It is important for ``3/4`` coding scheme.
Coding schemes are used to protect against data corruption. {IDF_TARGET_NAME} supports two coding schemes:
*``None``. EFUSE_BLK0 is stored with four backups, meaning each bit is stored four times. This backup scheme is automatically applied by the hardware and is not visible to software. EFUSE_BLK0 can be written many times.
*``RS``. EFUSE_BLK1 - EFUSE_BLK10 use Reed-Solomon coding scheme that supports up to 5 bytes of automatic error correction. Software will encode the 32-byte EFUSE_BLKx using RS (44, 32) to generate a 12-byte check code, and then burn the EFUSE_BLKx and the check code into eFuse at the same time. The eFuse Controller automatically decodes the RS encoding and applies error correction when reading back the eFuse block. Because the RS check codes are generated across the entire 256-bit eFuse block, each block can only be written to one time.
To write some fields into one block, or different blocks in one time, you need to use ``the batch writing mode``. Firstly set this mode through :cpp:func:`esp_efuse_batch_write_begin` function then write some fields as usual using the ``esp_efuse_write_...`` functions. At the end to burn them, call the :cpp:func:`esp_efuse_batch_write_commit` function. It burns prepared data to the eFuse blocks and disables the ``batch recording mode``.
EFUSE_BLK_KEY0 - EFUSE_BLK_KEY5 are intended to keep up to 6 keys with a length of 256-bits. Each key has an ``ESP_EFUSE_KEY_PURPOSE_x`` field which defines the purpose of these keys. The purpose field is described in :cpp:type:`esp_efuse_purpose_t`.
The purposes like ``ESP_EFUSE_KEY_PURPOSE_XTS_AES_...`` are used for flash encryption.
The purposes like ``ESP_EFUSE_KEY_PURPOSE_SECURE_BOOT_DIGEST...`` are used for secure boot.
There are some eFuse APIs useful to work with states of keys.
*:cpp:func:`esp_efuse_get_purpose_field` - Returns a pointer to a key purpose for an eFuse key block.
*:cpp:func:`esp_efuse_get_key` - Returns a pointer to a key block.
*:cpp:func:`esp_efuse_set_key_purpose` - Sets a key purpose for an eFuse key block.
*:cpp:func:`esp_efuse_set_keypurpose_dis_write` - Sets a write protection of the key purpose field for an eFuse key block.
*:cpp:func:`esp_efuse_find_unused_key_block` - Search for an unused key block and return the first one found.
*:cpp:func:`esp_efuse_count_unused_key_blocks` - Returns the number of unused eFuse key blocks in the range EFUSE_BLK_KEY0..EFUSE_BLK_KEY_MAX
*:cpp:func:`esp_efuse_get_digest_revoke` - Returns the status of the Secure Boot public key digest revocation bit.
*:cpp:func:`esp_efuse_set_digest_revoke` - Sets the Secure Boot public key digest revocation bit.
*:cpp:func:`esp_efuse_get_write_protect_of_digest_revoke` - Returns a write protection of the Secure Boot public key digest revocation bit.
*:cpp:func:`esp_efuse_set_write_protect_of_digest_revoke` - Sets a write protection of the Secure Boot public key digest revocation bit.
The Kconfig option :ref:`CONFIG_EFUSE_VIRTUAL` will virtualize eFuse values inside the eFuse Manager, so writes are emulated and no eFuse values are permanently changed. This can be useful for debugging app and unit tests.
In addition to the :ref:`CONFIG_EFUSE_VIRTUAL` option there is :ref:`CONFIG_EFUSE_VIRTUAL_KEEP_IN_FLASH` option that adds a feature to keep eFuses in flash memory. To use this mode the partition_table should have the `efuse` partition. partition.csv: ``"efuse_em, data, efuse, , 0x2000,"``.
During startup, the eFuses are copied from flash or, in case if flash is empty, from real eFuse to RAM and then update flash. This option allows keeping eFuses after reboots (possible to test secure_boot and flash_encryption features with this option).
Flash Encryption (FE) is a hardware feature that requires the physical burning of eFuses: key and FLASH_CRYPT_CNT. If FE is not actually enabled then enabling the :ref:`CONFIG_EFUSE_VIRTUAL_KEEP_IN_FLASH` option just gives testing possibilities and does not encrypt anything in the flash, even though the logs say encryption happens. The :cpp:func:`bootloader_flash_write` is adapted for this purpose. But if FE is already enabled on the chip and you run an application or bootloader created with the :ref:`CONFIG_EFUSE_VIRTUAL_KEEP_IN_FLASH` option then the flash encryption/decryption operations will work properly (data are encrypted as it is written into an encrypted flash partition and decrypted when they are read from an encrypted partition).
