{IDF_TARGET_NAME} may have different revisions. These revisions mainly fix some issues, and sometimes also bring new features to the chip. :ref:`versioning scheme` describes the versioning of these chip revisions, and the APIs to read the versions at runtime.
:ref:`revision_limitation` describes how the application can specify its chip revision requirements, and the way ESP-IDF checks the compatibility. After that, there is troubleshooting information for this mechanism.
A chip's revision number is typically expressed as ``vX.Y``, where:
-``X`` means a **Major** wafer version. If it is changed, it means that the current software version is not compatible with this released chip and the software must be updated to use this chip.
-``Y`` means a **Minor** wafer version. If it is changed that means the current software version is compatible with the released chip, and there is no need to update the software.
If a newly released chip does not contain breaking changes, the chip can run the same software as the previous chip. As such, the new chip's revision number will only increment the minor version while keeping the major version the same (e.g., ``v1.1`` to ``v1.2``).
Conversely, if a newly released chip contains breaking changes, the chip **cannot** run the same software as the previous chip. As such, the new chip's revision number will increment the major version and set the minor version to 0 (e.g., ``v1.1`` to ``v2.0``).
This versioning scheme was selected to indicate the derivation relationship of chip revisions, and clearly distinguish changes in chips between breaking changes and non-breaking changes.
ESP-IDF is designed to execute seamlessly on future chip minor revisions with the same logic as the chip's nearest previous minor revision. Thus，users can directly port their compiled binaries to newer MINOR chip revisions without upgrading their ESP-IDF version and re-compile the whole project.
When a binary is executed on a chip revision of unexpected MAJOR revision, the software is also able to report issues according to the MAJOR revision. The major and minor versioning scheme also allows hardware changes to be branchable.
The current chip revision scheme using major and minor versions was introduced from ESP-IDF v5.0 onwards. Thus bootloaders built using earlier versions of ESP-IDF will still use the legacy chip revision scheme of wafer versions.
- Major wafer version (``WAFER_VERSION_MAJOR`` eFuse)
- Minor wafer version (``WAFER_VERSION_MINOR`` eFuse)
- Ignore maximum revision (``DISABLE_WAFER_VERSION_MAJOR`` eFuse). See :ref:`revision_limitation` on how this is used.
..note::
The previous versioning logic was based on a single eFuse version field (``WAFER_VERSION``). This approach makes it impossible to mark chips as breaking or non-breaking changes, and the versioning logic becomes linear.
When building an application that needs to support multiple revisions of a particular chip, the minimum and maximum chip revision numbers supported by the build are specified via Kconfig.
The minimum chip revision can be configured via the :ref:`CONFIG_{IDF_TARGET_CFG_PREFIX}_REV_MIN` option. Specifying the minimum chip revision will limit the software to only run on a chip revisions that are high enough to support some features or bugfixes.
The maximum chip revision cannot be configured and is automatically determined by the current ESP-IDF version being used. ESP-IDF will refuse to boot any chip revision exceeding the maximum chip revision. Given that it is impossible for a particular ESP-IDF version to foresee all future chip revisions, the maximum chip revision is usually set to ``maximum supported MAJOR version + 99``. The "Ignore Maximum Revision" eFuse can be set to bypass the maximum revision limitation. However, the software is not guaranteed to work if the maximum revision is ignored.
Below is the information about troubleshooting when the chip revision fails the compatibility check. Then there are technical details of the checking and software behavior on earlier version of ESP-IDF.
1. If the 2nd stage bootloader is run on a chip revision smaller than minimum revision specified in the image (i.e., the application), a reboot occurs. The following message will be printed:
To resolve this issue, update ESP-IDF to a newer version that supports the chip's revision (``CONFIG_{IDF_TARGET_CFG_PREFIX}_REV_MAX_FULL``). Alternatively, set the ``Ignore maximal revision`` bit in eFuse or use a chip revision that is compatible with the current version of ESP-IDF.
The 2nd stage bootloader and the application binary images contain the :cpp:type:`esp_image_header_t` header, which stores information specifying the chip revisions that the image is permitted to run on. This header has 3 fields related to revisions:
-``min_chip_rev`` - Minimum chip MAJOR revision required by image (but for ESP32-C3 it is MINOR revision). Its value is determined by :ref:`CONFIG_{IDF_TARGET_CFG_PREFIX}_REV_MIN`.
-``min_chip_rev_full`` - Minimum chip MINOR revision required by image in format: ``major * 100 + minor``. Its value is determined by :ref:`CONFIG_{IDF_TARGET_CFG_PREFIX}_REV_MIN`.
-``max_chip_rev_full`` - Maximum chip revision required by image in format: ``major * 100 + minor``. Its value is determined by ``CONFIG_{IDF_TARGET_CFG_PREFIX}_REV_MAX_FULL``. It can not be changed by user. Only Espressif can change it when a new version will be supported in ESP-IDF.
1. The 1st stage bootloader (ROM bootloader) does not check minimum and maximum revision fields from :cpp:type:`esp_image_header_t` before running the 2nd stage bootloader.
2. The initialization phase of the 2nd stage bootloader checks that the 2nd stage bootloader itself can be launched on the chip of this revision. It extracts the minimum revision from the header of the bootloader image and checks against the chip revision from eFuses. If the chip revision is less than the minimum revision, the bootloader refuses to boot up and aborts. The maximum revision is not checked at this phase.
3. Then the 2nd stage bootloader checks the revision requirements of the application. It extracts the minimum and maximum revisions from the header of the application image and checks against the chip revision from eFuses. If the chip revision is less than the minimum revision or higher than the maximum revision, the bootloader refuses to boot up and aborts. However, if the Ignore maximum revision bit is set, the maximum revision constraint can be ignored. The ignore bit is set by the customer themselves when there is confirmation that the software is able to work with this chip revision.
4. Furthermore, at the OTA update stage, the running application checks if the new software matches the chip revision. It extracts the minimum and maximum revisions from the header of the new application image and checks against the chip revision from eFuses. It checks for revision matching in the same way that the bootloader does, so that the chip revision is between the min and max revisions (logic of ignoring max revision also applies).
The old bootloaders did not read the minor wafer version eFuse, and the major version can be only lower than or equal to v3. This means that the old bootloader can detect correctly only chip version in range ``v0.0`` to ``v3.0``, where the minor version is always set to ``0``.
{IDF_TARGET_NAME} chip support was added in ESP-IDF v4.3. The old bootloaders cannot read all bits of the wafer version eFuse, it can read only the first 3 least significant bits. This means that the old bootloader cannot detect chip version correctly. Chips ``v0.0`` to ``v0.8`` are detected correctly, but other chip versions will be recognized as a version from this range.
{IDF_TARGET_NAME} chip support was added in ESP-IDF v4.2. {IDF_TARGET_NAME} chips have ``rev_min`` in :cpp:type:`esp_image_header_t` header set to ``0`` because ``Minimum Supported ESP32-S2 Revision`` Kconfig option was not introduced, which means that the old bootloader does not check the chip revision. Any app can be loaded by such bootloader in range ``v0.0`` to ``v3.15``.
