It is recommended to update to newer :doc:`versions of ESP-IDF </versions>`: when they are released. The OTA (over the air) update process can flash new apps in the field but cannot flash a new bootloader. For this reason, the bootloader supports booting apps built from newer versions of ESP-IDF.
The bootloader does not support booting apps from older versions of ESP-IDF. When updating ESP-IDF manually on an existing product that might need to downgrade the app to an older version, keep using the older ESP-IDF bootloader binary as well.
If testing an OTA update for an existing product in production, always test it using the same ESP-IDF bootloader binary that is deployed in production.
..only:: esp32
Before ESP-IDF V2.1
^^^^^^^^^^^^^^^^^^^
Bootloaders built from very old versions of ESP-IDF (before ESP-IDF V2.1) perform less hardware configuration than newer versions. When using a bootloader from these early ESP-IDF versions and building a new app, enable the config option :ref:`CONFIG_ESP32_COMPATIBLE_PRE_V2_1_BOOTLOADERS`.
Bootloaders built from versions of ESP-IDF before V3.1 do not support MD5 checksums in the partition table binary. When using a bootloader from these ESP-IDF versions and building a new app, enable the config option :ref:`CONFIG_ESP32_COMPATIBLE_PRE_V3_1_BOOTLOADERS`.
Each ESP-IDF application or bootloader .bin file contains a header with :ref:`CONFIG_ESPTOOLPY_FLASHMODE`, :ref:`CONFIG_ESPTOOLPY_FLASHFREQ`, :ref:`CONFIG_ESPTOOLPY_FLASHSIZE` embedded in it. These are used to configure the SPI flash during boot.
The :ref:`first-stage-bootloader` in ROM reads the :ref:`second-stage-bootloader` header from flash and uses these settings to load it. However, at this time the system clock speed is lower than configured and not all flash modes are supported. When the :ref:`second-stage-bootloader` then runs and re-configures the flash, it reads values from the currently selected app binary header not the bootloader header. This allows an OTA update to change the SPI flash settings in use.
..only:: esp32
Bootloaders prior to ESP-IDF V4.0 used the bootloader's own header to configure the SPI flash, meaning these values could not be changed in an update. To maintain compatibility with older bootloaders, the app re-initializes the flash settings during app startup using the configuration found in the app header.
Next, update the firmware via OTA (over the air). The updated firmware will be loaded into an OTA app partition slot and the OTA data partition is updated to boot from this partition.
If you want to be able to roll back to the factory firmware and clear the settings, then you need to set :ref:`CONFIG_BOOTLOADER_FACTORY_RESET`. The factory reset mechanism allows to reset the device to factory settings:
:ref:`CONFIG_BOOTLOADER_NUM_PIN_FACTORY_RESET`- number of the GPIO input for factory reset uses to trigger a factory reset, this GPIO must be pulled low on reset to trigger this.
:ref:`CONFIG_BOOTLOADER_HOLD_TIME_GPIO`- this is hold time of GPIO for reset/test mode (by default 5 seconds). The GPIO must be held low continuously for this period of time after reset before a factory reset or test partition boot (as applicable) is performed.
The user can write a special firmware for testing in production, and run it as needed. The partition table also needs a dedicated partition for this testing firmware (See `partition table`).
:ref:`CONFIG_BOOTLOADER_NUM_PIN_APP_TEST` - GPIO number to boot TEST partition. The selected GPIO will be configured as an input with internal pull-up enabled. To trigger a test app, this GPIO must be pulled low on reset.
After the GPIO input is deactivated and the device reboots, the normally configured application will boot (factory or any OTA slot).
:ref:`CONFIG_BOOTLOADER_HOLD_TIME_GPIO` - this is hold time of GPIO for reset/test mode (by default 5 seconds). The GPIO must be held low continuously for this period of time after reset before a factory reset or test partition boot (as applicable) is performed.
The bootloader has the :ref:`CONFIG_BOOTLOADER_SKIP_VALIDATE_IN_DEEP_SLEEP` option which allows to reduce the wake-up time (useful to reduce consumption). This option is available when the :ref:`CONFIG_SECURE_BOOT` option is disabled. Reduction of time is achieved due to the lack of image verification. During the first boot, the bootloader stores the address of the application being launched in the RTC FAST memory. And during the awakening, this address is used for booting without any checks, thus fast loading is achieved.
The current bootloader implementation allows a project to override it. To do this, you must copy the directory ``/esp-idf/components/bootloader`` to your project components directory and then edit ``/your_project/components/bootloader/subproject/main/bootloader_start.c``.
In the bootloader space, you cannot use the drivers and functions from other components. If necessary, then the required functionality should be placed in the project's ``bootloader`` directory (note that this will increase its size).
If the bootloader grows too large then it can collide with the partition table, which is flashed at offset 0x8000 by default. Increase the :ref:`partition table offset <CONFIG_PARTITION_TABLE_OFFSET>` value to place the partition table later in the flash. This increases the space available for the bootloader.
..note:: The first time you copy the bootloader into an existing project, the project may fail to build as paths have changed unexpectedly. If this happens, run ``idf.py fullclean`` (or delete the project build directory) and then build again.
