Over The Air Updates (OTA)

OTA Process Overview

The OTA update mechanism allows a device to update itself based on data received while the normal firmware is running (for example, over WiFi or Bluetooth.)

OTA requires configuring the Partition Table of the device with at least two “OTA app slot” partitions (ie ota_0 and ota_1) and an “OTA Data Partition”.

The OTA operation functions write a new app firmware image to whichever OTA app slot is not currently being used for booting. Once the image is verified, the OTA Data partition is updated to specify that this image should be used for the next boot.

OTA Data Partition

An OTA data partition (type data, subtype ota) must be included in the Partition Table of any project which uses the OTA functions.

For factory boot settings, the OTA data partition should contain no data (all bytes erased to 0xFF). In this case the esp-idf software bootloader will boot the factory app if it is present in the the partition table. If no factory app is included in the partition table, the first available OTA slot (usually ota_0) is booted.

After the first OTA update, the OTA data partition is updated to specify which OTA app slot partition should be booted next.

The OTA data partition is two flash sectors (0x2000 bytes) in size, to prevent problems if there is a power failure while it is being written. Sectors are independently erased and written with matching data, and if they disagree a counter field is used to determine which sector was written more recently.

Secure OTA Updates Without Secure boot

The verification of signed OTA updates can be performed even without enabling hardware secure boot. For doing so, refer Signed App Verification Without Hardware Secure Boot

Application Example

End-to-end example of OTA firmware update workflow: system/ota.

API Reference


esp_err_t esp_ota_begin(const esp_partition_t *partition, size_t image_size, esp_ota_handle_t *out_handle)

Commence an OTA update writing to the specified partition.

The specified partition is erased to the specified image size.

If image size is not yet known, pass OTA_SIZE_UNKNOWN which will cause the entire partition to be erased.

On success, this function allocates memory that remains in use until esp_ota_end() is called with the returned handle.

  • ESP_OK: OTA operation commenced successfully.
  • ESP_ERR_INVALID_ARG: partition or out_handle arguments were NULL, or partition doesn’t point to an OTA app partition.
  • ESP_ERR_NO_MEM: Cannot allocate memory for OTA operation.
  • ESP_ERR_OTA_PARTITION_CONFLICT: Partition holds the currently running firmware, cannot update in place.
  • ESP_ERR_NOT_FOUND: Partition argument not found in partition table.
  • ESP_ERR_OTA_SELECT_INFO_INVALID: The OTA data partition contains invalid data.
  • ESP_ERR_INVALID_SIZE: Partition doesn’t fit in configured flash size.
  • partition: Pointer to info for partition which will receive the OTA update. Required.
  • image_size: Size of new OTA app image. Partition will be erased in order to receive this size of image. If 0 or OTA_SIZE_UNKNOWN, the entire partition is erased.
  • out_handle: On success, returns a handle which should be used for subsequent esp_ota_write() and esp_ota_end() calls.

esp_err_t esp_ota_write(esp_ota_handle_t handle, const void *data, size_t size)

Write OTA update data to partition.

This function can be called multiple times as data is received during the OTA operation. Data is written sequentially to the partition.

  • ESP_OK: Data was written to flash successfully.
  • ESP_ERR_INVALID_ARG: handle is invalid.
  • ESP_ERR_OTA_VALIDATE_FAILED: First byte of image contains invalid app image magic byte.
  • ESP_ERR_OTA_SELECT_INFO_INVALID: OTA data partition has invalid contents
  • handle: Handle obtained from esp_ota_begin
  • data: Data buffer to write
  • size: Size of data buffer in bytes.

esp_err_t esp_ota_end(esp_ota_handle_t handle)

Finish OTA update and validate newly written app image.

After calling esp_ota_end(), the handle is no longer valid and any memory associated with it is freed (regardless of result).
  • ESP_OK: Newly written OTA app image is valid.
  • ESP_ERR_NOT_FOUND: OTA handle was not found.
  • ESP_ERR_INVALID_ARG: Handle was never written to.
  • ESP_ERR_OTA_VALIDATE_FAILED: OTA image is invalid (either not a valid app image, or - if secure boot is enabled - signature failed to verify.)
  • ESP_ERR_INVALID_STATE: If flash encryption is enabled, this result indicates an internal error writing the final encrypted bytes to flash.
  • handle: Handle obtained from esp_ota_begin().

esp_err_t esp_ota_set_boot_partition(const esp_partition_t *partition)

Configure OTA data for a new boot partition.

If this function returns ESP_OK, calling esp_restart() will boot the newly configured app partition.
  • ESP_OK: OTA data updated, next reboot will use specified partition.
  • ESP_ERR_INVALID_ARG: partition argument was NULL or didn’t point to a valid OTA partition of type “app”.
  • ESP_ERR_OTA_VALIDATE_FAILED: Partition contained invalid app image. Also returned if secure boot is enabled and signature validation failed.
  • ESP_ERR_NOT_FOUND: OTA data partition not found.
  • ESP_ERR_FLASH_OP_TIMEOUT or ESP_ERR_FLASH_OP_FAIL: Flash erase or write failed.
  • partition: Pointer to info for partition containing app image to boot.

const esp_partition_t *esp_ota_get_boot_partition(void)

Get partition info of currently configured boot app.

If esp_ota_set_boot_partition() has been called, the partition which was set by that function will be returned.

If esp_ota_set_boot_partition() has not been called, the result is usually the same as esp_ota_get_running_partition(). The two results are not equal if the configured boot partition does not contain a valid app (meaning that the running partition will be an app that the bootloader chose via fallback).

If the OTA data partition is not present or not valid then the result is the first app partition found in the partition table. In priority order, this means: the factory app, the first OTA app slot, or the test app partition.

Note that there is no guarantee the returned partition is a valid app. Use esp_image_verify(ESP_IMAGE_VERIFY, …) to verify if the returned partition contains a bootable image.

Pointer to info for partition structure, or NULL if partition table is invalid or a flash read operation failed. Any returned pointer is valid for the lifetime of the application.

const esp_partition_t *esp_ota_get_running_partition(void)

Get partition info of currently running app.

This function is different to esp_ota_get_boot_partition() in that it ignores any change of selected boot partition caused by esp_ota_set_boot_partition(). Only the app whose code is currently running will have its partition information returned.

The partition returned by this function may also differ from esp_ota_get_boot_partition() if the configured boot partition is somehow invalid, and the bootloader fell back to a different app partition at boot.

Pointer to info for partition structure, or NULL if no partition is found or flash read operation failed. Returned pointer is valid for the lifetime of the application.

const esp_partition_t *esp_ota_get_next_update_partition(const esp_partition_t *start_from)

Return the next OTA app partition which should be written with a new firmware.

Call this function to find an OTA app partition which can be passed to esp_ota_begin().

Finds next partition round-robin, starting from the current running partition.

Pointer to info for partition which should be updated next. NULL result indicates invalid OTA data partition, or that no eligible OTA app slot partition was found.
  • start_from: If set, treat this partition info as describing the current running partition. Can be NULL, in which case esp_ota_get_running_partition() is used to find the currently running partition. The result of this function is never the same as this argument.



Used for esp_ota_begin() if new image size is unknown


Base error code for ota_ops api


Error if request was to write or erase the current running partition


Error if OTA data partition contains invalid content


Error if OTA app image is invalid

Type Definitions

typedef uint32_t esp_ota_handle_t

Opaque handle for an application OTA update.

esp_ota_begin() returns a handle which is then used for subsequent calls to esp_ota_write() and esp_ota_end().