Partition Tables

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Overview

A single ESP32’s flash can contain multiple apps, as well as many different kinds of data (calibration data, filesystems, parameter storage, etc). For this reason a partition table is flashed to (default offset) 0x8000 in the flash.

Partition table length is 0xC00 bytes (maximum 95 partition table entries). An MD5 checksum, which is used for checking the integrity of the partition table, is appended after the table data. If the partition table is signed due to secure boot, the signature is appended after the partition table.

Each entry in the partition table has a name (label), type (app, data, or something else), subtype and the offset in flash where the partition is loaded.

The simplest way to use the partition table is to make menuconfig and choose one of the simple predefined partition tables:

  • “Single factory app, no OTA”

  • “Factory app, two OTA definitions”

In both cases the factory app is flashed at offset 0x10000. If you make partition_table then it will print a summary of the partition table.

Built-in Partition Tables

Here is the summary printed for the “Single factory app, no OTA” configuration:

# Espressif ESP32 Partition Table
# Name,   Type, SubType, Offset,  Size, Flags
nvs,      data, nvs,     0x9000,  0x6000,
phy_init, data, phy,     0xf000,  0x1000,
factory,  app,  factory, 0x10000, 1M,

  • At a 0x10000 (64KB) offset in the flash is the app labelled “factory”. The bootloader will run this app by default.

  • There are also two data regions defined in the partition table for storing NVS library partition and PHY init data.

Here is the summary printed for the “Factory app, two OTA definitions” configuration:

# Espressif ESP32 Partition Table
# Name,   Type, SubType, Offset,  Size, Flags
nvs,      data, nvs,     0x9000,  0x4000,
otadata,  data, ota,     0xd000,  0x2000,
phy_init, data, phy,     0xf000,  0x1000,
factory,  0,    0,       0x10000, 1M,
ota_0,    0,    ota_0,  0x110000, 1M,
ota_1,    0,    ota_1,  0x210000, 1M,

  • There are now three app partition definitions. The type of the factory app (at 0x10000) and the next two “OTA” apps are all set to “app”, but their subtypes are different.

  • There is also a new “otadata” slot, which holds the data for OTA updates. The bootloader consults this data in order to know which app to execute. If “ota data” is empty, it will execute the factory app.

Creating Custom Tables

If you choose “Custom partition table CSV” in menuconfig then you can also enter the name of a CSV file (in the project directory) to use for your partition table. The CSV file can describe any number of definitions for the table you need.

The CSV format is the same format as printed in the summaries shown above. However, not all fields are required in the CSV. For example, here is the “input” CSV for the OTA partition table:

# Name,   Type, SubType,  Offset,   Size,  Flags
nvs,      data, nvs,      0x9000,  0x4000
otadata,  data, ota,      0xd000,  0x2000
phy_init, data, phy,      0xf000,  0x1000
factory,  app,  factory,  0x10000,  1M
ota_0,    app,  ota_0,    ,         1M
ota_1,    app,  ota_1,    ,         1M
nvs_key,  data, nvs_keys, ,        0x1000

  • Whitespace between fields is ignored, and so is any line starting with # (comments).

  • Each non-comment line in the CSV file is a partition definition.

  • The “Offset” field for each partition is empty. The gen_esp32part.py tool fills in each blank offset, starting after the partition table and making sure each partition is aligned correctly.

Name field

Name field can be any meaningful name. It is not significant to the ESP32. Names longer than 16 characters will be truncated.

Type field

Partition type field can be specified as app (0) or data (1). Or it can be a number 0-254 (or as hex 0x00-0xFE). Types 0x00-0x3F are reserved for esp-idf core functions.

If your application needs to store data, please add a custom partition type in the range 0x40-0xFE.

The bootloader ignores any partition types other than app (0) & data (1).

SubType

The 8-bit subtype field is specific to a given partition type. esp-idf currently only specifies the meaning of the subtype field for “app” and “data” partition types.

  • When type is “app”, the subtype field can be specified as factory (0), ota_0 (0x10) … ota_15 (0x1F) or test (0x20).

    • factory (0) is the default app partition. The bootloader will execute the factory app unless there it sees a partition of type data/ota, in which case it reads this partition to determine which OTA image to boot.

      • OTA never updates the factory partition.

      • If you want to conserve flash usage in an OTA project, you can remove the factory partition and use ota_0 instead.

    • ota_0 (0x10) … ota_15 (0x1F) are the OTA app slots. Refer to the OTA documentation for more details, which then use the OTA data partition to configure which app slot the bootloader should boot. If using OTA, an application should have at least two OTA application slots (ota_0 & ota_1). Refer to the OTA documentation for more details.

    • test (0x20) is a reserved subtype for factory test procedures. It will be used as the fallback boot partition if no other valid app partition is found. It is also possible to configure the bootloader to read a GPIO input during each boot, and boot this partition if the GPIO is held low, see Boot from TEST firmware.

  • When type is “data”, the subtype field can be specified as ota (0), phy (1), nvs (2), or nvs_keys (4).

    • ota (0) is the OTA data partition which stores information about the currently selected OTA application. This partition should be 0x2000 bytes in size. Refer to the OTA documentation for more details.

    • phy (1) is for storing PHY initialisation data. This allows PHY to be configured per-device, instead of in firmware.

      • In the default configuration, the phy partition is not used and PHY initialisation data is compiled into the app itself. As such, this partition can be removed from the partition table to save space.

      • To load PHY data from this partition, run make menuconfig and enable CONFIG_ESP32_PHY_INIT_DATA_IN_PARTITION option. You will also need to flash your devices with phy init data as the esp-idf build system does not do this automatically.

    • nvs (2) is for the Non-Volatile Storage (NVS) API.

      • NVS is used to store per-device PHY calibration data (different to initialisation data).

      • NVS is used to store WiFi data if the esp_wifi_set_storage(WIFI_STORAGE_FLASH) initialisation function is used.

      • The NVS API can also be used for other application data.

      • It is strongly recommended that you include an NVS partition of at least 0x3000 bytes in your project.

      • If using NVS API to store a lot of data, increase the NVS partition size from the default 0x6000 bytes.

    • nvs_keys (4) is for the NVS key partition. See Non-Volatile Storage (NVS) API for more details.

      • It is used to store NVS encryption keys when NVS Encryption feature is enabled.

      • The size of this partition should be 4096 bytes (minimum partition size).

Other data subtypes are reserved for future esp-idf uses.

Offset & Size

Partitions with blank offsets will start after the previous partition, or after the partition table in the case of the first partition.

App partitions have to be at offsets aligned to 0x10000 (64K). If you leave the offset field blank, gen_esp32part.py will automatically align the partition. If you specify an unaligned offset for an app partition, the tool will return an error.

Sizes and offsets can be specified as decimal numbers, hex numbers with the prefix 0x, or size multipliers K or M (1024 and 1024*1024 bytes).

If you want the partitions in the partition table to work with any starting offset (CONFIG_PARTITION_TABLE_OFFSET) of the table itself, leave the offset field (in CSV file) for all partitions blank. Similarly, if changing the partition table offset then be aware that all blank partition offsets may change to match, and that any fixed offsets may now collide with the partition table (causing an error).

Flags

Only one flag is currently supported, encrypted. If this field is set to encrypted, this partition will be encrypted if Flash Encryption is enabled.

(Note that app type partitions will always be encrypted, regardless of whether this flag is set or not.)

Generating Binary Partition Table

The partition table which is flashed to the ESP32 is in a binary format, not CSV. The tool partition_table/gen_esp32part.py is used to convert between CSV and binary formats.

If you configure the partition table CSV name in make menuconfig and then make partition_table, this conversion is done as part of the build process.

To convert CSV to Binary manually:

python gen_esp32part.py input_partitions.csv binary_partitions.bin

To convert binary format back to CSV manually:

python gen_esp32part.py binary_partitions.bin input_partitions.csv

To display the contents of a binary partition table on stdout (this is how the summaries displayed when running make partition_table are generated:

python gen_esp32part.py binary_partitions.bin

MD5 checksum

The binary format of the partition table contains an MD5 checksum computed based on the partition table. This checksum is used for checking the integrity of the partition table during the boot.

The MD5 checksum generation can be disabled by the --disable-md5sum option of gen_esp32part.py or by the CONFIG_PARTITION_TABLE_MD5 option. This is useful for example when one uses a bootloader from ESP-IDF before v3.1 which cannot process MD5 checksums and the boot fails with the error message invalid magic number 0xebeb.

Flashing the partition table

  • make partition_table-flash: will flash the partition table with esptool.py.

  • make flash: Will flash everything including the partition table.

A manual flashing command is also printed as part of make partition_table.

Note that updating the partition table doesn’t erase data that may have been stored according to the old partition table. You can use make erase_flash (or esptool.py erase_flash) to erase the entire flash contents.