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.

The partition table length is 0xC00 bytes, as we allow a maximum of 95 entries. An MD5 checksum, used for checking the integrity of the partition table at runtime, is appended after the table data. Thus, the partition table occupies an entire flash sector, which size is 0x1000 (4 KB). As a result, any partition following it must be at least located at (default offset) + 0x1000.

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 open the project configuration menu (idf.py menuconfig) and choose one of the simple predefined partition tables under CONFIG_PARTITION_TABLE_TYPE:

  • "Single factory app, no OTA"

  • "Factory app, two OTA definitions"

In both cases the factory app is flashed at offset 0x10000. If you execute idf.py 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:

# ESP-IDF 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 (64 KB) offset in the flash is the app labelled "factory". The bootloader runs 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:

# ESP-IDF 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,   0x110000, 1M,
ota_1,    app,  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. The maximum length of names is 16 bytes, including one null terminator. Names longer than the maximum length will be truncated.

Type Field

Partition type field can be specified as a name or a number 0-254 (or as hex 0x00-0xFE). Types 0x00-0x3F are reserved for ESP-IDF core functions.

  • app (0x00),

  • data (0x01),

  • bootloader (0x02). By default, this partition is not included in any CSV partition table files because it is not required and does not impact the system's functionality. It is only useful for the bootloader OTA update. Even if this partition is not present in the CSV file, it is still possible to perform the OTA. Please note that if you specify this partition in the CSV file, its address and size must match Kconfigs,

  • partition_table (0x03),

  • 0x40-0xFE are reserved for custom partition types. If your app needs to store data in a format not already supported by ESP-IDF, then use a value from this range.

See esp_partition_type_t for the enum definitions for app and data partitions.

If writing in C++ then specifying a application-defined partition type requires casting an integer to esp_partition_type_t in order to use it with the partition API. For example:

static const esp_partition_type_t APP_PARTITION_TYPE_A = (esp_partition_type_t)0x40;

The bootloader ignores any partition types other than app (0x00) and data (0x01).

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.

See enum esp_partition_subtype_t for the full list of subtypes defined by ESP-IDF, including the following:

  • When type is app, the SubType field can be specified as factory (0x00), ota_0 (0x10) ... ota_15 (0x1F) or test (0x20).

    • factory (0x00) 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. When OTA is in use, the OTA data partition configures which app slot the bootloader should boot. When 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 bootloader, the SubType field can be specified as:

    • primary (0x00). It is the so-called 2nd stage bootloader, which is placed at the 0x1000 address in the flash. The gen_esp32part.py does not allow to have this partition in the CSV file for now.

    • ota (0x01). It is a temporary bootloader partition used by the bootloader OTA update functionality for downloading a new image.

  • When type is partition_table, the SubType field can be specified as:

    • primary (0x00). It is the primary partition table, which is placed at the CONFIG_PARTITION_TABLE_OFFSET address in the flash. The gen_esp32part.py does not allow to have this partition in the CSV file for now.

    • ota (0x01). It is a temporary partition table partition used by the partition table OTA update functionality for downloading a new image.

  • When type is data, the subtype field can be specified as ota (0x00), phy (0x01), nvs (0x02), nvs_keys (0x04), or a range of other component-specific subtypes (see subtype enum).

    • ota (0) is the OTA data partition which stores information about the currently selected OTA app slot. 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, open the project configuration menu (idf.py menuconfig) and enable CONFIG_ESP_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).

      • 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).

    • There are other predefined data subtypes for data storage supported by ESP-IDF. These include:

      • coredump (0x03) is for storing core dumps while using a custom partition table CSV file. See Core Dump for more details.

      • efuse (0x05) is for emulating eFuse bits using Virtual eFuses.

      • undefined (0x06) is implicitly used for data partitions with unspecified (empty) subtype, but it is possible to explicitly mark them as undefined as well.

      • fat (0x81) is for FAT Filesystem Support.

      • spiffs (0x82) is for SPIFFS Filesystem.

      • littlefs (0x83) is for LittleFS filesystem. See storage/littlefs example for more details.

  • If the partition type is any application-defined value (range 0x40-0xFE), then subtype field can be any value chosen by the application (range 0x00-0xFE).

    Note that when writing in C++, an application-defined subtype value requires casting to type esp_partition_subtype_t in order to use it with the partition API.

Extra Partition SubTypes

A component can define a new partition subtype by setting the EXTRA_PARTITION_SUBTYPES property. This property is a CMake list, each entry of which is a comma separated string with <type>, <subtype>, <value> format. The build system uses this property to add extra subtypes and creates fields named ESP_PARTITION_SUBTYPE_<type>_<subtype> in esp_partition_subtype_t. The project can use this subtype to define partitions in the partitions table CSV file and use the new fields in esp_partition_subtype_t.

Offset & Size

  • The offset represents the partition address in the SPI flash, which sector size is 0x1000 (4 KB). Thus, the offset must be a multiple of 4 KB.

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

  • Partitions of type app have to be placed at offsets aligned to 0x10000 (64 KB). 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.

  • Partitions of type app should have the size aligned to the flash sector size (4 KB). If you specify an unaligned size for an app partition, the tool will return an error.

  • If Secure Boot V1 is enabled, then the partition of type app needs to have size aligned to 0x10000 (64 KB) boundary.

  • 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 relative to any placement (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

Two flags are currently supported, encrypted and readonly:

  • If encrypted flag is set, the partition will be encrypted if Flash Encryption is enabled.

Note

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

  • If readonly flag is set, the partition will be read-only. This flag is only supported for data type partitions except ota` and coredump` subtypes. This flag can help to protect against accidental writes to a partition that contains critical device-specific configuration data, e.g., factory data partition.

Note

Using C file I/O API to open a file (fopen`) in any write mode (w, w+, a, a+, r+) will fail and return NULL. Using open with any other flag than O_RDONLY will fail and return -1 while errno global variable will be set to EROFS. This is also true for any other POSIX syscall function performing write or erase operations. Opening a handle in read-write mode for NVS on a read-only partition will fail and return ESP_ERR_NOT_ALLOWED error code. Using a lower level API like esp_partition, spi_flash, etc. to write to a read-only partition will result in ESP_ERR_NOT_ALLOWED error code.

You can specify multiple flags by separating them with a colon. For example, encrypted:readonly.

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 the project configuration (idf.py menuconfig) and then build the project or run idf.py 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 idf.py partition-table are generated:

python gen_esp32part.py binary_partitions.bin

Partition Size Checks

The ESP-IDF build system will automatically check if generated binaries fit in the available partition space, and will fail with an error if a binary is too large.

Currently these checks are performed for the following binaries:

  • Bootloader binary must fit in space before partition table (see Bootloader Size).

  • App binary should fit in at least one partition of type "app". If the app binary does not fit in any app partition, the build will fail. If it only fits in some of the app partitions, a warning is printed about this.

Note

Although the build process will fail if the size check returns an error, the binary files are still generated and can be flashed (although they may not work if they are too large for the available space.)

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

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

  • idf.py flash: Will flash everything including the partition table.

A manual flashing command is also printed as part of idf.py partition-table output.

Note

Note that updating the partition table does not erase data that may have been stored according to the old partition table. You can use idf.py erase-flash (or esptool.py erase_flash) to erase the entire flash contents.

Partition Tool (parttool.py)

The component partition_table provides a tool parttool.py for performing partition-related operations on a target device. The following operations can be performed using the tool:

  • reading a partition and saving the contents to a file (read_partition)

  • writing the contents of a file to a partition (write_partition)

  • erasing a partition (erase_partition)

  • retrieving info such as name, offset, size and flag ("encrypted") of a given partition (get_partition_info)

The tool can either be imported and used from another Python script or invoked from shell script for users wanting to perform operation programmatically. This is facilitated by the tool's Python API and command-line interface, respectively.

Python API

Before anything else, make sure that the parttool module is imported.

import sys
import os

idf_path = os.environ["IDF_PATH"]  # get value of IDF_PATH from environment
parttool_dir = os.path.join(idf_path, "components", "partition_table")  # parttool.py lives in $IDF_PATH/components/partition_table

sys.path.append(parttool_dir)  # this enables Python to find parttool module
from parttool import *  # import all names inside parttool module

The starting point for using the tool's Python API to do is create a ParttoolTarget object:

# Create a parttool.py target device connected on serial port /dev/ttyUSB1
target = ParttoolTarget("/dev/ttyUSB1")

The created object can now be used to perform operations on the target device:

# Erase partition with name 'storage'
target.erase_partition(PartitionName("storage"))

# Read partition with type 'data' and subtype 'spiffs' and save to file 'spiffs.bin'
target.read_partition(PartitionType("data", "spiffs"), "spiffs.bin")

# Write to partition 'factory' the contents of a file named 'factory.bin'
target.write_partition(PartitionName("factory"), "factory.bin")

# Print the size of default boot partition
storage = target.get_partition_info(PARTITION_BOOT_DEFAULT)
print(storage.size)

The partition to operate on is specified using PartitionName or PartitionType or PARTITION_BOOT_DEFAULT. As the name implies, these can be used to refer to partitions of a particular name, type-subtype combination, or the default boot partition.

More information on the Python API is available in the docstrings for the tool.

Command-line Interface

The command-line interface of parttool.py has the following structure:

parttool.py [command-args] [subcommand] [subcommand-args]

- command-args - These are arguments that are needed for executing the main command (parttool.py), mostly pertaining to the target device
- subcommand - This is the operation to be performed
- subcommand-args - These are arguments that are specific to the chosen operation

# Erase partition with name 'storage'
parttool.py --port "/dev/ttyUSB1" erase_partition --partition-name=storage

# Read partition with type 'data' and subtype 'spiffs' and save to file 'spiffs.bin'
parttool.py --port "/dev/ttyUSB1" read_partition --partition-type=data --partition-subtype=spiffs --output "spiffs.bin"

# Write to partition 'factory' the contents of a file named 'factory.bin'
parttool.py --port "/dev/ttyUSB1" write_partition --partition-name=factory --input "factory.bin"

# Print the size of default boot partition
parttool.py --port "/dev/ttyUSB1" get_partition_info --partition-boot-default --info size

Note

If the device has already enabled Flash Encryption or Secure Boot, attempting to use commands that modify the flash content, such as erase_partition or write_partition, will result in an error. This error is generated by the erase command of esptool.py, which is called first before writing. This error is done as a safety measure to prevent bricking your device.

A fatal error occurred: Active security features detected, erasing flash is disabled as a safety measure. Use --force to override, please use with caution, otherwise it may brick your device!

To work around this, you need use the --force flag with esptool.py. Specifically, the parttool.py provides the --esptool-erase-args argument that help to pass this flag to esptool.py.

# Erase partition with name 'storage'
# If Flash Encryption or Secure Boot are enabled then add "--esptool-erase-args=force"
parttool.py --port "/dev/ttyUSB1" --esptool-erase-args=force erase_partition --partition-name=storage

# Write to partition 'factory' the contents of a file named 'factory.bin'
# If Flash Encryption or Secure Boot are enabled then add "--esptool-erase-args=force"
parttool.py --port "/dev/ttyUSB1" --esptool-erase-args=force write_partition --partition-name=factory --input "factory.bin"

More information can be obtained by specifying --help as argument:

# Display possible subcommands and show main command argument descriptions
parttool.py --help

# Show descriptions for specific subcommand arguments
parttool.py [subcommand] --help

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