Espressif ESP32

The ESP32 is a series of single and dual-core SoCs from Espressif based on Harvard architecture Xtensa LX6 CPUs and with on-chip support for Bluetooth and Wi-Fi.

All embedded memory, external memory and peripherals are located on the data bus and/or the instruction bus of these CPUs. With some minor exceptions, the address mapping of two CPUs is symmetric, meaning they use the same addresses to access the same memory. Multiple peripherals in the system can access embedded memory via DMA.

On dual-core SoCs, the two CPUs are typically named “PRO_CPU” and “APP_CPU” (for “protocol” and “application”), however for most purposes the two CPUs are interchangeable.

Toolchain

You can use the prebuilt toolchain for Xtensa architecture and OpenOCD for ESP32 by Espressif.

For flashing firmware, you will need to install esptool.py by running:

$ pip install esptool

Building from source

You can also build the toolchain yourself. The steps to build the toolchain with crosstool-NG on Linux are as follows

$ git clone https://github.com/espressif/crosstool-NG.git
$ cd crosstool-NG
$ git checkout esp-2021r1
$ git submodule update --init

$ ./bootstrap && ./configure --enable-local && make

$ ./ct-ng xtensa-esp32-elf
$ ./ct-ng build

$ chmod -R u+w builds/xtensa-esp32-elf

$ export PATH="crosstool-NG/builds/xtensa-esp32-elf/bin:$PATH"

These steps are given in the setup guide in ESP-IDF documentation.

Flashing

Firmware for ESP32 is flashed via the USB/UART interface using the esptool.py tool. It’s a two step process where the first converts the ELF file into a ESP32-compatible binary and the second flashes it to the board. These steps are included into the build system and you can flash your NuttX firmware simply by running:

$ make flash ESPTOOL_PORT=<port>

where <port> is typically /dev/ttyUSB0 or similar. You can change the baudrate by passing ESPTOOL_BAUD.

Bootloader and partitions

ESP32 requires a bootloader to be flashed as well as a set of FLASH partitions. This is only needed the first time (or any time you which to modify either of these). An easy way is to use prebuilt binaries for NuttX from here. In there you will find instructions to rebuild these if necessary. Once you downloaded both binaries, you can flash them by adding an ESPTOOL_BINDIR parameter, pointing to the directory where these binaries were downloaded:

$ make flash ESPTOOL_PORT=<port> ESPTOOL_BINDIR=<dir>

Note

It is recommended that if this is the first time you are using the board with NuttX that you perform a complete SPI FLASH erase.

$ esptool.py erase_flash

Peripheral Support

The following list indicates the state of peripherals’ support in NuttX:

Peripheral

Support

NOTES

ADC

No

AES

Yes

Bluetooth

Yes

CAN/TWAI

Yes

DMA

Yes

eFuse

Yes

Ethernet

Yes

GPIO

Yes

I2C

Yes

I2S

Yes

LED_PWM

Yes

MCPWM

No

Pulse_CNT

No

RMT

No

RNG

Yes

RSA

No

RTC

Yes

SD/MMC

No

SDIO

No

SHA

No

SPI

Yes

SPIFLASH

Yes

SPIRAM

Yes

Timers

Yes

Touch

Yes

UART

Yes

Watchdog

Yes

Wifi

Yes

Memory Map

Address Mapping

BUS TYPE

START

LAST

DESCRIPTION

NOTES

Data

0x3F400000

0x3F7FFFFF

External Memory

Data

0x3F800000 0x3FC00000

0x3FBFFFFF 0x3FEFFFFF

External Memory

Reserved

Data

0x3FF00000

0x3FF7FFFF

Peripheral

Data

0x3FF80000

0x3FFFFFFF

Embedded Memory

Instruction

0x40000000

0x400C1FFF

Embedded Memory

Instruction

0x400C2000

0x40BFFFFF

External Memory

.

0x40C00000

0x4FFFFFFF

Reserved

Data / Instruction

0x50000000

0x50001FFF

Embedded Memory

.

0x50002000

0xFFFFFFFF

Reserved

Embedded Memory

BUS TYPE

START

LAST

DESCRIPTION

NOTES

Data

0x3ff80000

0x3ff81fff

RTC FAST Memory

PRO_CPU Only

.

0x3ff82000

0x3ff8ffff

Reserved

Data

0x3ff90000

0x3ff9ffff

Internal ROM 1

.

0x3ffa0000

0x3ffadfff

Reserved

Data

0x3ffae000

0x3ffdffff

Internal SRAM 2

DMA

Data

0x3ffe0000

0x3fffffff

Internal SRAM 1

DMA

Boundary Address (Embedded)

BUS TYPE

START

LAST

DESCRIPTION

NOTES

Instruction

0x40000000

0x40007fff

Internal ROM 0

Remap

Instruction

0x40008000

0x4005ffff

Internal ROM 0

.

0x40060000

0x4006ffff

Reserved

Instruction

0x40070000

0x4007ffff

Internal SRAM 0

Cache

Instruction

0x40080000

0x4009ffff

Internal SRAM 0

Instruction

0x400a0000

0x400affff

Internal SRAM 1

Instruction

0x400b0000

0x400b7FFF

Internal SRAM 1

Remap

Instruction

0x400b8000

0x400bffff

Internal SRAM 1

Instruction

0x400c0000

0x400c1FFF

RTC FAST Memory

PRO_CPU Only

Data / Instruction

0x50000000

0x50001fff

RTC SLOW Memory

External Memory

BUS TYPE

START

LAST

DESCRIPTION

NOTES

Data

0x3f400000

0x3f7fffff

External Flash

Read

Data

0x3f800000

0x3fbfffff

External SRAM

Read and Write

Boundary Address (External)

Instruction 0x400c2000 0x40bfffff 11512 KB External Flash Read

Linker Segments

DESCRIPTION

START

END

ATTR

LINKER SEGMENT NAME
FLASH mapped data:

  • .rodata

  • Constructors /destructors

0x3f400010

0x3fc00010

R

drom0_0_seg
COMMON data RAM:

  • .bss/.data

0x3ffb0000

0x40000000

RW

dram0_0_seg (NOTE 1,2,3)
IRAM for PRO cpu:

  • Interrupt Vectors

  • Low level handlers

  • Xtensa/Espressif libraries

0x40080000

0x400a0000

RX

iram0_0_seg
RTC fast memory:

  • .rtc.text (unused?)

0x400c0000

0x400c2000

RWX

rtc_iram_seg (PRO_CPU only)
FLASH:

  • .text

0x400d0018

0x40400018

RX

iram0_2_seg (actually FLASH)
RTC slow memory:

  • .rtc.data/rodata (unused?)

0x50000000

0x50001000

RW

rtc_slow_seg (NOTE 4)

Note

  1. Linker script will reserve space at the beginning of the segment for BT and at the end for trace memory.

  2. Heap ends at the top of dram_0_seg.

  3. Parts of this region is reserved for the ROM bootloader.

  4. Linker script will reserve space at the beginning of the segment for co-processor reserve memory and at the end for ULP coprocessor reserve memory.

64-bit Timers

ESP32 has 4 generic timers of 64 bits (2 from Group 0 and 2 from Group 1). They’re accessible as character drivers, the configuration along with a guidance on how to run the example and the description of the application level interface can be found here.

Watchdog Timers

ESP32 has 3 WDTs. 2 MWDTS from the Timers Module and 1 RWDT from the RTC Module (Currently not supported yet). They’re accessible as character drivers, The configuration along with a guidance on how to run the example and the description of the application level interface can be found here.

SMP

The ESP32 has 2 CPUs. Support is included for testing an SMP configuration. That configuration is still not yet ready for usage but can be enabled with the following configuration settings, in RTOS Features ‣ Tasks and Scheduling, with:

CONFIG_SPINLOCK=y
CONFIG_SMP=y
CONFIG_SMP_NCPUS=2

Debug Tip: During debug session, OpenOCD may mysteriously switch from one CPU to another. This behavior can be eliminated by uncommenting one of the following in scripts/esp32.cfg:

# Only configure the PRO CPU
#set ESP32_ONLYCPU 1
# Only configure the APP CPU
#set ESP32_ONLYCPU 2

Wi-Fi

A standard network interface will be configured and can be initialized such as:

nsh> ifup wlan0
nsh> wapi psk wlan0 mypasswd 3
nsh> wapi essid wlan0 myssid 1
nsh> renew wlan0

In this case a connection to AP with SSID myssid is done, using mypasswd as password. IP address is obtained via DHCP using renew command. You can check the result by running ifconfig afterwards.

Tip

Boards usually expose a wifi defconfig which enables Wi-Fi

Wi-Fi SoftAP

It is possible to use ESP32 as an Access Point (SoftAP). Actually there are some boards config examples called sta_softap which enables this support

If you are using this board config profile you can run these commands to be able to connect your smartphone or laptop to your board:

nsh> ifup wlan1
nsh> dhcpd_start wlan1
nsh> wapi psk wlan1 mypasswd 3
nsh> wapi essid wlan1 nuttxap 1

In this case, you are creating the access point nuttxapp in your board and to connect to it on your smartphone you will be required to type the password mypasswd using WPA2. The dhcpd_start is necessary to let your board to associate an IP to your smartphone.

Bluetooth

These are the steps to test Bluetooth Low Energy (BLE) scan on ESP32 (i.e. Devkit board). First configure to use the BLE board profile:

$ make distclean
$ ./tools/configure.sh esp32-devkitc:ble
$ make flash ESPTOOL_PORT=/dev/ttyUSB0

Enter in the NSH shell using your preferred serial console tool and run the scan command:

NuttShell (NSH) NuttX-10.2.0
nsh> ifconfig
bnep0   Link encap:UNSPEC at DOWN
        inet addr:0.0.0.0 DRaddr:0.0.0.0 Mask:0.0.0.0

wlan0   Link encap:Ethernet HWaddr ac:67:b2:53:8b:ec at UP
        inet addr:10.0.0.2 DRaddr:10.0.0.1 Mask:255.255.255.0

nsh> bt bnep0 scan start
nsh> bt bnep0 scan stop
nsh> bt bnep0 scan get
Scan result:
1.     addr:           63:14:2f:b9:9f:83 type: 1
       rssi:            -90
       response type:   3
       advertiser data: 1e ff 06 00 01 09 20 02 7c 33 a3 a7 cd c9 44 5b
2.     addr:           52:ca:05:b5:ad:77 type: 1
       rssi:            -82
       response type:   3
       advertiser data: 1e ff 06 00 01 09 20 02 03 d1 21 57 bf 19 b3 7a
3.     addr:           46:8e:b2:cd:94:27 type: 1
       rssi:            -92
       response type:   2
       advertiser data: 02 01 1a 09 ff c4 00 10 33 14 12 16 80 02 0a d4
4.     addr:           46:8e:b2:cd:94:27 type: 1
       rssi:            -92
       response type:   4
       advertiser data: 18 09 5b 4c 47 5d 20 77 65 62 4f 53 20 54 56 20
5.     addr:           63:14:2f:b9:9f:83 type: 1
       rssi:            -80
       response type:   3
    advertiser data: 1e ff 06 00 01 09 20 02 7c 33 a3 a7 cd c9 44 5b
nsh>

I2S

The I2S peripheral is accessible using either the generic I2S audio driver or a specific audio codec driver. Also, it’s possible to use the I2S character driver to bypass the audio subsystem and develop specific usages of the I2S peripheral.

Note

Note that the bit-width and sample rate can be modified “on-the-go” when using audio-related drivers. That is not the case for the I2S character device driver and such parameters are set on compile time through make menuconfig.

Warning

Some upper driver implementations might not handle both transmission and reception configured at the same time on the same peripheral.

Please check for usage examples using the ESP32 DevKitC.

Using QEMU

First follow the instructions here to build QEMU.

Enable the ESP32_QEMU_IMAGE config found in Board Selection ‣ ESP32 binary image for QEMU.

Download the bootloader and the partition table from https://github.com/espressif/esp-nuttx-bootloader/releases and place them in a directory, say ../esp-bins.

Build and generate the QEMU image:

$ make ESPTOOL_BINDIR=../esp-bins

A QEMU-compatible nuttx.merged.bin binary image will be created. It can be run as:

$ qemu-system-xtensa -nographic -machine esp32 -drive file=nuttx.merged.bin,if=mtd,format=raw

Secure Boot and Flash Encryption

Secure Boot

Secure Boot protects a device from running any unauthorized (i.e., unsigned) code by checking that each piece of software that is being booted is signed. On an ESP32, these pieces of software include the second stage bootloader and each application binary. Note that the first stage bootloader does not require signing as it is ROM code thus cannot be changed. This is achieved using specific hardware in conjunction with MCUboot (read more about MCUboot here).

The Secure Boot process on the ESP32 involves the following steps performed:

  1. The first stage bootloader verifies the second stage bootloader’s RSA-PSS signature. If the verification is successful, the first stage bootloader loads and executes the second stage bootloader.

  2. When the second stage bootloader loads a particular application image, the application’s signature (RSA, ECDSA or ED25519) is verified by MCUboot. If the verification is successful, the application image is executed.

Warning

Once enabled, Secure Boot will not boot a modified bootloader. The bootloader will only boot an application firmware image if it has a verified digital signature. There are implications for reflashing updated images once Secure Boot is enabled. You can find more information about the ESP32’s Secure boot here.

Note

As the bootloader image is built on top of the Hardware Abstraction Layer component of ESP-IDF, the API port by Espressif will be used by MCUboot rather than the original NuttX port.

Flash Encryption

Flash encryption is intended for encrypting the contents of the ESP32’s off-chip flash memory. Once this feature is enabled, firmware is flashed as plaintext, and then the data is encrypted in place on the first boot. As a result, physical readout of flash will not be sufficient to recover most flash contents.

Warning

After enabling Flash Encryption, an encryption key is generated internally by the device and cannot be accessed by the user for re-encrypting data and re-flashing the system, hence it will be permanently encrypted. Re-flashing an encrypted system is complicated and not always possible. You can find more information about the ESP32’s Flash Encryption here.

Prerequisites

First of all, we need to install imgtool (a MCUboot utility application to manipulate binary images) and esptool (the ESP32 toolkit):

$ pip install imgtool esptool

We also need to make sure that the python modules are added to PATH:

$ echo "PATH=$PATH:/home/$USER/.local/bin" >> ~/.bashrc

Now, we will create a folder to store the generated keys (such as ~/signing_keys):

$ mkdir ~/signing_keys && cd ~/signing_keys

With all set up, we can now generate keys to sign the bootloader and application binary images, respectively, of the compiled project:

$ espsecure.py generate_signing_key --version 2 bootloader_signing_key.pem
$ imgtool keygen --key app_signing_key.pem --type rsa-3072

Important

The contents of the key files must be stored securely and kept secret.

Enabling Secure Boot and Flash Encryption

To enable Secure Boot for the current project, go to the project’s NuttX directory, execute make menuconfig and the following steps:

  1. Enable experimental features in Build Setup ‣ Show experimental options;

  2. Enable MCUboot in Application Configuration ‣ Bootloader Utilities ‣ MCUboot;

  3. Change image type to MCUboot-bootable format in System Type ‣ Application Image Configuration ‣ Application Image Format;

  4. Enable building MCUboot from the source code by selecting Build binaries from source; in System Type ‣ Application Image Configuration ‣ Source for bootloader binaries;

  5. Enable Secure Boot in System Type ‣ Application Image Configuration ‣ Enable hardware Secure Boot in bootloader;

  6. If you want to protect the SPI Bus against data sniffing, you can enable Flash Encryption in System Type ‣ Application Image Configuration ‣ Enable Flash Encryption on boot.

Now you can design an update and confirm agent to your application. Check the MCUboot design guide and the MCUboot Espressif port documentation for more information on how to apply MCUboot. Also check some notes about the NuttX MCUboot port, the MCUboot porting guide and some examples of MCUboot applied in Nuttx applications.

After you developed an application which implements all desired functions, you need to flash it into the primary image slot of the device (it will automatically be in the confirmed state, you can learn more about image confirmation here). To flash to the primary image slot, select Application image primary slot in System Type ‣ Application Image Configuration ‣ Target slot for image flashing and compile it using make -j ESPSEC_KEYDIR=~/signing_keys.

When creating update images, make sure to change System Type ‣ Application Image Configuration ‣ Target slot for image flashing to Application image secondary slot.

Important

When deploying your application, make sure to disable UART Download Mode by selecting Permanently disabled in System Type ‣ Application Image Configuration ‣ UART ROM download mode and change usage mode to Release in System Type –> Application Image Configuration –> Enable usage mode. After disabling UART Download Mode you will not be able to flash other images through UART.

Things to Do

  1. Lazy co-processor save logic supported by Xtensa. That logic works like this:

    1. CPENABLE is set to zero on each context switch, disabling all co- processors.

    2. If/when the task attempts to use the disabled co-processor, an exception occurs

    3. The co-processor exception handler re-enables the co-processor.

    Instead, the NuttX logic saves and restores CPENABLE on each context switch. This has disadvantages in that (1) co-processor context will be saved and restored even if the co-processor was never used, and (2) tasks must explicitly enable and disable co-processors.

  2. Currently the Xtensa port copies register state save information from the stack into the TCB. A more efficient alternative would be to just save a pointer to a register state save area in the TCB. This would add some complexity to signal handling and also to up_initialstate(). But the performance improvement might be worth the effort.

  3. See SMP-related issues above

Supported Boards