ESP32

Overview

ESP32 is a series of low cost, low power system on a chip microcontrollers with integrated Wi-Fi & dual-mode Bluetooth. The ESP32 series employs a Tensilica Xtensa LX6 microprocessor in both dual-core and single-core variations. ESP32 is created and developed by Espressif Systems, a Shanghai-based Chinese company, and is manufactured by TSMC using their 40nm process. [1]

The features include the following:

  • Dual core Xtensa microprocessor (LX6), running at 160 or 240MHz
  • 520KB of SRAM
  • 802.11b/g/n/e/i
  • Bluetooth v4.2 BR/EDR and BLE
  • Various peripherals:
    • 12-bit ADC with up to 18 channels
    • 2x 8-bit DACs
    • 10x touch sensors
    • Temperature sensor
    • 4x SPI
    • 2x I2S
    • 2x I2C
    • 3x UART
    • SD/SDIO/MMC host
    • Slave (SDIO/SPI)
    • Ethernet MAC
    • CAN bus 2.0
    • IR (RX/TX)
    • Motor PWM
    • LED PWM with up to 16 channels
    • Hall effect sensor
  • Cryptographic hardware acceleration (RNG, ECC, RSA, SHA-2, AES)
  • 5uA deep sleep current

System requirements

Prerequisites

Two components are required in order to build this port: the toolchain and the SDK. Both are provided by the SoC manufacturer.

The SDK contains headers and a hardware abstraction layer library (provided only as object files) that are required for the port to function.

The toolchain is available for Linux, Windows, and Mac hosts and instructions to obtain and set them up are available in the ESP-IDF repository, using the toolchain and SDK links above.

Set up build environment

With both the toolchain and SDK installed, the Zephyr build system must be instructed to use this particular variant by setting the ZEPHYR_TOOLCHAIN_VARIANT shell variable. One more other environment variables should also be set, pointing to where the toolchain has been installed:

export ZEPHYR_TOOLCHAIN_VARIANT="espressif"
export ESPRESSIF_TOOLCHAIN_PATH="/path/to/xtensa-esp32-elf/"

Flashing

The usual flash target will work with the esp32 board configuration. Here is an example for the Hello World application.

Using west:

# From the root of the zephyr repository
west flash

Using CMake and ninja:

# From the root of the zephyr repository
# Use cmake to configure a Ninja-based buildsystem:
cmake -B build -GNinja -DBOARD=esp32 samples/hello_world

# Now run ninja on the generated build system:
ninja -C build flash

Refer to Build an Application and Run an Application for more details.

It’s impossible to determine which serial port the ESP32 board is connected to, as it uses a generic RS232-USB converter. The default of /dev/ttyUSB0 is provided as that’s often the assigned name on a Linux machine without any other such converters.

The baud rate of 921600bps is recommended. If experiencing issues when flashing, try halving the value a few times (460800, 230400, 115200, etc). It might be necessary to change the flash frequency or the flash mode; please refer to the esptool documentation for guidance on these settings.

All flashing options are now handled by the West (Zephyr’s meta-tool) tool, including flashing with custom options such as a different serial port. The west tool supports specific options for the ESP32 board, as listed here:

--esp-idf-path ESP_IDF_PATH
 path to ESP-IDF
--esp-device ESP_DEVICE
 serial port to flash, default /dev/ttyUSB0
--esp-baud-rate ESP_BAUD_RATE
 serial baud rate, default 921600
--esp-flash-size ESP_FLASH_SIZE
 flash size, default “detect”
--esp-flash-freq ESP_FLASH_FREQ
 flash frequency, default “40m”
--esp-flash-mode ESP_FLASH_MODE
 flash mode, default “dio”
--esp-tool ESP_TOOL
 if given, complete path to espidf. default is to search for it in [ESP_IDF_PATH]/components/esptool_py/ esptool/esptool.py
--esp-flash-bootloader ESP_FLASH_BOOTLOADER
 Bootloader image to flash
--esp-flash-partition_table ESP_FLASH_PARTITION_TABLE
 Partition table to flash

For example, to flash to /dev/ttyUSB2, use the following command after having build the application in the build directory:

west flash -d build/ --skip-rebuild --esp-device /dev/ttyUSB2

Using JTAG

As with much custom hardware, the ESP-32 modules require patches to OpenOCD that are not upstream. Espressif maintains their own fork of the project here. By convention they put it in ~/esp next to the installations of their toolchain and SDK:

cd ~/esp

git clone https://github.com/espressif/openocd-esp32

cd openocd-esp32
./bootstrap
./configure
make

On the ESP-WROVER-KIT board, the JTAG pins are connected internally to a USB serial port on the same device as the console. These boards require no external hardware and are debuggable as-is. The JTAG signals, however, must be jumpered closed to connect the internal controller (the default is to leave them disconnected). The jumper headers are on the right side of the board as viewed from the power switch, next to similar headers for SPI and UART. See ESP-WROVER-32 V3 Getting Started Guide for details.

On the ESP-WROOM-32 DevKitC board, the JTAG pins are not run to a standard connector (e.g. ARM 20-pin) and need to be manually connected to the external programmer (e.g. a Flyswatter2):

ESP32 pin JTAG pin
3V3 VTRef
EN nTRST
IO14 TMS
IO12 TDI
GND GND
IO13 TCK
IO15 TDO

Once the device is connected, you should be able to connect with (for a DevKitC board, replace with esp32-wrover.cfg for WROVER):

cd ~/esp/openocd-esp32
src/openocd -f interface/ftdi/flyswatter2.cfg -c 'set ESP32_ONLYCPU 1' -c 'set ESP32_RTOS none' -f board/esp-wroom-32.cfg -s tcl

The ESP32_ONLYCPU setting is critical: without it OpenOCD will present only the “APP_CPU” via the gdbserver, and not the “PRO_CPU” on which Zephyr is running. It’s currently unexplored as to whether the CPU can be switched at runtime or if breakpoints can be set for either/both.

Now you can connect to openocd with gdb and point it to the OpenOCD gdbserver running (by default) on localhost port 3333. Note that you must use the gdb distributed with the ESP-32 SDK. Builds off of the FSF mainline get inexplicable protocol errors when connecting.

~/esp/xtensa-esp32-elf/bin/xtensa-esp32-elf-gdb outdir/esp32/zephyr.elf
(gdb) target remote localhost:3333

Further documentation can be obtained from the SoC vendor in JTAG debugging for ESP32.