.. _esp32:

ESP32
#####

Overview
********

From Wikipedia:

::

   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_GCC_VARIANT`` shell variable.  Two other environment variables
should also be set, pointing to, respectively, the path where ESP-IDF can be
located, and where the toolchain has been installed:

.. code-block:: console

   export ZEPHYR_GCC_VARIANT="espressif"
   export ESP_IDF_PATH="/path/to/esp-idf"
   export ESPRESSIF_TOOLCHAIN_PATH="/path/to/xtensa-esp32-elf/"

Since ESP-IDF is an external project in constant development, it's possible
that files that Zephyr depends on will be moved, removed, or renamed.  Those
files are mostly header files containing hardware definitions, which are
unlikely to change and require fixes from the vendor.  In addition to
setting the environment variables above, also check out an earlier version
of ESP-IDF:

.. code-block:: console

   cd ${ESP_IDF_PATH}
   git checkout dc8c33892e0

Flashing
========

Issuing ``make BOARD=esp32 flash`` should work as usual.  Environment
variables can be set to set the serial port device, baud rate, and
other options.  Please refer to the following table for details.

+----------------+---------------+
| Variable       | Default value |
+================+===============+
| ESP_DEVICE     | /dev/ttyUSB0  |
+----------------+---------------+
| ESP_BAUD_RATE  | 921600        |
+----------------+---------------+
| ESP_FLASH_SIZE | detect        |
+----------------+---------------+
| ESP_FLASH_FREQ | 40m           |
+----------------+---------------+
| ESP_FLASH_MODE | dio           |
+----------------+---------------+
| ESP_TOOL       | espidf        |
+----------------+---------------+

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.

If ``ESP_TOOL`` is set to "espidf", the `esptool.py`_ script found within
ESP-IDF will be used.  Otherwise, this variable is handled as a path to
the tool.


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 the `~/esp` next to the
installations of their toolchain and SDK:

.. code-block:: console

   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):

.. code-block:: console

    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.

.. code-block:: console

    ~/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`_.

References
**********

.. [1] https://en.wikipedia.org/wiki/ESP32
.. _`ESP32 Technical Reference Manual`: https://espressif.com/sites/default/files/documentation/esp32_technical_reference_manual_en.pdf
.. _`JTAG debugging for ESP32`: http://esp-idf.readthedocs.io/en/latest/api-guides/jtag-debugging/index.html
.. _`toolchain`: https://esp-idf.readthedocs.io/en/latest/get-started/index.html#get-started-setup-toochain
.. _`SDK`: https://esp-idf.readthedocs.io/en/latest/get-started/index.html#get-started-get-esp-idf
.. _`Hardware Referecne`: https://esp-idf.readthedocs.io/en/latest/hw-reference/index.html
.. _`esptool documentation`: https://github.com/espressif/esptool/blob/master/README.md
.. _`esptool.py`: https://github.com/espressif/esptool
.. _`ESP-WROVER-32 V3 Getting Started Guide`: https://dl.espressif.com/doc/esp-idf/latest/get-started/get-started-wrover-kit.html