Basic M5Stack Core2 Example

This repository is intended as a straightforward example or base project for using the M5Stack Core2 with ESP-IDF directly in C - i.e. No Arduino.

Because it's intended as an example to learn and copy-paste from, the code favors simplicity and "obviousness" over clean abstractions and good practice. It exists only to provide a minimum-viable reference for how to bring up the board in ESP-IDF.

Functionality

At present, a subset of the board's features are supported:

• AXP192 PMIC
  • Voltage rail adjustment, enable/disable
  • Configure charger(s)
  • Read battery voltage
  • GPIOs
  • etc...
• LED control
  • via AXP192 GPIO1
• Vibration motor control
  • via AXP192 LDO3
• LCD backlight control
  • via AXP192 DCDC3
• 5 V "Bus"/Grove power control
  • via AXP192 EXTEN / N_VBUSEN
• LCD and Touchscreen
  • GUI supported using lvgl and lvgl_esp32_drivers

"Out of the box", it will boot to a simple GUI where you can toggle the LED, vibration motor, and 5 V bus power on and off.

Building

This project builds using ESP-IDF. I have been building it with a random git snapshot of ESP-IDF from mid 2020. Specifically, the version is reported as:

ESP-IDF v4.2-dev-1099-g38102d0e4

I believe it should work with any v4 version (where idf.py is used instead of make), but I will be happy to receive pull requests to fix the build if there are any issues.

To build it, in a shell where you have already set up ESP-IDF (i.e. have sourced export.sh):

# Clone the repository, and its submodules
git clone https://github.com/usedbytes/m5core2-basic-idf.git
cd m5core2-basic-idf
git submodule update --init --recursive

# Build it, flash it, and see the serial output
idf.py build flash monitor

The sdkconfig is set up for the M5Core2, and so it should work out-of-the box.

External Dependencies

To provide drivers for the different devices, this project pulls in a few external components as sub-modules. I have had to make adjustments to most of them to make them work with this board. Some would be suitable for upstreaming, others are quick hacks.

components/esp_i2c_hal

This provides a dead simple i2c abstraction, which is used by the AXP192 driver.

• Upstream: https://github.com/tuupola/esp_i2c_hal
• Downstream: https://github.com/usedbytes/esp_i2c_hal

Modifications:

• Not using the most recent upstream version, which changed the API in a non-backwards compatible way, and I didn't update my axp192 fork to support it yet
• Switch the i2c bus to 0. Needed to match the (non-configurable) lvgl_esp_drivers bus.

components/axp192

Driver for the AXP192 PMIC.

• Upstream: https://github.com/tuupola/axp192
• Downstream: https://github.com/usedbytes/axp192

Modifications:

• Not using the "init commands" functionality from upstream, which isn't very flexible
• Ignoring most of the Kconfig support, which is a bit cumbersome
• Add voltage rail configuration and direct register access
• Add IRQ handling (though not used in this project! See also my T-Beam project. The IRQ pin isn't wired up on the M5Core2 board.

components/lvgl

Light and Versatile Graphics Library

• Upstream: https://github.com/lvgl/lvgl

Modifications:

• None!

components/lvgl_esp32_drivers

Drivers for common ESP32 LCD and touch controllers, for use with LVGL. See "known issues".

• Upstream: https://github.com/lvgl/lvgl_esp32_drivers
• Downstream: https://github.com/usedbytes/lvgl_esp32_drivers

Modifications:

• Hack the correct screen orientation for M5Core2
• Allow skipping LCD reset control (upstreamable)
  • The reset pin isn't connected to the ESP32, it's connected to the AXP192.

Known Issues

PMIC / LCD initialisation

The primary issue is that the lvgl_esp32_drivers implementation is very "opinionated", and almost everything is done via KConfig. That means that if your board isn't already supported, it's quite likely that you're going to have trouble supporting it cleanly.

Also, it takes complete ownership of setting up the i2c and SPI buses, which doesn't gel well with the shared i2c bus of the M5Core2.

The main manifestation of this is related to the PMIC and LCD initialisation, which is a bit hacked. See the comment above lvgl_driver_init() in main.c for more details.

Apparent high CPU usage

The LVGL CPU-usage icon shows ~50% CPU utilisation most of the time. However during development, I have seen this sometimes show ~0% most of the time. I'm not sure if there's some strangeness around how FreeRTOS is scheduling the LVGL work which means it really is using 50% CPU, or if it's just a quirk of how LVGL is measuring utilisation.

Needs more investigation.