ESP32 implementation for an HID over GATT Keyboard (Bluetooth Low Energy).
Any tips for improvments would be greatly appriciated.

• Benjamin Aigner for his work on implementing HID over BLE on the ESP32: https://github.com/asterics/esp32_mouse_keyboard.
• Neil Kolban for his great contributions to the ESP32 SW (in particular the Bluetooth support): https://github.com/nkolban
• QMK for their layouts and inspiration for this project: https://github.com/qmk/qmk_firmware/

In order to wire your desired keyboard (if handwired) I reccomend checking out: https://geekhack.org/index.php?topic=87689.0

In order to define your keyboard:

• Modify keyboard_config.h for your desired keyboard size, features, etc.
• modify matrix.c for your gpio layout
• Modify keymap.c for your desired layouts and macros.

• When uploading code to main board, make sure MASTER and SPLIT_MASTER are defined in keyboard_config.h.
• When uploading code to slave board, make sure SLAVE is defined in keyboard_config.h, currently you will also need to update the correct Mac adress of your main board in espnow_send.c.

• You can add an OLED display to your keyboard by defining ENABLE_OLED in keyboard_config.h and setting the correct i2c pins.
• The graphics are currently configured for an SSD1306 128x64 OLED but can be modified by changing your display in oled_tasks.c. please refer to the u8g2 wiki for additional information: https://github.com/olikraus/u8g2/wiki
• The oled currently responds relativley slowly (probably an issue with i2c over 100khz on the esp32).

• Battery level indication is by approximate values (you can look at LiPo voltage discharge graphs and adjust using an appropriate voltage-discharge capacity table)
• You can add battery monitoring by selecting an adc pin and using a voltage divider.
• See battery_monitor.h for resistor values.
• I have not tested the accuracy of the battery monitoring as of yet.

• Define R_ENCODER or (R_ENCODER_SLAVE if on slave pad) and set encoder pins in keyboard_config.h (undefine ENCODER_S_PIN if your encoder does not have a switch).
• You can modify encoder actions in keymap.c (set it as a mouse, consumer device, ordinary keyboard keys etc).

• For better battery usage you can define the number of inactive minutes you would like your device to wait before sleeping in keyboard_config.h.
• The controller is defined to wake up when the capacity on GPIO pin 2 is modified ("touch sensor"), if you do not want to enable deep sleep simply undefine SLEEP_MINS.
• Notice that on windows reconnection after waking from deep sleep might take a couple of minutes, no problem on linux. (on windows you can rescan for bluetooth devices and it should recconect without waiting).
• Tested current (hopefully will improve in future): 40 mA for BLE (without split functionality). 100 mA with split mode enabled (possible to reduce if I change from ESP-NOW to BLE).
  <1 mA while in deep sleep.

• More than 2 pads (currently supports 2 pads).
• Connection switching.
• Modify keymap via webserver.
• Security (?).