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The application on the phone sends instructions to ESP32 using Bluetooth Low Energy:

• The phone acts as BLE Central (also called Master, Client)
• ESP32 acts as BLE Peripheral (also called Slave, Server)

Currently the only compatible application is available on iOS: Sygic GPS Navigation & Maps

To enable BLE HUD in the app (it's an unofficial prototype feature):

• Menu / Settings / Info / About / tap 3 times on any item (new line About appears at the top) / About / BLE HUD / Start
• Bluetooth permission required

Notes:

• When BLE HUD enabled, the application automatically scans and connects to ESP32 module (the app must be in foreground)
• When already connected and the route is set, the phone screen can be turned off
• The feature becomes off after restarting the application
• During route recompute (e.g. when I miss my turn) "No route" message can temporarily appear on ESP32

• ESP32 with external OLED display 128x128 (enabled by-default), how to connect
• ESP32 TTGO T-Display with embedded TFT 135x240, how to enable

1. Connection flow:
   1. Central (the application on the phone) scans for a Peripheral with wanted service UUID (see Table of UUIDs) - it's going to be our ESP32
   2. The application connects to the first found device
2. Sending instruction:
   1. Central writes new data to the characteristic (see Table of UUIDs) as soon as the data changes (current speed limit or an instruction)
   2. Data example: 0x01320A3335306D, meaning: basic data (0x01), current speed limit is 50km/h (0x32), turn right (0x0A) in 350m (0x3335306D, not null-terminated string)
   3. If the app doesn't send an update for a few seconds, ESP32 sends an empty indication (see Table of UUIDs), letting the app know that it wants a data update

To enable TTGO T-Display:

1. Download library TFT_eSPI adjusted by TTGO, copy TFT_eSPI folder to Arduino/libraries
2. Uncomment display in code

// comment out these lines 
#include "OLED_SSD1351_nolib.h"
OLED_SSD1351_nolib selectedDisplay;
constexpr bool ENABLE_VOLTAGE_MEASUREMENT = false;

// uncomment these lines
//#include "TFT_TTGO.h"
//TFT_TTGO selectedDisplay;
//constexpr bool ENABLE_VOLTAGE_MEASUREMENT = true;

Display: Waveshare 14747 128x128 OLED RGB (link)

Protocol: SSD1351

Note: there is a great graphics library for this display: Adafruit-SSD1351-library, but it's not used in this project.

Connected this way: