# Bluetooth Low-Energy : 
## Introduction
This page describes the BLE implementation and API built in this firmware.

**Note** : I'm a beginner in BLE related technologies and the information of this document reflect my current knowledge and understanding of the BLE stack. These informations might be erroneous or incomplete. Feel free to submit a PR if you think you can improve these.  

## BLE Connection
When starting the firmware start a BLE advertising : it send small messages that can be received by any *central* device in range. This allows the device to announce its presence to other devices.

A companion application (running on a PC, RasberryPi, smartphone) which received this avertising packet can request a connection to the device. This connection procedure allows the 2 devices to negociate communication parameters, security keys,...

When the connection is established, the pinetime will try to discover services running on the companion application. For now **CTS** (**C**urrent **T**ime **S**ervice) and **ANS** (**A**lert **N**otification **S**ervice) are supported.

If **CTS** is detected, it'll request the current time to the companion application. If **ANS** is detected, it will listen to new notifications coming from the companion application.

![BLE connection sequence diagram](ble/connection_sequence.png "BLE connection sequence diagram")

## BLE services
[List of standard BLE services](https://www.bluetooth.com/specifications/gatt/services/)

### CTS
[Current Time Service](https://www.bluetooth.com/wp-content/uploads/Sitecore-Media-Library/Gatt/Xml/Services/org.bluetooth.service.alert_notification.xml)

### ANS
[Alert Notification Service](https://www.bluetooth.com/wp-content/uploads/Sitecore-Media-Library/Gatt/Xml/Services/org.bluetooth.service.current_time.xml)

![ANS sequence diagram](./ble/ans_sequence.png "ANS sequence diagram")