说明：  蓝牙V4.2官方标准文档。对于低功耗的蓝牙开发，除了厂商提供的芯片说明文档，最权威的就是官方的文档。
(Bluetooth V4.2 official standard documentation. For low-power Bluetooth development, in addition to the chip manufacturer s documentation, it is the most authoritative official documents.)