转自：https://blog.csdn.net/chengdong1314/article/details/62038429
SYD8801是一款低功耗高性能蓝牙低功耗SOC，集成了高性能2.4GHz射频收发机、32位ARM Cortex-M0处理器、128kB Flash存储器、以及丰富的数字接口。SYD8801片上集成了Balun无需阻抗匹配网络、高效率DCDC降压转换器，适合用于可穿戴、物联网设备等。具体可咨询：http://www.syd-tek.com/
蓝牙数据包大小的请教
在4.0蓝牙规范的《2 AIR INTERFACE PACKETS》这一节中定义BLE蓝牙链路层数据包的结构，如下：
规定了PUD的大小最大是39个byte,这里分析数据通道的PUD大小，我们看看如下定义：
而下面关于payload的长度 Length有如下解释：
也就说明payload长度最大是27个byte!
再后来在来l2cap的定义如下：
这里再减去4个字节，所以真正上层的数据大小只有23个字节！
而对于notified，att中定义如下：
这里再减去3个字节，所以对于notify真正有效的数据大小就是20个byte
这里上传上蓝牙4.0规范：http://download.csdn.net/detail/chengdong1314/9780390
BLE PACKET FORMAT
Preamble
Advertising channel packetsshall have 10101010b as the preamble.
The data channel packetpreambleis either 10101010b or 01010101b, depending on the LSB of the AccessAddress. If the LSB of the Access Address is 1, the preamble shall be01010101b,otherwise the preamble shall be 10101010b.
AccessAddress
The Access Address for all advertising channel packets shall be 0x8E89BED6.
The Access Address in data channel packets shall be different foreach Link Layer connection between any two devices with certain restrictionsasdefined .
ADVERTISINGCHANNEL PDU
Header field
PDU Type
TxAdd andRxAdd
The TxAdd and RxAdd fields of the advertising channel PDU that arecontained in the header contain information specific to the PDU type definedfor each advertising channel PDU separately. If the TxAdd orRxAdd fields arenot defined as used in a given PDU then they shall be considered Reserved for FutureUse.
DATA CHANNEL PDU
