SX127x

来自Jack's Lab
(版本间的差异)
跳转到: 导航, 搜索
(CAD)
(CAD)
第32行: 第32行:
  
 
   SymbolTime = (2^SF + 32) / BW seconds
 
   SymbolTime = (2^SF + 32) / BW seconds
 +
 +
Refer to: https://www.semtech.com/uploads/documents/an1200.21_std.pdf
  
 
<br>
 
<br>

2019年5月23日 (四) 22:01的版本

LoRa-packet-structure.jpg

目录

1 RSSI

Received Signal Strength Indication,接收信号强度指示

Antenna Diversity,天线分集

The RSSI information is available during CAD and there is no need to leave the receiver on for some additional time


2 CAD

Channel Activity Detection. It's designed to detect a LoRa preamble on the radio channel with the best possible power efficiency.

Once in CAD mode, the SX127x will perform a very quick scan of the band to detect a LoRa packet preamble.

During CAD, the following operation take place:

  • The PLL locks
  • The radio receiver captures LoRa preamble symbol of data from the channel. The radio current consumption during that phase is approximately 10mA.
  • The radio receiver and the PLL turn off and the modem digital processing starts.
  • The modem searches for a correlation between the Radio captured samples and the ideal preamble waveform. This correlation process takes a little bit less than a symbol period toperform. The radio current consumption during that phase is greatly reduced.
  • Once the calculation is finished the modem generates the CadDone interrupt. If the correlation was successful, the CadDetected is generated simultaneously.
  • The chip goes back to stand-by mode.
  • If a preamble was detected, clear the interrupt, then initiate the reception by putting the radio in RX single mode or RX continuous mode.


The typical CAD detection time:

 SymbolTime = (2^SF + 32) / BW seconds

Refer to: https://www.semtech.com/uploads/documents/an1200.21_std.pdf


3 Preamble

LoRa 数据包前导码,用于接收机辩识 rx data flow。内容固定,长度可选。

默认 12 个 symbol(Bytes?), 0x8 (RegPreambleLsb) + 4

可写 RegPreambleMsb 和 RegPreambleLsb 这两个寄存器,将其改为 6 +4 ~ 65535 +4

10 symbol 的短前导码,通信够用但具有更短的接收周期。。。

当前系统一般皆适用 12 (4 + 8) 的长度:

......
^$Get Preamble Length: state 0
^$Preamble Length: 8
......


4 Header

Explicit Header & Implicit Header

可通过寄存器 RegModemConfig1 内的位 ImplicitHeaderModeOn 来控制


Explicit Header:

Reset 默认的模式。其能提供的信息:

  • Payload 长度
  • The forward error correction code rate
  • The presence of an optional 16-bits CRC for the payload

The head is transmitted with maximum error correction code (4/8). It also has its own CRC to allow the receiver to discard invalid headers.


Implicit Header:

Header is removed from the LoRa Packet.

Payload, Coding Rate and CRC presence are fixed.

Be advantageous to reduce transmission time



5 CRC

RegPayloadCrcOn: RegModemConfig2[2:2]

  • 0 ---> CRC disable
  • 1 ---> CRC enable


In Implicit Header Mode: must be set on Tx and Rx side

In Explicit Header Mode: on the Tx side alone (recovered from the header in Rx side)


Explicit Header Mode Checking process:

Rx 用户负责检查 CrcOnPayload (RegHopChannel[6:6], CRC information extracted from the rx packet header)

如果 CrcOnPayload == 1,需要检查 IRQ 标志位 PayloadCrcError,以确保没有 CRC 错误


Implicit Header Mode 则直接去检查 IRQ 标志位 PayloadCrcError,以确保没有 CRC 错误



个人工具
名字空间

变换
操作
导航
工具箱