Figure 8-5: Variable-Length Packet - Semtech LR1110 User Manual

The preamble length is set from 8 to 65535 bits using the parameter PreambleLen. It is usually recommended to use a
minimum of 16 bits for the preamble to guarantee a valid reception of the packet on the receiver side. The CRC operation,
packet length and preamble length are defined using the command SetPacketParams( ).
8.4.3.2 Variable-Length Packet
Where the packet is of uncertain or variable size, then information about the packet length must be transmitted within the
packet. The format of the variable-length packet is shown below.
8.4.3.3 Setting The Packet Length Or Node Address
The packet length and Node or Broadcast address are not considered part of the payload and they are added automatically
in hardware.
The packet length is added automatically in the packet when the PacketType field is set to variable size in the command
SetPacketParam( ) .
The node or broadcast address can be enabled by using the AddrComp field is in the command SetPacketParam( ) . This field
allow the user to enable and select an additional packet filtering at the payload level.
8.4.3.4 Whitening
The whitening process is built around a 9-bit LFSR which is used to generate a random sequence and the payload (including
the payload length, the Node or Broadcase address and CRC checksum when needed) is then XORed with this random
sequence to generate the whitened payload. The data is de-whitened on the receiver side by XORing with the same
random sequence. This setup limits the number of consecutive 1's or 0's to 9. Note that the data whitening is only required
when the user data has high correlation with long strings of 0's and 1's. If the data is already random then the whitening is
not required.
LR1110
User Manual Rev.1.0
UM.LR1110.W.APP March 2020

Figure 8-5: Variable-Length Packet

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