Sx128X Compatibility - Semtech LR1121 User Manual

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 are added automatically in
hardware. The packet length is added automatically in the packet when the PacketType field is set to variable size in
command SetPacketParams(...).
The node or broadcast address can be enabled by the AddrComp field in command SetPacketParams(...). This field
allows the user to enable and select an additional packet filtering at the payload level.
8.4.3.4 Whitening
The whitening is based around the 9-bit LFSR polynomial x^9+x^5+1 for sub-GHz and x^7 + x^4 + 1 for HF which generates
a random sequence. With this structure, the LSB at the output of the LFSR is XORed with the MSB of the data. At the initial
stage, command SetGfskWhitParams(...) sets the whitening Seed.
The payload (including the payload length, the Node or Broadcast 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 process limits the number
of consecutive 1's or 0's to 9.
Note that 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.
8.4.3.5 CRC
The LR1121 offers full flexibility to select the CRC polynomial and initial value of the selected polynomial. The user can also
select a complete inversion of the computed CRC to comply with some international standards.
The CRC can be enabled and configured in the CrcType field in command SetPacketParams(...). This field allows the
user to enable and select the length and configuration of the CRC.
Command SetGfskCrcParams(...) configures the CRC polynomial and initial value.

8.4.4 SX128x Compatibility

The following limitations apply when using the SX128x 9-bit header PacketType 0x02:
•
Payload length: payload length is limited to max. 254 - CrcLength (Ex: 252 bytes with a 16-bit crc)
•
CRC length: 0,1 or 2 bytes
•
Syncword length: only allowed syncword length values are 8,16,24, 32 or 40 bits
•
AddrComp: has to be disabled (feature not present on SX128x)
•
DcFree has to be either disabled (0x00) or SX128x compatible (0x03). 0x01 is not accepted
In order to make the LR1121 to be air-compatible with the SX128x:
•
Set the whitening seed to 0x0001 with the SetGfskWhitParams command (default SX128x value)
•
SetGfskCrcParams: (default SX128x values):
1 byte crc: seed = 0xFF, poly = 0x07

2 byte crc: seed = 0xFFFF, poly = 0x1021

Note that for transmitted frames, 7 additional bits are transmitted and for received frames 23 additional bits are received
(delaying the RX_DONE IRQ).
LR1121
User Manual Rev 1.1
UM.LR1121.W.APP Mar 2023
www.semtech.com 73 of 130
Semtech
Proprietary & Confidential