SX1231-31SKB433 Semtech, SX1231-31SKB433 Datasheet - Page 52

• Manufacturer Part Number: SX1231-31SKB433
• Description: USB Kit - SX1231 433Mhz
• Manufacturer: Semtech

5.5.2.2. Variable Length Packet Format
Variable length packet format is selected when bit PacketFormat is set to 1.
This mode is useful in applications where the length of the packet is not known in advance and can vary over time. It is then
necessary for the transmitter to send the length information together with each packet in order for the receiver to operate
properly.
In this mode the length of the payload, indicated by the length byte, is given by the first byte of the FIFO and is limited to
255 bytes if AES is not enabled else the message is limited to 64 bytes i.e. max 66 bytes payload if Address byte is
enabled. Note that the length byte itself is not included in its calculation. In this mode, the payload must contain at least 2
bytes, i.e. length + address or message byte.
An illustration of a variable length packet is shown below. It contains the following fields:
Rev 4 - Nov 2010
ADVANCED COMMUNICATIONS & SENSING
Preamble (1010...)
Sync word (Network ID)
Length byte
Optional Address byte (Node ID)
Message data
Optional 2-bytes CRC checksum
0 to 65535
Preamble
bytes
Optional User provided fields which are part of the payload
Message part of the payload
Fields added by the packet handler in Tx and processed and removed in Rx
0 to 65535
Preamble
bytes
Optional User provided fields which are part of the payload
Message part of the payload
Fields added by the packet handler in Tx and processed and removed in Rx
0 to 8 bytes
Sync Word
Figure 32. Variable Length Packet Format
0 to 8 bytes
Figure 31. Fixed Length Packet Format
Sync Word
Length
byte
Address
byte
CRC checksum calculation
CRC checksum calculation
Address
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byte
(min 2 bytes)
DC free Data encoding
(min 1 byte)
DC free Data encoding
Payload
Payload
Up to 255 bytes
AES Enc/Dec
Message
Up to 255 bytes
AES Enc/Dec
Message
2-bytes
CRC
2-bytes
CRC
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