G)Fsk Packet Engine; G)Fsk Packet Format; Figure 8-4: Fixed-Length Packet - Semtech LR1110 User Manual

8.4.2 (G)FSK Packet Engine

The LR1110 is designed for packet-based transmission. The packet controller block is responsible for assembly of received
data bit-stream into packets and their storage into the data buffer. It also performs the bit-stream decoding operations such
as de-whitening & CRC-checks on the received bit-stream.
On the transmit side, the packet handler can construct a packet and send it bit by bit to the modulator for transmission. It
can whiten the payload and append the CRC-checksum to the end of the packet. The packet controller only works in
half-duplex mode i.e. either in transmit or receive at a time.
The packet controller is configured using the command SetPacketParams( ) . This function can be called only after defining
the protocol.
Preamble Detection in Receiver Mode
The LR1110 is able to gate the reception of a packet if an insufficient number of alternating preamble symbols (usually
referred to 0x55 or 0xAA in hexadecimal form) has been detected. This can be selected by the user by using the parameter
PreambleDetectorLength used in the command SetPacketParams( ) . The user can select a value ranging from "Preamble
detector length off" - where the radio will not perform any gating and will try to lock directly on the following Syncword -to
"Preamble detector length 32 bits" where the radio will be expecting to receive 32 bits of preamble before the following
Syncword. In this case, if the 32 bits of preamble are not detected, the radio will either drop the reception in RxSingle mode,
or restart its tracking loop in RxContinuous mode.
To achieve best performance of the device, it is recommended to set PreambleDetectorLength to "Preamble detector length
8 bits" or "Preamble detector length 16 bits" depending of the complete size of preamble which is sent by the transmitter.
Note: In all cases, PreambleDetectorLength must be smaller than the size of the following Syncword to achieve proper
detection of the packets. If the preamble length is greater than the following Syncword length (typically when no Syncword
is used) the user should fill some of the Syncword bytes with 0x55.

8.4.3 (G)FSK Packet Format

The (G)FSK packet format provides a conventional packet format for application in proprietary NRZ coded, low energy
communication links. The packet format has built in facilities for CRC checking of the payload, dynamic payload size and
packet acknowledgement. Optionally whitening based upon pseudo random number generation can be enabled. Two
principle packet formats are available in the (G)FSK protocol: fixed length and variable length packets.
8.4.3.1 Fixed-Length Packet
If the packet length is fixed and known on both sides of the link then knowledge of the packet length does not need to be
transmitted over the air. Instead the packet length can be written to the parameter packetLength which determines the
packet length in bytes (0 to 255).
LR1110
User Manual Rev.1.0
UM.LR1110.W.APP March 2020

Figure 8-4: Fixed-Length Packet

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