SP2-433-160 Datasheet, PDF (19/27 Pages) Radiometrix Ltd – UHF SpacePort Transceiver

The SP2's Packet Decoder
Signal Decoding is in 4 stages :-
Search
Initially the SP2's decoder searches the radio noise on the RXD line for the 80kHz preamble signal. The
search is performed by a 16 times over-sampling detector which computes the spectral level of 80kHz in
192 samples of the RXD signal (156µs window).If the level exceeds a pre-set threshold the decoder will
attempt to decode a packet.
Lock-in
The same set of 192 samples are used to compute the phase of the incoming preamble and synchronise
the internal recovery clock to an accuracy of +/- 2µs. The recovery clock samples the mid point of each
incoming data bit and shifts the samples trough an 8 bit serial comparator. The comparator searches
the data on a bit by bit basis for the frame sync byte. While the search is in progress, the decode will
abort if the preamble fails to maintain a certain level of integrity. If the comparator finds the 'please
hold the line' code used during extended wake-up preamble a phase re-lock is triggered to ensure
accurate phase tracking until the actual packet arrives. When the frame sync is detected the decoder
attains full synchronisation and will move to the Decode state.
Decode
Data is now taken in 12 bits at a time (one symbol), decoded into the original byte and placed in the
receive buffer. The symbol decoder verifies each received symbol as valid (only 256 out of a possible
4096 are valid) and will immediately abort the decode on a symbol failure. The first byte contains the
byte count and is used to determine the end of message.
Check Sum
The last byte is the received check sum, this is verified against a locally generated sum of all the
received bytes in the packet. If it matches the packet is valid and RXR line will be pulled low to inform
the Host that a packet awaits uploading.
Notes on error handling
The SP2’s' decoder is deliberately non bit error tolerant, i.e.. no attempt is made to repair corrupt data
bits. All of the redundancy in the code is directed towards error checking. For an FM radio link using
short packet lengths, e.g. SP2 + BiM2 , packets are either 100% or so grossly corrupt as to be
unrecoverable. By the same reasoning, the Host is not informed when the SP2 decoder aborts a packet
decode since corrupt information is of little value. A packet acknowledge Time-Out and re-transmission
is the preferred strategy for error handling.
Radiometrix Ltd, SP2 data sheet
page 19