MRF24XA_15 Datasheet, PDF (223/258 Pages) Microchip Technology – Low-Power, 2.4 GHz ISM-Band IEEE 802.15.4™ RF

9.8 Start-of-Frame Delimiter (SFD)
Detection
The following sections describe the SFD detection
mechanism for the different data rates.
Header processing is required to work at least as reli-
ably as the demodulation. To meet this requirement,
longer preamble and 16-bit SFD is defined for frames
where the payload data rate is lower than the air-data-
rate of the preamble.
9.8.1 SFD DETECTION AT 125 kbps
The input contains a nibble of bits decoded from the
received DSSS symbol. This input is updated on every
new DSSS symbol received.
In each update, the latest four received nibbles are
compared against the nibbles contained in the 16-bit
SFD pattern that the host MC configures. At least three
out of four nibbles must match to trigger an SFD_-
FOUND event.
SFD_TIMEOUT occurs if the latest five nibbles are
different from “0000” (preamble lost) while SFD_-
FOUND is not triggered. Reception is reset on SFD_-
TIMEOUT.
For the 125 kbps data rate the last decoded four
nibbles and the nibbles of pattern_125 must match in at
least three nibble positions.
MRF24XA
9.8.2 SFD DETECTION AT 1 Mbps
The input contains a byte, which is updated on every
new byte received after the first preamble byte has
been detected (this identifies the byte boundary). The
two latest received bytes form a Word of 16 bits is
denoted by W.
SFD_FOUND event is reported if W exactly matches
the host configured 16-bit preamble pattern (SFD), or if
an approximate match is found with the following error
patterns:
• SFD XOR W = 110...0110…0 (two error bursts of
length 2)
• SFD XOR W = 10...0110…01 (single error burst of
length 2, and single error on either or both ends)
SFD XOR W = 0...0110…0 (single error burst of
length 2)
• SFD XOR W = 0...010…0 (single error)
The rationale behind selecting these patterns is that the
maximum-likelihood demodulator tends to produce
error bursts of length 2 due to the trellis of the MSK
modulation (this particular tolerance scheme seems to
be novel).
SFD_TIMEOUT event is reported if the latest three
octets are different from 0x0F0F0F, while SFD_-
FOUND is not triggered. Reception is reset on SFD_-
TIMEOUT.
At 1 Mbps the match tolerates single bit or maximum 2
non-contiguous 2-bit burst differences in the
comparison of the last received 16 bits and
pattern_1000 (simultaneously isolated single bit
mismatches at both ends of the pattern constitute a
single 2-bit mismatch burst).
 2015 Microchip Technology Inc.
Preliminary
DS70005023C-page 223