CC2420 Datasheet, PDF (37/92 Pages) List of Unclassifed Manufacturers – 2.4 GHz IEEE 802.15.4 / ZigBee-ready RF Transceiver

additional zero symbols in SYNCWORD
make CC2420 compliant with [1].
In reception, CC2420 synchronises to
received zero-symbols and searches for
the SFD sequence defined by the
SYNCWORD register. The least significant
symbols in SYNCWORD set to 0xF will be
ignored, while symbols different from 0xF
will be required for synchronisation. The
default setting of 0xA70F thereby requires
one additional zero-symbol for
synchronisation. This will reduce the
number of false frames detected due to
noise.
The following illustrates how the
programmed synch word is interpreted
during reception by CC2420: If SYNCWORD
= 0xA7FF, CC2420 will require the
incoming symbol sequence of (from left to
CC2420
right) 0 7 A. If SYNCWORD = 0xA70F,
CC2420 will require the incoming symbol
sequence of (from left to right) 0 0 7 A. If
SYNCWORD = 0xA700, CC2420 will require
the incoming symbol sequence of (from
left to right) 0 0 0 7 A.
In receive mode CC2420 uses the
preamble sequence for symbol
synchronisation and frequency offset
adjustments. The SFD is used for byte
synchronisation, and is not part of the data
stored in the receive buffer (RXFIFO).
Synchronisation Header
Preamble
SFD
IEEE 802.15.4 0
0
0
0
0
0
0
0
7
A
CC2420
2·(PREAMBLE_LENGTH + 1) zero symbols SW0 SW1 SW2 SW3
Each box corresponds to 4 bits. Hence the preamble corresponds to 8 x 4 ''0' s or 4 bytes with the value 0.
SW0 = SYNCWORD[3:0] if different from 'F', else '0'
SW1 = SYNCWORD[7:4] if different from 'F', else '0'
SW2 = SYNCWORD[11:8] if different from 'F', else '0'
SW3 = SYNCWORD[15:12] if different from 'F', else '0'
Figure 18. Transmitted Synchronisation Header
16.2 Length field
The frame length field shown in Figure 17
defines the number of bytes in the MPDU.
Note that the length field does not include
the length field itself. It does however
include the FCS (Frame Check
Sequence), even if this is inserted
automatically by CC2420 hardware. It also
includes the MIC if authentication is used.
The length field is 7 bits and has a
maximum value of 127. The most
significant bit in the length field is reserved
[1], and should be set to zero.
CC2420 uses the length field both for
transmission and reception, so this field
must always be included. In transmit
mode, the length field is used for
underflow detection, as described in the
FIFO access section on page 31.
16.3 MAC protocol data unit
The FCF, data sequence number and
address information follows the length field
as shown in Figure 17. Together with the
MAC data payload and Frame Check
Sequence, they form the MAC Protocol
Data Unit (MPDU).
The format of the FCF is shown in Figure
19. Please refer to [1] for details.
SWRS041B
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