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Length field, Mac protocol data unit, Cc2420 – Texas Instruments 3138 155 232931 User Manual

Page 37

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CC2420

SWRS041B Page 37 of 89

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

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).

0

7

A

IEEE 802.15.4

Preamble

SFD

CC2420

2·(PREAMBLE_LENGTH + 1) zero symbols

0

0

0

0

0

0

0

SW0

SW0 = SYNCWORD[3:0]

SW1 = SYNCWORD[7:4]

SW2 = SYNCWORD[11:8]

SW3 = SYNCWORD[15:12]

SW1

SW2

SW3

if different from 'F', else '0'

if different from 'F', else '0'

if different from 'F', else '0'

if different from 'F', else '0'

Synchronisation Header

Each box corresponds to 4 bits. Hence the preamble corresponds to 8 x 4 ''0' s or 4 bytes with the value 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. Pleas
e refer to [1] for details.