CC2420Z-RTR1 datasheet(37/92 Pages) TI

37 / 92 page

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

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.

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).

IEEE 802.15.4

Preamble

SFD

CC2420

2·(PREAMBLE_LENGTH + 1) zero symbols

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'

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. Please refer to [1] for details.

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