Texas Instruments 3138 155 232931 manual Transmitted Synchronisation Header Length field

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

IEEE 802.15.4

Preamble

SFD

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 Page 37 of 89