Texas Instruments CC2500, Manual

Page: 28 / 96

CC2500
SWRS040C Page 28 of 89
14.3 Byte Synchronization
Byte synchronization is achieved by a
continuous sync word search. The sync word
is a 16 bit configurable field (can be repeated
to get a 32 bit) that is automatically inserted at
the start of the packet by the modulator in
transmit mode. The demodulator uses this
field to find the byte boundaries in the stream
of bits. The sync word will also function as a
system identifier, since only packets with the
correct predefined sync word will be received if
the sync word detection in RX is enabled in
register
MDMCFG2
(see Section 17.1). The
sync word detector correlates against the
user-configured 16 or 32 bit sync word. The
correlation threshold can be set to 15/16,
16/16, or 30/32 bits match. The sync word can
be further qualified using the preamble quality
indicator mechanism described below and/or a
carrier sense condition. The sync word is
configured through the
SYNC1
and
SYNC0
registers.
In order to make false detections of sync
words
less likely, a mechanism called
preamble quality indication (PQI) can be used
to qualify the sync word. A threshold value for
the preamble quality must be exceeded in
order for a detected sync word to be accepted.
See Section 17.2 on page 34 for more details.
15 Packet Handling Hardware Support
The
CC2500
has built-in hardware support for
packet oriented radio protocols.
In transmit mode, the packet handler can be
configured to add the following elements to the
packet stored in the TX FIFO:

A programmable number of preamble
bytes

A two byte synchronization (sync) word.
Can be duplicated to give a 4-byte sync
word (recommended). It is not possible to
only insert preamble or only insert a sync
word.

A CRC checksum computed over the data
field
The recommended setting is 4-byte preamble
and 4-byte sync word, except for 500 kBaud
data rate where the recommended preamble
length is 8 bytes.
In addition, the following can be implemented
on the data field and the optional 2-byte CRC
checksum:

Whitening of the data with a PN9
sequence.

Forward error correction by the use of
interleaving
and coding of the data
(convolutional coding).
In receive mode, the packet handling support
will
de-construct the data packet by
implementing the following (if enabled):

Preamble detection

Sync word detection

CRC computation and CRC check

One byte address check

Packet length check (length byte checked
against a programmable maximum length)

De-whitening

De-interleaving and decoding
Optionally, two status bytes (see Table 21 and
Table 22) with RSSI value, Link Quality
Indication, and CRC status can be appended
in the RX FIFO.
Bit Field Name Description
7:0 RSSI RSSI value
Table 21: Received Packet Status Byte 1
(first byte appended after the data)
Bit Field Name Description
7 CRC_OK 1: CRC for received data OK (or
CRC disabled)
0: CRC error in received data
6:0 LQI The Link Quality Indicator
estimates how easily a received
signal can be demodulated
Table 22: Received Packet Status Byte 2
(second byte appended after the data)
Note that register fields that control the packet
handling features should only be altered when
CC2500
is in the IDLE state.