Texas Instruments CC2500 User Manual

CC2500
S
WRS040
C
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 re
peated
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
-
c
onfigured 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 wo
rd 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 Ha
rdware 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 progra
mmable 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 checks
um computed over the data
field
The recommended setting is 4
-
byte preamb
le
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.

Fo
rward 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 detec
tion

CRC computation and CRC chec
k

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 N
ame
Description
7:0
RSSI
RSSI value
Table
21
: Received Packet
S
tatus
B
yte 1
(first byte appended after the data)
Bit
Field N
ame
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 demodu
lated
Table
22
: Received
P
acket
S
tatus
B
yte 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.

Texas Instruments CC2500 Specifications

General

Brand	Texas Instruments
Model	CC2500
Category	Transceiver
Language	English