CC2500 Datasheet, PDF (26/76 Pages) Texas Instruments – Single Chip Low Cost Low Power RF Transceiver

CC2500
In receive mode, the packet handling support
will de-construct the data packet:
• Preamble detection.
• Sync word detection.
• Optional one byte address check.
• Optionally compute and check CRC.
• Optionally append two status bytes (see
Table 21 and Table 22) with RSSI value,
Link Quality Indication and CRC status.
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.
15.1 Data Whitening
From a radio perspective, the ideal over the air
data are random and DC free. This results in
the smoothest power distribution over the
occupied bandwidth. This also gives the
regulation loops in the receiver uniform
operation conditions (no data dependencies).
Real world data often contain long sequences
of zeros and ones. Performance can then be
improved by whitening the data before
transmitting, and de-whitening in the receiver.
With CC2500, this can be done automatically
by setting PKTCTRL0.WHITE_DATA=1. All
data, except the preamble and the sync word,
are then XOR-ed with a 9-bit pseudo-random
(PN9) sequence before being transmitted. At
the receiver end, the data are XOR-ed with the
same pseudo-random sequence. This way, the
whitening is reversed, and the original data
appear in the receiver.
Setting PKTCTRL0.WHITE_DATA=1 is recom-
mended for all uses, except when over-the-air
compatibility with other systems is needed.
15.2 Packet Format
The format of the data packet can be
configured and consists of the following items:
• Preamble
• Synchronization word
• Length byte or constant programmable
packet length
• Optional address byte
• Payload
• Optional 2 byte CRC
The preamble pattern is an alternating
sequence of ones and zeros (01010101…).
The minimum length of the preamble is
programmable. When enabling TX, the
modulator will start transmitting the preamble.
When the programmed number of preamble
bytes has been transmitted, the modulator will
send the sync word and then data from the TX
FIFO if data is available. If the TX FIFO is
empty, the modulator will continue to send
preamble bytes until the first byte is written to
the TX FIFO. The modulator will then send the
sync word and then the data bytes. The
number of preamble bytes is programmed with
the MDMCFG1.NUM_PREAMBLE value.
The synchronization word is a two-byte value
set in the SYNC1 and SYNC0 registers. The
sync word provides byte synchronization of the
incoming packet. A one-byte sync word can be
emulated by setting the SYNC1 value to the
preamble pattern. It is also possible to emulate
a 32 bit sync word by using
MDMCFG2.SYNC_MODE=3 or 7. The sync word
will then be repeated twice.
CC2500 supports both fixed packet length
protocols and variable packet length protocols.
Variable or fixed packet length mode can be
used for packet up to 255 bytes. For longer
packets, infinite packet length mode must be
used.
Fixed packet length mode is selected by
setting PKTCTRL0.LENGTH_CONFIG=0. The
desired packet length is set by the PKTLEN
register. The packet length is defined as the
payload data, excluding the length byte and
the optional automatic CRC. In variable length
mode, PKTCTRL0.LENGTH_CONFIG=1, the
packet length is configured by the first byte
after the sync word. The PKTLEN register is
Preliminary Data Sheet (rev.1.1.) SWRS040
Page 26 of 77