CC2550 Datasheet, PDF (19/50 Pages) Texas Instruments – Single Chip Low Cost Low Power RF-Transmitter

CC2550
With CC2550, in combination with a CC2500 at
the receiver end, this can be done
automatically by setting WHITE_DATA=1 in the
PKTCTRL0 register. 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.
20.2 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 synch 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.
CC2550 supports both constant packet length
protocols and variable 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.
With PKTCTRL0.LENGTH_CONFIG=2, the
packet length is set to infinite and transmission
will continue until turned off manually. The
infinite mode can be turned off while a packet
is being transmitted. As described in the next
section, this can be used to support packet
formats with different length configuration than
natively supported by CC2550.
20.2.1 Arbitrary length field configuration
By utilizing the infinite packet length option,
arbitrary packet length is available. At the start
of the packet, the infinite mode must be active.
When less than 256 bytes remains of the
packet, the MCU sets the PKTLEN register to
mod(length, 256), disables infinite packet
length and activates fixed length packets.
When the internal byte counter reaches the
PKTLEN value, the packet transmission ends.
Automatic CRC appending can be used (by
setting PKTCTRL0.CRC_EN to 1).
When for example a 454-byte packet is to be
transmitted, the MCU does the following:
• Set PKTCTRL0.LENGTH_CONFIG=2 (10).
• Pre-program the PKTLEN register to
mod(454,256)=198.
• Transmit at least 198 bytes, for example
by filling the 64-byte TX FIFO four times
(256 bytes transmitted).
• Set PKTCTRL0.LENGTH_CONFIG=0 (00).
• The transmission ends when the packet
counter reaches 198. A total of
256+198=454 bytes are transmitted.
Preliminary Data Sheet (rev. 1.1) SWRS039
Page 19 of 51