CC2550_06 Datasheet, PDF (18/55 Pages) Texas Instruments – Single Chip Low Cost Low Power RF Transmitter

CC2550
12 Data Rate Programming
The data rate used when transmitting is
programmed by the MDMCFG3.DRATE_M and
the MDMCFG4.DRATE_E configuration
registers. The data rate is given by the formula
below. As the formula shows, the programmed
data rate depends on the crystal frequency.
( ) _{RDATA =}
256 + DRATE _ M
2²⁸
⋅ 2^{DRATE _ E} _{⋅ f XOSC}
The following approach can be used to find
suitable values for a given data rate:
DRATE
_
E
=
⎢
⎢^log
⎢⎣
2
⎜⎜⎝⎛
R_DATA ⋅ 2 ²⁰
f _XOSC
⎟⎟⎠⎞⎥⎥⎦⎥
DRATE _ M = ^{RDATA ⋅ 228} − 256
f _XOSC ⋅ 2 ^{DRATE _ E}
If DRATE_M is rounded to the nearest integer
and becomes 256, increment DRATE_E and
use DRATE_M=0.
The data rate can programmed from 1.2 kbps
to 500 kbps with a minimum step size of:
Data rate
start
[kbps]
0.8
3.17
6.35
12.7
25.4
50.8
101.6
203.1
406.3
Typical
data rate
[kbps]
1.2/2.4
4.8
9.6
19.6
38.4
76.8
153.6
250
500
Data rate
stop [kbps]
3.17
6.35
12.7
25.4
50.8
101.6
203.1
406.3
500
Data rate
step size
[kbps]
0.0062
0.0124
0.0248
0.0496
0.0992
0.1984
0.3967
0.7935
1.5869
Table 16: Data rate step size
13 Packet Handling Hardware Support
The CC2550 has built-in hardware support for
packet oriented radio protocols.
In transmit mode, the packet handler will 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.
• Optionally whiten the data with a PN9
sequence.
• Optionally Interleave and Forward Error
Code the data.
• Optionally compute and add a CRC
checksum over the data field.
In a system where CC2550 is used as the
transmitter and CC2500 as the receiver the
recommended setting is 4-byte preamble and
4-byte sync word except for 500 kbps data rate
where the recommended preamble length is 8
bytes.
13.1 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 CC2550, in combination with a CC2500 at
the receiver end, 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 as shown in Figure 9.
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.
Data whitening can only be used when
PKTCTRL0.CC2400_EN = 0 (default).
PRELIMINARY Data Sheet (Rev.1.2) SWRS039A
Page 18 of 54