CC2500 Datasheet, PDF (24/76 Pages) Texas Instruments – Single Chip Low Cost Low Power RF Transceiver

CC2500
12 Data Rate Programming
The data rate used when transmitting, or the
data rate expected in receive 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 be set from 1.2 kbps to 500
kbps with the minimum step size of:
Data rate
start
0.8 kbps
3.17 kbps
6.35 kbps
12.7 kbps
25.4 kbps
50.8 kbps
101.6 kbps
203.1 kbps
406.3 kbps
Typical
data rate
1.2/2.4
kbps
4.8 kbps
9.6 kbps
19.6 kbps
38.4 kbps
76.8 kbps
153.6 kbps
250 kbps
500 kbps
Data rate
stop
3.17 kbps
6.35 kbps
12.7 kbps
25.4 kbps
50.8 kbps
101.6 kbps
203.1 kbps
406.3 kbps
500 kbps
Data rate
step size
0.0062 kbps
0.0124 kbps
0.0248 kbps
0.0496 kbps
0.0992 kbps
0.1984 kbps
0.3967 kbps
0.7935 kbps
1.5869 kbps
Table 19: Data rate step size
13 Receiver Channel Filter Bandwidth
In order to meet different channel width
requirements, the receiver channel filter is
programmable. The MDMCFG4.CHANBW_E and
MDMCFG4.CHANBW_M configuration registers
control the receiver channel filter bandwidth,
which scales with the crystal oscillator
frequency. The following formula gives the
relation between the register settings and the
channel filter bandwidth:
BW_channel
=
f _XOSC
8 ⋅ (4 + CHANBW _ M )·2^{CHANBW_ E}
The CC2500 supports the following channel
filter bandwidths:
MDMCFG4.
CHANBW_M
00
01
10
11
MDMCFG4.CHANBW_E
00 01 10 11
812 406 203 102
650 325 162 81
541 270 135 68
464 232 116 58
Table 20: Channel filter bandwidths [kHz]
(assuming a 26 MHz crystal)
Above 300 kHz bandwidth, however, the
sensitivity and blocking performance may be
somewhat degraded. For best performance,
the channel filter bandwidth should be
selected so that the signal bandwidth occupies
at most 80% of the channel filter bandwidth.
The channel centre tolerance due to crystal
accuracy should also be subtracted from the
signal bandwidth. The following example
illustrates this:
With the channel filter bandwidth set to 600
kHz, the signal should stay within 80% of 600
Preliminary Data Sheet (rev.1.1.) SWRS040
Page 24 of 77