Texas Instruments CC2500 User Manual

CC2500
S
WRS040
C
Page
35
of
89
automatically added to the
first byt
e appended
after the data
payload
.
The RSSI value read from the RSSI status
register is a 2’s complement number. The
following procedure can be used to convert the
RSSI reading to an absolute power level
(RSSI_dBm).
1)
Read the RSSI status register
2)
Convert t
he reading from a hexadecimal
number to a decimal number (RSSI_dec)
3)
If RSSI_dec
≥ 128 then RSSI_dBm =
(RSSI_dec
-
256)/2
–
RSSI_offset
4)
Else if RSSI_dec < 128 then RSSI_dBm =
(RSSI_dec)/2
–
RSSI_offset
Table
25
prov
ides
typical values for the
RSSI_offset.
Figure
13
shows typical plots of RSSI reading
s
as a function of input power level for different
data rates.
Data R
ate
[
kBaud
]
RSSI_offset [
dB
]
2.4
71
10
69
250
72
500
72
Table
25
: Typical RSSI_offset
V
alues
-120,0
-110,0
-100,0
-90,0
-80,0
-70,0
-60,0
-50,0
-40,0
-30,0
-20,0
-10,0
0,0
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
Input power [dBm]
RSSI readout [dBm]
2.4 kBaud
10 kBaud
250 kBaud
250 kBaud, reduced current
500 kBaud
Figure
13
: Typical RSSI
V
alue vs.
I
nput
P
ower
L
evel for
S
ome
T
ypical
D
ata
R
ates
17.4
Carrier Sense (CS)
The Carrier Sense
(CS)
flag is used as a sync
word qualifier
and for CCA. The CS flag can
be set based on two conditions, which can be
individually adjusted:

CS is asserted when the RSSI is above a
programmable absolute threshold, and de
-
asserted when RSSI is below the same
threshold (with hysteresis).

CS is asserte
d when the RSSI has
increased with a programmable number of
dB from one RSSI sample to the next, and
de
-
asserted when RSSI has decreased
with the same number of dB. This setting
is not dependent on the absolute signal
level and is thus useful to detect sig
nals in
environments with a time varying noise
floor.
Carrier Sense can be used as a sync word
qualifier that requires the signal level to be
higher than the threshold for a sync word
search to be performed.
The signal can also
be observed on one of the
GDO
pins by setting

Texas Instruments CC2500 Specifications

General

Brand	Texas Instruments
Model	CC2500
Category	Transceiver
Language	English