Texas Instruments CC2500 - Page 36

CC2500
S
WRS040
C
Page
36
of
89
IOCFGx.GDOx_CFG
=14
and in the status
register bit
PKTSTATUS.CS
.
Other uses of Carrier Sense include th
e TX
-
i
f
-
CCA function (see Section
17.5
on page
37
)
and the optional fast RX termination (see
Section
19.7
on page
43
).
CS can be used to avoid int
erference from e.g.
WLAN.
17.4.1
CS Absolute Threshold
The absolute threshold related to the RSSI
value depends on the following register fields:

AGCCTRL2.MAX_LNA_GAIN

AGCCTRL2.MAX_DVGA_GAIN

AGCCTRL1.C
ARRIER_SENSE_ABS_THR

AGCCTRL2.M
AGN_TARGET
For a given
AGCCTRL2.MAX_LNA_GAIN
and
AGCCTRL2.MAX_DVGA_GAIN
setting the
absolute threshold can be adjusted ±7 dB in
steps of 1 dB using
CARRIER_SENSE_A
BS_THR
.
The
MAGN_TARGET
setting is a compromise
between blocker tolerance/selectivity and
sensitivity. The value sets the desired signal
level in the channel into the demodulator.
Increasing this value reduces the headroom
for
blockers, and therefore close
-
in selectivity.
It is strongly recommended to use SmartRF
®
Studio
[5]
to generate the correct
MAGN_TARGET
setting.
Table
26
and
Table
27
show the
typical
RSSI
readout values at the CS threshold at 2.4
kBaud
and 250
kBaud
data rate respectively.
The default
CARRIER_SENSE_ABS_THR
=0
(0
dB) and
MAGN_TARGET
=
3
(33 dB) have been
used.
For other data rates the user must generate
similar tables to find the CS absolute
threshold.
MAX_DVGA_GAIN[1:0]
00
01
10
11
000
-
99
-
93
-
87
-
81.5
001
-
97
-
90.5
-
85
-
78.5
010
-
93.5
-
87
-
82
-
76
011
-
91.5
-
86
-
80
-
74
100
-
90.5
-
84
-
78
-
72.5
101
-
88
-
82.5
-
76
-
70
110
-
84.5
-
78.5
-
73
-
67
MAX_LNA_GAIN[2:0]
111
-
82.5
-
76
-
70
-
64
Table
26
:
Typical RSSI Value in dBm at CS
Threshold with Default
MAGN_TARGET
at
2.4
kBaud
MAX_DVGA_GAIN[1:0]
00
01
10
11
000
-
96
-
90
-
84
-
78.5
001
-
94.5
-
89
-
83
-
77.5
010
-
92.5
-
87
-
81
-
75
011
-
91
-
85
-
78.5
-
73
100
-
87.5
-
82
-
76
-
70
101
-
85
-
79.5
-
73.5
-
67.5
110
-
83
-
76.5
-
70.5
-
65
MAX_LN
A_GAIN[2:0]
111
-
78
-
72
-
66
-
60
Table
27
:
Typical RSSI
V
alue in dBm at CS
T
hreshold with
D
efault
MAGN_TARGET
at 250
kBaud
If the threshold is set high, i.e. only strong
signals are wanted, the threshold should be
adjusted upwards by first reducing the
MAX_LNA_GAIN
value and then the
MAX_DVGA_GAIN
value. This
will reduce
power consumption in the receiver front end,
since the highest gain settings are avoided.
17.4.2
CS R
elative
T
hreshold
The relative threshold detects sudden changes
in the measured signal level. This setting is not
dependent on the absolute signal le
vel and is
thus useful to detect signals in environments
with a time varying noise floor. The register
field
AGCCTRL1.CARRIER_SENSE_REL_THR
is used to enable/disable relative CS, and to
select threshold of 6 dB, 10 dB or 14 dB
RSSI
change