Texas Instruments CC2500, Manual

Page: 36 / 96

CC2500
SWRS040C Page 36 of 89
IOCFGx.GDOx_CFG
=14 and in the status
register bit
PKTSTATUS.CS
.
Other uses of Carrier Sense include the TX-if-
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 interference 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.CARRIER_SENSE_ABS_THR

AGCCTRL2.MAGN_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_ABS_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
M
A
X
_
L
N
A
_
G
A
IN
[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
M
A
X
_
L
N
A
_
G
A
IN
[2
:0
]
111 -78 -72 -66 -60
Table 27: Typical RSSI Value in dBm at CS
Threshold with Default
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 Relative Threshold
The relative threshold detects sudden changes
in the measured signal level. This setting is not
dependent on the absolute signal level 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