CC2500 Datasheet, PDF (31/76 Pages) Texas Instruments – Single Chip Low Cost Low Power RF Transceiver

CC2500
Data rate
2.4 kbps
10 kbps
250 kbps
500 kbps
RSSI_offset (decimal)
71
69
72
72
Table 25: Typical RSSI_offset values
0.0
-10.0
-20.0
-30.0
-40.0
-50.0
-60.0
-70.0
-80.0
-90.0
-100.0
-110.0
-120.0
-120 -110 -100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0
Input pow er [dBm]
2.4 kbps
10 kbps
250 kbps
250 kbps, reduced current
500 kbps
Figure 9: Typical RSSI value vs. input power level for some typical data rates
17.4 Carrier Sense (CS)
The Carrier Sense 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 asserted 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 signals in
environments with a time varying noise
floor.
Carrier Sense (CS) 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 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 32)
and the optional fast RX termination (see
Section 19.7 on page 37).
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 is given by:
THR_RSSI = MAGN _ TARGET +
CARRIER _ SENSE _ ABS _ THR − GAIN_MAX
The maximum possible gain can be reduced
using the AGCCTRL2.MAX_LNA_GAIN and
AGCCTRL2.MAX_DVGA_GAIN register fields.
CARRIER_SENSE_ABS_THR is programmable
in 1 dB steps from -7 dB to + 7dB. Table 26
and Table 27 show the RSSI readout values
Preliminary Data Sheet (rev.1.1.) SWRS040
Page 31 of 77