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automatically added to the first byte 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 the 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
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
2.4 kBaud
Figure 13: Typical RSSI Value vs. Input Power Level for Some Typical Data Rates
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 asserted when the RSSI has
increased with a programmable number of
dB from one RSSI sample to the next, and
-110
-100
-90
-80
Input power [dBm]
10 kBaud
250 kBaud
SWRS040C
Table 25 provides typical values for the
RSSI_offset.
Figure 13 shows typical plots of RSSI readings
as a function of input power level for different
data rates.
Data Rate [kBaud]
2.4
10
250
500
Table 25: Typical RSSI_offset Values
-70
-60
-50
-40
-30
250 kBaud, reduced current
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 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
CC2500
RSSI_offset [dB]
71
69
72
72
-20
-10
0
500 kBaud
Page 35 of 89
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