CC2500_06 Datasheet, PDF (32/84 Pages) Texas Instruments – Single Chip Low Cost Low Power RF Transceiver

CC2500
If PKTCTRL1.APPEND_STATUS is enabled the
RSSI value at sync word detection is
automatically added to the first byte appended
after the data.
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
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
[kbps]
2.4
10
250
500
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 13: 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.
PRELIMINARY Data Sheet (Rev.1.2) SWRS040A
Page 32 of 83