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CC113L
SWRS108A
Page 32 of 68
17.2
RSSI
The RSSI value is an estimate of the signal
power level in the chosen channel. This value
is based on the current gain setting in the RX
chain and the measured signal level in the
channel.
In RX mode, the RSSI value can be read
continuously from the
RSSI status register
until the demodulator detects a sync word
(when sync word detection is enabled). At that
point the RSSI readout value is frozen until the
next time the chip enters the RX state.
The RSSI value is given in dBm with a dB
resolution.
The
RSSI
update
rate,
fRSSI,
depends on the receiver filter bandwidth
(BWchannel is defined in Section 13) and LENGTH
FILTER
channel
RSSI
BW
f
_
2
8
2
the
last
RSSI
value
of
the
packet
is
automatically added to the first byte appended
after the payload.
The RSSI value read from the
RSSI status
register is a 2s 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
gives
typical
values
for
the
typical plots of RSSI readings as a function of
input power level for different data rates.
Data rate [kBaud]
RSSI_offset [dB], 433 MHz
RSSI_offset [dB], 868 MHz
1.2
74
38.4
74
250
74
500
74
Table 23: Typical RSSI_offset Values
Figure 19: Typical RSSI Value vs. Input Power Level for Different Data Rates at 433 MHz
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
Input Power [dBm]
RS
S
I
R
e
a
d
o
u
t
[d
Bm
]
1.2 kBaud
38.4 kBaud
250 kBaud
500 kBaud
Note: It takes some time from the radio
enters RX mode until a valid RSSI value is
present in the RSSI register. Please see
DN505
[7] for details on how the RSSI
response time can be estimated.