CC112X/CC1175
SWRU295C
Page 38 of 108
In cases where
is set to freeze the RSSI value after a sync word
is detected, it is important that preamble and sync word is long enough so that the RSSI represent the
RSSI of the packet and not of noise received prior to the preamble.
Assume a data rate of 1.2 ksps and the following register configurations:
Equation 16 can be used to find the max RSSI update rate. For this example it is assumed that the
radio has been in RX for some time before a packet is received so T0 can be ignored.
)
1
2
(
)
2
1
(
Rate
Update
RSSI
_
_
CNT
VALID
RSSI
T
T
us
320
10
32
20
8
)
48
1
16
(
Factor
Decimation
FACT
BB_CIC_DEC
)
48
_WAIT
AGC_SETTLE
16
(
T1
6
XOSC
f
us
321.44
10
32
46
20
8
2
46
Factor
Decimation
FACT
BB_CIC_DEC
2
T2
6
4
2
4
ZE
AGC_WIN_SI
XOSC
f
ms
5.77
)
1
2
(
)
44
.
321
320
(
Rate
Update
RSSI
3
To make sure that the RSSI measurement is done during the packet and not on noise, preamble +
sync word should be greater than twice the RSSI update rate.
With a data rate of 2.4 ksps this means that a minimum of 28 bits preamble/sync must be received for
the RSSI readout to be correct. For this example it is assumed that the radio has been in RX for a
time longer than the max CS response time (see Equation 16) when the preamble and sync word is
received.
5.8.1
Carrier Sense (CS)
Carrier Sense (CS) is asserted when the RSSI is above a programmable CS threshold,
AGC_CS_THR
,
and de-asserted when RSSI is below the same threshold. The CS threshold should be set high
enough so that CS is de-asserted when only background noise is present and low enough so that CS
is asserted when a wanted signal is present. Different usage of CS includes:
Sync word qualifier: When
MDMCFG1.CARRIER_SENSE_GATE = 1
, the demodulator will not
start to look for a sync word before CS is asserted
Clear Channel Assessment (CCA) and TX on CCA
Avoid interference from other RF sources in the ISM band
RX termination
The latter is described in Section 8.5. By setting the
IOCFGX.GPIOX_CFG = CARRIER_SENSE
(17),
GPIOx will indicate if a carrier is present. The CS signal is evaluated each time a new internal RSSI
estimate is computed. The
signal must only be interpreted when it is valid, as
indicated by
. This signal can be routed to GPIOX by setting
IOCFGX.GPIOX_CFG = CARRIER_SENSE_VALID
(16) to help evaluate this in real-time. The two
signals can also be read from the
5.9
Collision Detector
If a packet is currently being received by the radio (data is put in the RX FIFO) when a new packet,
with higher signal energy, appears on the air and blocks the packet being received, a collision has
found place. A collision can be handled in several different ways.
Not handled. The current packet received will have errors and can be discarded. This method will
work well in most systems as RF protocols have capabilities for re-transmissions to solve these
issues.