MRF24XA_15 Datasheet, PDF (214/258 Pages) Microchip Technology – Low-Power, 2.4 GHz ISM-Band IEEE 802.15.4™ RF

MRF24XA
9.6 Clear Channel Assessment (CCA)
Clear Channel Assessment is a function within CSMA/
CA to determine whether the wireless medium is ready
and able to receive data, therefore the transmitter can
start sending the data.
CCA is implemented outside of the MAC. This enables
the radio to transmit in the presence of interference
from other wireless protocols that operate on the same
frequency.
CCA may be performed using either Energy Detection
(ED), Carrier Sense (CS) or a combination of both.
9.6.1 CCA CONFIGURATION
CCA is automatically executed (potentially multiple
times) as part of the CSMA-CA procedure when the
TXST register bit is set and CSMA-CA is enabled.
The following register bits are used in the Configuration
of CCA:
• CCAMODE<1:0>
• CCALEN<1:0>
• CSTHR<3:0>
• RSSITHR<5:0>
9.6.1.1 Energy Detection (ED)
When CCAMODE<1:0> = 10, the CCA reports a busy
medium upon detecting energy above the energy
detection threshold defined in the RSSITHR<5:0> reg-
ister bits.
To use this method of CCA, the following Configuration
must be used:
• CCALEN<1:0> = Measurement duration
• CCAMODE<1:0> = 10
• RSSITHR<5:0> = RSSI threshold value
The mapping between the RSSITHR threshold and the
power level is shown in Figure 9-4 and Equation 9-1.
9.6.1.2 Carrier Sense (CS)
When CCAMODE<1:0> = 01, the CCA reports a busy
medium upon detecting of a signal with particular mod-
ulation and spreading characteristics.
To use this method of CCA, the following Configuration
must be used:
• CCALEN<1:0> = Measurement duration
• CCAMODE<1:0> = 01
• CSTHR<3:0> = Carrier sense threshold
9.6.1.3 Carrier Sense with Energy Detection
When CCAMODE<1:0> = 11, the CCA reports a busy
medium upon detecting of a signal with particular mod-
ulation and spreading characteristics and energy
above the energy detection threshold defined in the
RSSITHR<5:0> register bits. To use this method of
CCA, the following Configuration must be used:
• CCALEN<1:0> = Measurement duration
• CCAMODE<1:0> = 11
• RSSITHR<5:0> = RSSI threshold value
• CSTHR<3:0> = Carrier sense threshold
Note:
In the corresponding CCA modes the
radio measures EDMEAN<7:0>. If
EDMEAN<7:2> is greater than RSSI-
THR<5:0>, CCABUSY is set. Example: To
set the RSSI threshold where the chip
measures 0x30 EDMEAN, RSSI-
THR<5:0> must be 0x0C.
9.6.2 CCA OPERATION
MRF24XA automatically initiates CCA as part of the
CSMA-CA algorithm. CCA operation is requested inde-
pendently for software CSMA-CA, or for test purpose
through the CCAST bit.
DS70005023C-page 214
Preliminary
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