NRF52820-QDAA-R datasheet(180/437 Pages) NORDIC

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Peripherals

IEEE 802.15.4 implements a listen-before-talk channel access method to avoid collisions when

transmitting, known as carrier sense multiple access with collision avoidance (CSMA-CA). The key part of

this is measuring if the wireless medium is busy or not.

The following clear channel assesment modes are supported:

• CCA Mode 1 (energy above threshold) - The medium is reported busy upon detecting any energy above

the ED threshold.

• CCA Mode 2 (carrier sense only) - The medium is reported busy upon detection of a signal compliant

with the IEEE 802.15.4 standard with the same modulation and spreading characteristics.

• CCA Mode 3 (carrier sense with energy above threshold) - The medium is reported busy using a logical

combination (AND/OR) between the results from CCA Mode 1 and CCA Mode 2.

The clear channel assessment should survey a period equal to 8 symbols or 128 µs.

The RADIO must be in receive mode and be able to receive correct packets when performing the CCA. The

shortcut between READY and START must be disabled if baseband processing is not to be performed while

the measurement is running.

CCA Mode 1

CCA Mode 1 is enabled by first configuring the field CCACTRL.CCAMODE=EdMode and writing the

CCACTRL.CCAEDTHRES field to a chosen value. Once the CCASTART task is written, the RADIO will

perform a ED measurement for 8 symbols and compare the measured level with that found in the

CCACTRL.CCAEDTHRES field. If the measured value is higher than or equal to this threshold, the CCABUSY

event is generated. If the measured level is less than the threshold, the CCAIDLE event is generated.

CCA Mode 2

CCA Mode 2 is enabled by configuring CCACTRL.CCAMODE=CarrierMode. The RADIO will sample to see

if a valid SFD is found during the 8 symbols. If a valid SFD is detected, the CCABUSY event is generated

and the device should not send any data. The CCABUSY event is also generated if the scan was performed

during an ongoing frame reception. In the case where the measurement period completes with no SFD

detection, the CCAIDLE event is generated. When CCACTRL.CCACORRCNT is not zero, the algorithm will

look at the correlator output in addition to the SFD detection signal. If a SFD is reported during the scan

period, it will terminate immidiately indicating busy medium. Similarly, if the number of peaks above

CCACTRL.CCACORRTHRES crosses the CCACTRL.CCACORRCNT, the CCACTRL.CCABUSY event is generated.

If less than CCACORRCOUNT crossings are found and no SFD is reported, the CCAIDLE event will be

generated and the device can send data.

CCA Mode 3

CCA Mode 3 is enabled by configuring CCACTRL.CCAMODE=CarrierAndEdMode or

CCACTRL.CCAMODE=CarrierOrEdMode, performing the required logical combination of the result from

CCA Mode 1 and 2. The CCABUSY or CCAIDLE events are generated by ANDing or ORing the energy above

threshold and carrier detection scans.

Shortcuts

An ongoing CCA can always be stopped by issuing the CCASTOP task. This will trigger the associated

CCASTOPPED event.

For CCA mode automation, a number of shortcuts are available.

• To automatically switch between RX (when performing the CCA) and to TX where the packet is sent,

the shortcut between CCAIDLE and TXEN, in conjunction with the short between CCAIDLE and STOP

muse be used.

4463_156 v1.0

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NRF52820-QDAA-R Datasheet(PDF) 180 Page - Nordic Semiconductor

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