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1501

IEEE 802.15.4

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• If ccaEnEnergy = 0 and ccaEnCorr = 0 and ccaEnSync = 1, then ccaState = ccaSync

• Otherwise, if ccaEnSync = 1 and ccaSyncOp = 0, then:

– If either ccaTmp or ccaSync is Busy, then ccaState = Busy;

– Otherwise, if ccaTmp is Invalid, then ccaState = Invalid;

– Otherwise, ccaState = Idle

• Otherwise, if ccaEnSync = 1 and ccaSyncOp = 1, then:

– If either ccaTmp or ccaSync is Idle, then ccaState = Idle;

– Otherwise, if ccaTmp is Invalid, then ccaState = Invalid;

– Otherwise, ccaState = Busy

The ccaSync CCA state is required to be Idle for the overall CCA state to be Idle, according to

Table 23-68. Thus, to comply, ccaEnSync is 1 and cceSyncOp is 0.

CCA mode 1, as defined in Table 23-68, is implemented by setting ccaEnEnergy = 1 and ccaEnCorr = 0.

CCA mode 2 is implemented by setting ccaEnEnergy = 0 and ccaEnCorr = 1. CCA mode 3 is

implemented by setting ccaEnEnergy = 1 and ccaEnCorr = 1. With CCA mode 3, ccaCorrOp is allowed to

be either 0 or 1; this distinguishes between the logical operator AND (1) and OR (0) as described in

Table 23-68.

The CCA states and the current RSSI can be read by the system CPU by issuing the immediate

command CMD_IEEE_CCA_REQ. If a CMD_IEEE_CSMA operation is running in the foreground, the

radio CPU also monitors the CCA autonomously.

23.5.4.2 Energy Detect Scan Operation

The energy detect scan radio operation is a background-level operation that starts with the

CMD_IEEE_ED_SCAN command, and uses a command structure as given in Table 23-57.

At the start of an RX operation, the radio CPU waits for the start trigger, then programs the frequency

based on the channel parameter. If channel is 0xFF, the operation keeps running on an already-

configured channel. This requires that the operation follows another receive operation or a synthesizer

programming operation. If the frequency synthesizer is not running, the operation ends with an error. After

programming the frequency, the radio CPU configures the receiver to receive IEEE 802.15.4 packets, but

it does not store any received data.

While the receiver is running, CCA is updated as described in Section 23.5.4.1.5, CCA monitoring. When

the demodulator obtains sync on a frame, the PHY header is read. This is used only for determining the

carrier sense based on sync found, and sync search restarts immediately afterwards.

The energy detect scan operation ends under the same conditions as the RX operation, as described in

Section 23.5.4.1.4, End of Receive Operation. Before the operation ends, the radio CPU writes the

maximum-observed RSSI during the energy detect scan operation to maxRssi.

23.5.4.3 CSMA-CA Operation

The CSMA-CA operation is a foreground-level operation that runs on top of a receive or energy-detect

scan operation. If run on top of an energy-detect scan operation, this does not perform the energy-detect

scan procedure, but starts a receiver without having to receive packets. This operation starts with the

CMD_IEEE_CSMA command, and uses the command structure given in Table 23-58, IEEE 802.15.4

CSMA-CA Command Structure.

At the start of a CSMA-CA operation, the radio CPU waits for the start trigger.

The radio CPU maintains a variable CW which initializes to csmaConfig.initCW.

If remainingPeriods is nonzero at the start of the command, the radio CPU delays for that number of

backoff periods (default 320 µs) measured from the start trigger before proceeding. Otherwise, the radio

CPU draws a pseudo-random number in the range 0 to 2

(BE)–1

, where BE is given by (Table 23-58) . The

radio CPU then waits that number of backoff periods from the start trigger before proceeding.

1508

Radio SWCU117A–February 2015–Revised March 2015

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