While the receiver is running, the radio CPU monitors some signals for use in clear-channel assessment. This monitoring is controlled by ccaOpt. There are three sources for CCA: RSSI above level (ccaEnergy), carrier sense based on the correlation value (ccaCorr), and carrier sense based on sync found (ccaSync). Each of these may have the state BUSY, IDLE, or INVALID.

The RSSI above-level is maintained by monitoring the RSSI. If the RSSI is greater than or equal to ccaRssiThr, ccaEnergy is busy. If the RSSI is smaller than ccaRssiThr, ccaEnergy is IDLE. When an RSSI calculation has not yet been completed because the receiver started, ccaEnergy is INVALID.

The carrier-sense monitoring based on correlation value uses correlation peaks as defined for use in the SFD search algorithm in the receiver. If the number of correlation peaks observed in the last 8 symbol periods (32 µs) is greater than ccaOpt.corrThr, ccaCorr is BUSY; otherwise ccaCorr is IDLE. The value of ccaOpt.corrThr can be from 0 to 3. While the receiver is receiving a frame, ccaCorr is BUSY regardless of the observed correlation peaks. If the time since the receiver started is less than 8 symbol periods and the number of correlation peaks observed since the receiver started is less than or equal to ccaOpt.corrThr, ccaCorr is INVALID.

The carrier-sense monitoring based on sync found is maintained by the radio CPU as follows. If sync is obtained on the receiver, the radio CPU checks the PHY header to find the frame length. The radio CPU considers the channel to be busy for the duration of this frame. This check is done even if reception of the frame is stopped due to the frame filtering and sync search is restarted. If sync is found again while the channel is viewed as BUSY, the channel is viewed as Busy until both these frames have ended according to the observed frame lengths. The INVALID state is not used for ccaSync.

If the radio is transmitting an ACK or is suspended for running a TX operation, ccaEnergy, ccaCorr, and ccaSync are all BUSY.

The overall CCA state ccaState depends on the ccaEnEnergy, ccaEnCorr, and ccaEnSync bits of ccaOpt together with the ccaCorrOp and ccaSyncOp bits. The following rules apply for finding the ccaState (ccaTmp is a helper state in the description):

• If ccaEnEnergy = 0 and ccaEnCorr = 0 and ccaEnSync = 0, then ccaState = IDLE
• If ccaEnEnergy = 1 and ccaEnCorr = 0, then ccaTmp = ccaEnergy
• If ccaEnEnergy = 0 and ccaEnCorr = 1, then ccaTmp = ccaCorr
• If ccaEnEnergy = 1 and ccaEnCorr = 1 and ccaCorrOp = 0, then:
  • If either ccaEnergy or ccaCorr is BUSY, then ccaTmp = BUSY;
  • Otherwise, if either ccaEnergy or ccaCorr is INVALID, then ccaTmp = INVALID;
  • Otherwise, ccaTmp = IDLE
• If ccaEnEnergy = 1 and ccaEnCorr = 1 and ccaCorrOp = 1, then:
  • If either ccaEnergy or ccaCorr is IDLE, then ccaTmp = IDLE;
  • Otherwise, if either ccaEnergy or ccaCorr is Invalid, then ccaTmp = INVALID;
  • Otherwise, ccaTmp = BUSY
• 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 the IEEE 802.15.4 standard. Thus, to comply, ccaEnSync is 1 and cceSyncOp is 0.

CCA mode 1, as defined in the IEEE 802.15.4 standard, 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 the IEEE 802.15.4 standard.

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.