CCA Measurement

This section describes, how the CCA measurement for the current channel is done. CCA stands for "clear channel assessment" and is a procedure which is used before transmitting a frame in order to ensure, that the transmission of another station is not disturbed.

The CCA measurement is controlled with two parameters mode and edthrs:

Parameters:

	mode	The CCA-Mode parameter is set in the sub register SR_CCA_MODE and has the following meaning: mode=1 Energy above threshold. CCA shall report a busy medium upon detecting any energy above the ED threshold. mode=2 Carrier sense only. CCA shall report a busy medium only upon the detection of a signal with the modulation and spreading characteristics of IEEE 802.15.4. This signal may be above or below the ED threshold. mode=3 Carrier sense with energy above threshold. CCA shall report a busy medium only upon the detection of a signal with the modulation and spreading characteristics of IEEE 802.15.4 with energy above the ED threshold.
	edthrs	If a signal is present, which has an energy value above the energy threshold, the CCA measurement reports a busy medium. The edthrs value is a 4 bit value, which is converted to an energy value =RSSI_BASE_VAL+2*edthrs. This parameter is only relevant for mode=1 and mode=3.

The CCA measurement itself is carried out with one of the sequences PHY_DO_CCA. It returns two values cca and done:

Return values:

	cca	This parameter is retrieved from the sub register SR_CCA_STATUS and has the following meaning: 0 : medium is busy 1 : medium is free
	done	This parameter is retrieved from the sub register SR_CCA_DONE. The value 1 means, that a previously started CCA measurement is completed correctly, otherwise the CCA measurement is not (yet) completed.

Note:

Each read access to register RG_TRX_STATUS resets the sub registers SR_CCA_STATUS and SR_CCA_DONE. Therefore the register RG_TRX_STATUS needs to be read only once to a variable trx_status and the values for cca and done needs to be extracted by computing cca = (trx_status>>6)&1 and done = (trx_status>>7)&1.

The following methods are defined:

• PHY_SET_CCA_MODE
• PHY_GET_CCA_MODE
• PHY_DO_CCA

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PHY_SET_CCA_MODE

This sequence sets the parameters mode and edthrs for the CCA measurement.

Code example

    trx_bit_write(SR_CCA_MODE, mode);
    trx_bit_write(SR_CCA_ED_THRES, edthrs);

PHY_GET_CCA_MODE

This sequence returns the currently configured parameters mode and edthrs .

Code example

    mode = trx_bit_read(SR_CCA_MODE);
    edthrs = trx_bit_read(SR_CCA_ED_THRES);

PHY_DO_CCA

This sequence assumes that the CCA measurement was previously configured according to PHY_SET_CCA_MODE.

CCA measurements can be done from state RX_ON. The measurement is started by writing the value 1 to the sub register SR_CCA_REQUEST. The measurement is finished after 140us and the radio transceiver is switched back to state RX_ON.

The completion of the measurement can be verified in the sub register SR_CCA_DONE. In general, the result parameters for the measurement need special handling, which is described here.

Code example

    trx_bit_write(SR_CCA_REQUEST, 1);
    delay_us(140);
    stat = trx_reg_read(RG_TRX_STATUS);
    cca=(stat>>6)&1;
    done=(stat>>7)&1;
    ASSERT(done==1);

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