Texas Instruments 3138 155 232931 Clear Channel Assessment, Frequency and Channel Programming

25 Clear Channel Assessment

The clear channel assessment signal is based on the measured RSSI value and a programmable threshold. The clear channel assessment function is used to implement the CSMA-CA functionality specified in [1]. CCA is valid when the receiver has been enabled for at least 8 symbol periods.

Carrier sense threshold level is programmed by RSSI.CCA_THR. The threshold value can be programmed in steps of 1 dB. A CCA hysteresis can also

be programmed in the MDMCTRL0.CCA_HYST control bits.

All 3 CCA modes specified by [1] are implemented in CC2420. They are set in MDMCTRL0.CCA_MODE, as can be seen in the register description. The different modes are:

CC2420

0Reserved

1Clear channel when received energy is below threshold.

2Clear channel when not receiving valid IEEE 802.15.4 data.

3Clear channel when energy is below threshold and not receiving valid IEEE 802.15.4 data

Clear channel assessment is available on the CCA output pin. CCA is active high, but the polarity may be changed by setting the IOCFG0.CCA_POLARITY control bit.

Implementing CSMA-CA may easiest be done by using the STXONCCA command strobe, as described in the Radio control state machine section on page 43. Transmission will then only start if the channel is clear. The TX_ACTIVE status bit (see Table 5) may be used to detect the result of the CCA.

26 Frequency and Channel Programming

The operating frequency is set by programming the 10 bit frequency word located in FSCTRL.FREQ[9:0]. The operating frequency FC in MHz is given by:

FC = 2048 + FSCTRL.FREQ[9:0] MHz

The frequency can be programmed with 1 MHz resolution. In receive mode the actual LO frequency is FC – 2 MHz, since a 2 MHz IF is used. Direct conversion is used for transmission, so here the LO frequency equals FC. The 2 MHz IF is automatically set by CC2420, so the frequency programming is equal for RX and TX.

IEEE 802.15.4 specifies 16 channels within the 2.4 GHz band, in 5 MHz steps, numbered 11 through 26. The RF frequency of channel k is given by [1]:

FC = 2405 + 5 (k-11) MHz, k=11, 12, ..., 26

For operation in channel k, the FSCTRL.FREQ register should therefore be set to:

FSCTRL.FREQ = 357 + 5 (k-11)

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