CC2420-RTR1 datasheet(50/92 Pages) TI

50 / 92 page

CC2420

SWRS041B

Page 50 of 89

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

programmed by RSSI.CCA_THR. The

threshold value can be programmed in

steps of 1 dB. A CCA hysteresis can also

programmed

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:

Reserved

Clear channel when received energy is below

threshold.

Clear channel when not receiving valid IEEE

802.15.4 data.

Clear 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

channel

the

FSCTRL.FREQ

be set to:

FSCTRL.FREQ

= 357 + 5 (k-11)

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