CC2430 Datasheet, PDF (169/212 Pages) Texas Instruments – A True System-on-Chip solution for 2.4 GHz IEEE 802.15.4 / ZigBee-TM

CC2430
Radio : Clear Channel Assessment
therefore also provides an average correlation
value for each incoming packet, based on the
eight first symbols following the SFD. This
unsigned 7-bit value, which should be as high
as possible, can be looked upon as a
indication of the “chip error rate,” although
CC2430 does not perform chip decision.
As described in the Frame check sequence
section on page 162, the average correlation
value for the eight first symbols is appended to
each received frame together with the RSSI
and CRC OK/not OK when
MDMCTRL0L.AUTOCRC is set. A correlation
value of approx. 110 indicates a maximum
quality frame while a value of approx. 50 is
typically the lowest quality frames detectable
by CC2430.
Software must convert the correlation value to
the range 0-255 defined by [1], e.g. by
calculating:
LQI = (CORR – a) · b
limited to the range 0-255, where a and b are
found empirically based on PER
measurements as a function of the correlation
value.
A combination of RSSI and correlation values
may also be used to generate the LQI value.
14.25 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
MDMCTRL0H.CCA_HYST control bits.
All three CCA modes specified by [1] are
implemented in CC2430. These are set in
MDMCTRL0L.CCA_MODE, as can be seen in
the register description. The different modes
are:
00 Reserved
01 Clear channel when received energy
is below threshold.
10 Clear channel when not receiving
valid IEEE 802.15.4 data.
11 Clear channel when energy is below
threshold and not receiving valid
IEEE 802.15.4 data
Clear channel assessment is available on the
RFSTATUS.CCA RF register bit.
RFSTATUS.CCA is active high. This register bit
will also set the interrupt flag RFIF.IRQ_CCA.
Implementing CSMA-CA may easiest be done
by using the STXONCCA command strobe
given by the CSMA-CA/strobe processor, as
shown in the Radio control state machine
section on page 166. Transmission will then
only start if the channel is clear. The
TX_ACTIVE status bit in the RFSTATUS RF
register may be used to detect the result of the
CCA.
14.26 Frequency and Channel Programming
The operating frequency is set by
programming the 10 bit frequency word
located in FSCTRLH.FREQ[9:8] and
FSCTRLL.FREQ[7:0]. The operating
frequency F_C in MHz is given by:
F_C = 2048 + FREQ[9:0] MHz
where FREQ[9:0] is the value given by
FSCTRLH.FREQ[9:8]:FSCTRLL.FREQ[7:0]
In receive mode the actual LO frequency is F_C
– 2 MHz, since a 2 MHz IF is used. Direct
conversion is used for transmission, so here
the LO frequency equals F_C. The 2 MHz IF is
14.27 VCO and PLL Self-Calibration
automatically set by CC2430, so the frequency
programming is equal for RX and TX.
IEEE 802.15.4 specifies 16 channels within
the 2.4 GHz band, numbered 11 through 26.
The RF frequency of channel k is given by [1] :
F_C = 2405 + 5 (k-11) MHz, k=11, 12, ..., 26
For operation in channel
FSCTRLH.FREQ:FSCTRLL.FREQ
should therefore be set to:
k, the
register
FSCTRLH.FREQ:FSCTRLL.FREQ = 357 + 5 (k-11)
CC2430 Data Sheet (rev. 2.1) SWRS036F
Page 169 of 211