CC2500_06 Datasheet, PDF (33/84 Pages) Texas Instruments – Single Chip Low Cost Low Power RF Transceiver

CC2500
Carrier Sense (CS) can be used as a sync
word qualifier that requires the signal level to
be higher than the threshold for a sync word
search to be performed. The signal can also
be observed on one of the GDO pins and in
the status register bit PKTSTATUS.CS.
Other uses of Carrier Sense include the TX-If-
CCA function (see Section 17.5 on page 33)
and the optional fast RX termination (see
Section 19.7 on page 38).
CS can be used to avoid interference from e.g.
WLAN.
17.4.1 CS Absolute Threshold
The absolute threshold related to the RSSI
value is given by:
THR_RSSI = MAGN _ TARGET +
CARRIER _ SENSE _ ABS _ THR − GAIN_MAX
The maximum possible gain can be reduced
using the AGCCTRL2.MAX_LNA_GAIN and
AGCCTRL2.MAX_DVGA_GAIN register fields.
CARRIER_SENSE_ABS_THR is programmable
in 1 dB steps from -7 dB to + 7dB. Table 26
and Table 27 show the RSSI readout values
at the CS threshold at 2.4 kbps and 250 kbps
data rate respectively. The default
CARRIER_SENSE_ABS_THR = 0 (0 dB) and
MAGN_TARGET = 3 (33 dB) have been used.
MAX_DVGA_GAIN[1:0]
00 01 10 11
000 -96 -90 -84 -78.5
001 -94.5 -89 -83 -77.5
010 -92.5 -87 -81 -75
011 -91 -85 -78.5 -73
100 -87.5 -82 -76 -70
101 -85 -79.5 -73.5 -67.5
110 -83 -76.5 -70.5 -65
111 -78 -72 -66 -60
Table 27: Typical RSSI value in dBm at CS
threshold with default MAGN_TARGET at
250 kbps
If the threshold is set high, i.e. only strong
signals are wanted, the threshold should be
adjusted upwards by first reducing the
MAX_LNA_GAIN value and then the
MAX_DVGA_GAIN value. This will reduce
power consumption in the receiver front end,
since the highest gain settings are avoided.
The MAGN_TARGET setting is a compromise
between blocker tolerance/selectivity and
sensitivity. The value sets the desired signal
level in the channel into the demodulator.
Increasing this value reduces the headroom
for blockers, and therefore close-in selectivity.
MAX_DVGA_GAIN[1:0]
00 01 10 11
000 -99 -93 -87 -81.5
001 -97 -90.5 -85 -78.5
010 -93.5 -87 -82 -76
011 -91.5 -86 -80 -74
100 -90.5 -84 -78 -72.5
101 -88 -82.5 -76 -70
110 -84.5 -78.5 -73 -67
111 -82.5 -76 -70 -64
Table 26: Typical RSSI value in dBm at CS
threshold with default MAGN_TARGET at 2.4
kbps
17.4.2 CS relative threshold
The relative threshold detects sudden changes
in the measured signal level. This setting is not
dependent on the absolute signal level and is
thus useful to detect signals in environments
with a time varying noise floor. The register
field AGCCTRL1.CARRIER_SENSE_REL_THR
is used to enable/disable relative CS, and to
select threshold of 6 dB, 10 dB or 14 dB RSSI
change
17.5 Clear Channel Assessment (CCA)
The Clear Channel Assessment is used to
indicate if the current channel is free or busy.
The current CCA state is viewable on any of
the GDO pins.
MCSM1.CCA_MODE selects the mode to use
when determining CCA.
When the STX or SFSTXON command strobe is
given while CC2500 is in the RX state, the TX
state is only entered if the clear channel
requirements are fulfilled. The chip will
PRELIMINARY Data Sheet (Rev.1.2) SWRS040A
Page 33 of 83