CC2500 Datasheet, PDF (32/76 Pages) Texas Instruments – Single Chip Low Cost Low Power RF Transceiver

CC2500
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 -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
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 to be set high, e.g. only
signals with good strength 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.
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
otherwise remain in RX. This feature is called
TX if CCA.
Four CCA requirements can be programmed:
• Always (CCA disabled, always goes to TX)
• If RSSI is below threshold
• Unless currently receiving a packet
• Both the above (RSSI below threshold and
not currently receiving a packet)
17.6 Link Quality Indicator (LQI)
The Link Quality Indicator is a metric of the
current quality of the received signal. If
PKTCTRL1.APPEND_STATUS is enabled, the
value is automatically appended to the end of
each received packet. The value can also be
read from the LQI status register. The LQI is
calculated over the 64 symbols following the
sync word (first 8 packet bytes for 2-ary
modulation). LQI is best used as a relative
measurement of the link quality, since the
value is dependent on the modulation format.
Preliminary Data Sheet (rev.1.1.) SWRS040
Page 32 of 77