CC1100-RTY1 datasheet(39/100 Pages) TI1

39 / 100 page

CC1100

SWRS038D

Page 39 of 92

17.4 Carrier Sense (CS)

Carrier Sense (CS) is used as a sync word

qualifier and for CCA and can be asserted

based on two conditions, which can be

individually adjusted:

•

CS is asserted when the RSSI is above a

programmable absolute threshold, and de-

asserted when RSSI is below the same

threshold (with hysteresis).

•

CS is asserted when the RSSI has

increased with a programmable number of

dB from one RSSI sample to the next, and

de-asserted when RSSI has decreased

with the same number of dB. This setting

is not dependent on the absolute signal

level and is thus useful to detect signals in

environments with time varying noise floor.

Carrier Sense 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 by

setting IOCFGx.GDOx_CFG=14 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 40)

and the optional fast RX termination (see

Section 19.7 on page 46).

CS can be used to avoid interference from

other RF sources in the ISM bands.

17.4.1 CS Absolute Threshold

The absolute threshold related to the RSSI

value depends on the following register fields:

•

AGCCTRL2.MAX_LNA_GAIN

•

AGCCTRL2.MAX_DVGA_GAIN

•

AGCCTRL1.CARRIER_SENSE_ABS_THR

•

AGCCTRL2.MAGN_TARGET

•

For a given AGCCTRL2.MAX_LNA_GAIN

and AGCCTRL2.MAX_DVGA_GAIN setting the

absolute threshold can be adjusted ±7 dB in

steps

using

CARRIER_SENSE_ABS_THR

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.

It is strongly recommended to use SmartRF

Studio to generate the correct MAGN_TARGET

setting.

Table 26 and Table 27 show the typical RSSI

readout values at the CS threshold at 2.4

kBaud and 250 kBaud data rate respectively.

The default CARRIER_SENSE_ABS_THR=0 (0

dB) and MAGN_TARGET=3 (33 dB) have been

used.

For other data rates the user must generate

similar tables to find the CS absolute

threshold.

MAX_DVGA_GAIN[1:0]

000

-97.5

-91.5

-85.5

-79.5

001

-94

-88

-82.5

-76

010

-90.5

-84.5

-78.5

-72.5

011

-88

-82.5

-76.5

-70.5

100

-85.5

-80

-73.5

-68

101

-84

-78

-72

-66

110

-82

-76

-70

-64

111

-79

-73.5

-67

-61

Table 26: Typical RSSI Value in dBm at CS

Threshold with Default MAGN_TARGET at 2.4

kBaud, 868 MHz

MAX_DVGA_GAIN[1:0]

000

-90.5

-84.5

-78.5

-72.5

001

-88

-82

-76

-70

010

-84.5

-78.5

-72

-66

011

-82.5

-76.5

-70

-64

100

-80.5

-74.5

-68

-62

101

-78

-72

-66

-60

110

-76.5

-70

-64

-58

111

-74.5

-68

-62

-56

Table 27: Typical RSSI Value in dBm at CS

Threshold with Default MAGN_TARGET at 250

kBaud, 868 MHz

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.

CC1100-RTY1 Datasheet(PDF) 39 Page - Texas Instruments

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