CC2500-RTR1 datasheet(33/84 Pages) TI

33 / 84 page

CC2500

PRELIMINARY Data Sheet (Rev.1.2)

SWRS040A

Page 33 of 83

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:

MAX

RSSI

GAIN

THR

ABS

SENSE

CARRIER

TARGET

MAGN

THR

−

The maximum possible gain can be reduced

using the AGCCTRL2.MAX_LNA_GAIN and

AGCCTRL2.MAX_DVGA_GAIN

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]

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]

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.

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

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