CC2500 datasheet(31/76 Pages) TI

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CC2500

Preliminary Data Sheet (rev.1.1.)

SWRS040

Page 31 of 77

Data rate

RSSI_offset (decimal)

2.4 kbps

10 kbps

250 kbps

500 kbps

Table 25: Typical RSSI_offset values

-120.0

-110.0

-100.0

-90.0

-80.0

-70.0

-60.0

-50.0

-40.0

-30.0

-20.0

-10.0

0.0

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

Input pow er [dBm]

2.4 kbps

10 kbps

250 kbps

250 kbps, reduced current

500 kbps

Figure 9: Typical RSSI value vs. input power level for some typical data rates

17.4 Carrier Sense (CS)

The Carrier Sense flag is used as a sync word

qualifier and for CCA. The CS flag can be set

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 a time varying noise

floor.

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 32)

and the optional fast RX termination (see

Section 19.7 on page 37).

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 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

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