CC2500 datasheet(32/76 Pages) TI

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CC2500

Preliminary Data Sheet (rev.1.1.)

SWRS040

Page 32 of 77

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 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.

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