CC1020-RTR1 datasheet(39/91 Pages) TI

39 / 91 page

CC1020

SWRS046

Page 39 of 92

FILTER_CLK periods, depending of the

AGC_AVG setting in the VGA2 register.

D) If the RSSI value is higher than

CS_LEVEL+8, then the carrier sense

indicator is set (if CS_SET = 0). If the

RSSI value is too high according to the

CS_LEVEL, VGA_UP and VGA_DOWN

settings, and the VGA gain is not already

at minimum, then the VGA gain is reduced

and the AGC continues from B).

E) If the RSSI value is too low according to

the CS_LEVEL and VGA_UP settings, and

the VGA gain is not already at maximum

(given by VGA_SETTING), then the VGA

gain is increased and the AGC continues

from B).

2-3

VGA

gain

changes

should

expected before the AGC has settled.

Increasing

AGC_AVG

increases

the

settling time, but may be worthwhile if

there is the time in the protocol, and for

reducing false wake-up events when

setting the carrier sense close to the noise

floor.

The AGC settling time depends on the

FILTER_CLK (= 2·ChBW). Thus, there is a

trade off between AGC settling time and

receiver sensitivity because the AGC

settling time can be reduced for data rates

lower than 76.8 kBaud by using a wider

receiver channel filter bandwidth (i.e.

larger ChBW).

12.10. Preamble Length and Sync Word

The rules for choosing a good sync word

are as follows:

1. The sync word should be significantly

different from the preamble

2. A large number of transitions is good for

the bit synchronization or clock recovery.

Equal

bits

reduce

the

number

transitions. The recommended sync word

has at the most 3 equal bits in a row.

3. Autocorrelation. The sync word should

not repeat itself, as this will increase the

likelihood for errors.

4. In general the first bit of sync should be

opposite of last bit in preamble, to achieve

one more transition.

The recommended sync words for

CC1020

are 2 bytes (D391), 3 bytes (D391DA) or 4

bytes (D391DA26) and are selected as the

best compromise of the above criteria.

Using the register settings provided by the

SmartRF® Studio software, packet error

rates (PER) less than 0.5% can be

achieved when using 24 bits of preamble

and a 16 bit sync word (D391). Using a

preamble longer than 24 bits will improve

the PER.

When performing the PER measurements

described

above

the

packet

format

consisted of 10 bytes of random data, 2

bytes CRC and 1 dummy byte in addition

to the sync word and preamble at the start

of each package.

For the test 1000 packets were sent 10

times. The transmitter was put in power

down between each packet. Any bit error

in the packet, either in the sync word, in

the data or in the CRC caused the packet

to be counted as a failed packet.

12.11. Carrier Sense

The carrier sense signal is based on the

RSSI

value

and

programmable

threshold. The carrier sense function can

be used to simplify the implementation of a

CSMA (Carrier Sense Multiple Access)

medium access protocol.

Carrier

sense

threshold

level

programmed by CS_LEVEL[4:0] in the

VGA4 register and VGA_SETTING[4:0] in

the VGA3 register.

VGA_SETTING[4:0] sets the maximum

gain in the VGA. This value must be set so

that the ADC works with optimum dynamic

range

for

certain

channel

filter

bandwidth. The detected signal strength

(after the ADC) will therefore depend on

this setting.

CS_LEVEL[4:0] sets the threshold for this

specific VGA_SETTING[4:0] value. If the

VGA_SETTING[4:0]

changed,

the

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