CC2420-RTR1 datasheet(57/92 Pages) TI

57 / 92 page

CC2420

SWRS041B

Page 57 of 89

34 System Considerations and Guidelines

SRD regulations

International regulations and national laws

regulate the use of radio receivers and

transmitters. SRDs (Short Range Devices)

for license free operation are allowed to

operate in the 2.4 GHz band worldwide.

The most important regulations are ETSI

EN 300 328 and EN 300 440 (Europe),

FCC CFR-47 part 15.247 and 15.249

(USA), and ARIB STD-T66 (Japan).

34.1

Frequency hopping and multi-

channel systems

The 2.4 GHz band is shared by many

systems both in industrial, office and home

environments.

CC2420

uses

direct

sequence spread spectrum (DSSS) as

defined by [1] to spread the output power,

thereby making the communication link

more robust even in a noisy environment.

With

CC2420 it is also possible to combine

both DSSS and FHSS (frequency hopping

spread spectrum) in a proprietary non-

IEEE 802.15.4 system. This is achieved

by reprogramming the operating frequency

(see

the

Frequency

and

Channel

Programming section on page 50) before

enabling

TX.

frequency

synchronisation scheme must then be

implemented within the proprietary MAC

layer to make the transmitter and receiver

operate on the same RF channel.

34.2

Data burst transmissions

The data buffering in

CC2420 lets the user

have a lower data rate link between the

microcontroller and the RF device than the

RF bit rate of 250 kbps. This allows the

microcontroller to buffer data at its own

speed, reducing the workload and timing

requirements.

The relatively high data rate of

CC2420

also

reduces

the

average

power

consumption compared to the 868 / 915

MHz bands defined by [1], where only 20 /

40 kbps are available.

CC2420 may be

powered up a smaller portion of the time,

so that the average power consumption is

reduced for a given amount of data to be

transferred.

34.3

Crystal accuracy and drift

A crystal accuracy of ±40 ppm is required

for compliance with IEEE 802.15.4 [1].

This accuracy must also take ageing and

temperature drift into consideration.

A crystal with low temperature drift and

low aging could be used without further

compensation. A trimmer capacitor in the

crystal oscillator circuit (in parallel with C7)

could be used to set the initial frequency

accurately.

For non-IEEE 802.15.4 systems, the

robust demodulator in

CC2420 allows up to

120 ppm total frequency offset between

the transmitter and receiver. This could

e.g. relax the accuracy requirement to 60

ppm for each of the devices.

Optionally in a star network topology, the

FFD could be equipped with a more

accurate crystal thereby relaxing the

requirement on the RFD. This can make

sense in systems where the RFDs ship in

higher volumes than the FFDs.

34.4

Communication robustness

CC2420 provides very good adjacent,

alternate and co channel rejection, image

frequency

suppression

and

blocking

properties. The

CC2420 performance is

significantly better than the requirements

imposed by [1]. These are highly important

parameters for reliable operation in the 2.4

GHz band, since an increasing number of

devices/systems are using this license

free frequency band.

34.5

Communication security

The

hardware

encryption

and

authentication

operations

CC2420

enable secure communication, which is

required for many applications. Security

operations

require

lot

data

processing, which is costly in an 8-bit

microcontroller system. The hardware

support within

CC2420 enables a high level

of security even with a low-cost 8 bit

controller.

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