Texas Instruments 3138 155 232931 - Manual (Page 57)

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 RX or TX. A 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 in
CC2420
enable secure communication, which is
required for many applications. Security
operations require a lot of 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.