low-energy 8-bit device or a 32-bit ARMCortex-
M class device. Considerations for selecting
the optimal MCU include memory and pro-
cessing requirements for the RF stack and
sensor management, energy consumption,
device footprint and cost. Considerations for
selecting the RF protocol include energy
consumption, link budget and cost. Typical
wireless connectivity options include a propri-
etary sub-GHz/ISM stack, ZigBee, Bluetooth
orWi-Fi. Of these options, sub-GHz and ZigBee
are the most commonly used protocols for
home automation as they provide the energy
efficiency, long battery life (typically 3-5 years)
and extended range required to locate a sensor
node anywhere in a house without the incon-
venience of having to change batteries frequently.
Bluetooth lacks adequate range for many wire-
less sensor node applications because it there
is no provision for repeaters. The power re-
quirements of Bluetooth are also significantly
greater than ZigBee. Wi-Fi requires higher
power consumption than ZigBee and sub-GHz
and is thus not appropriate for battery-powered
applications in which the battery cannot be
easily recharged.
For sub-GHz star endpoints or flooding-capa-
ble RF stacks and space-constrained applica-
tions such as sensor nodes, a small footprint,
ultra-low energy 8-bit MCU and RF transceiver,
or SoC with integrated MCU and transceiver
may offer the most cost-effective solution. For
ZigBee mesh networking applications, an SoC
with integrated MCU and RF subsystems
might be the best option, particularly where
PCB area is at a premium. Look for MCU and
RF transceiver suppliers who offer low-energy
8-bit and 32-bit Cortex-MMCUs and wireless
SoCs along with the development tools to sim-
plify implementing the RF stack requirements.
The advanced IoT end node might be a smart
thermostat, wireless camera or a white goods
device such as a washing machine, as shown
in figure 4. The main system MCU is likely to
be a 32-bit ARM Cortex-M or Cortex-A class
device combined with one or more secondary
32-bit Cortex-M class or 8-bit MCUs used to
offload the primary processor, provide features
such as capacitive touch sensing, or optimize
the energy efficiency of the system by consoli-
dating sensor functions. Key considerations
for selecting the primary MCU include mem-
ory and processing requirements for the RF
stacks, sensor and system management, and
cost. Energy consumption will be of concern
for battery-powered-solutions. Considerations
for selecting the secondary MCU include inte-
grated features and energy efficiency. Look for
MCU suppliers that offer the most energy-
friendly 8-bit and 32-bit MCUs. Considerations
for selecting the optimal RF connectivity solu-
tion include bandwidth, energy consumption
link budget and cost, with ZigBee, Bluetooth
and Wi-Fi being the most common options.
Wi-Fi is the most widely used protocol for
bandwidth-intensive applications such as a
wireless camera, while ZigBee is ideal for ther-
mostat applications with multiple nodes and
lower data rates. Wi-Fi or Bluetooth provide
easy connectivity with smart phones and tablets,
which end users typically use to control their
connected home applications.
IoT developers must consider this question
when optimizing the energy efficiency of their
end node application: “Which is more impor-
I
NTERNET
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OF
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HINGS
Figure 2. Example of connected home gateway architecture
Figure 3. Basic sensor node architecture
