In addition to the criteria discussed in Part 1 in this series, there are several other factors that also need to be considered carefully including ease of use and hardware and RF considerations. After reviewing them, the rest of this second part in this series will discuss 802.15.4 and how it compares against these selection criteria.

Ease of Use
Ease of use is a term that is defined by a subjective analysis of a protocol's usability. Many factors, including code readability, supporting documentation, direct engineering support, and simplified APIs can reduce the learning curve for even the most complex software systems; however, this essay measures ease of use by a protocol's complexity.

According to the specified application considerations, a protocol of high complexity can be deemed necessary by the designer, but it should be understood that rich feature sets often come hand-in-hand with complex software implementations.

Low power wireless protocols like 802.15.4, ZigBee, and SimpliciTI find themselves in a sweet spot in that the complete protocol architectures are accessible enough to be completely understood in order to fully leverage the feature sets, but are just large enough that the learning process can be challenging to even experienced embedded developers. Therefore, the balance between protocol features and ease of use is an important selection criteria to keep in mind.

Hardware and RF Considerations
Some hardware and application questions that should be considered are physical size of the system, distance to transmit, cost budget, and power budget. What are the features of the application such as will it require voice recognition or a user interface?

The answers to these questions will not only help determine the wireless protocol but also the microcontroller features. A list of key hardware selection criteria that should be considered in the design process of ultra-low power protocols is presented in Figure 5 below.

Figure 5 -- Key low power wireless hardware selection criteria

These selection criteria enable compatibility with the protocols presented in this paper and are good points for discussion when evaluating the hardware to implement the final solutions. There are also system-level concerns that should be addressed, such as the physical size of the hardware, which could limit the MCU and/or radio selection.

In some cases a system-on-chip (SOC) which integrates the MCU and radio into a single device will give the optimal size and feature solution. In other cases, the size restrictions will mean that having integrated analog functionality like an ADC will drive the MCU selection.

Figure 6 -- Microcontroller memory requirements for various wireless protocols

Also, the hardware selection could be influenced by the memory and MCU resource requirements of the protocol itself. In the case that the resource requirements of the protocol implementation are restrictive to the MCU's application performance, the designer could choose to use a wireless application processor dedicated to implementing, for example, the ZigBee stack, leaving the "application" MCU to implement custom application functions.

Before presenting the protocols in detail, example compilations of the protocols are shown in Figure 6 above using an MSP4304618 MCU and a CC2420 radio to give the reader a general feel for the memory footprints that can be expected from real-world implementations.