Products based on wireless technology have been steadily insinuating their way into our lives since the 1980s. Wireless devices such as Apple’s iPhone and Amazon’s Kindle have become common items in many of today’s households. Similarly, within the industrial market segment, the use of small wireless sensor devices is becoming more widespread within office buildings and on factory floors.
Equipment manufacturers are incorporating and connecting these inconspicuous technologies into wireless sensor networks (WSNs). A WSN consists of a set of spatially distributed devices that work together to send data and control information among each other, as well as to a main gateway, to perform some meaningful function. Each device can operate autonomously under the control of an embedded firmware application. Sensors are used to monitor physical or environmental conditions, such as temperature and motion.
WSN devices have proliferated via one of two technological trends. The first is the introduction of new products. The second pathway is the upgrading of established products. In the latter case, the primary driver is usually the need to convert the product’s previously wired interface into a wireless one. A furnace and its wireless, wall-mounted temperature-control unit interface is one such example.
In high-node-count networks, there’s a strong need to hammer down the cost of the individual node as low as possible. A primary cost driver for each device is the amount of memory required to operate the application.
Wireless device manufacturers and network designers should consider three strategies as they strive to achieve a WSN device with a low memory footprint (LMF): simplifying the topology and routing, device feature selection, and distributing some of the specialized tasks among different devices in a network.
MEMORY PARTITIONS IN WSN MCUs
WSN device applications reside in, and are executed from, what is generically referred to as an MCU’s “program memory.” In practice, program memory is a well-organized structure, comprising several functional partitions that must be maintained to ensure the device’s proper operation. These functional areas consist of different types of storage:
• The Program Text and Data sections of memory contain the application’s executable machine code instructions. This is generally flash memory, the contents of which are only changed during firmware updates. The size of the text area is fixed, and it is determined at the time the code is compiled and linked. The Data memory contains the variables used by the application at run time. This size is fixed at link time as well.
• The Stack and Heap sections of memory are two runtime mechanisms used by modern programming languages to store information that’s dynamic in nature. The Stack stores parameters that are passed during subroutines or function calls, for example. The size of the Stack varies during the execution lifecycle of an application. For applications that use dynamic memory allocation at runtime, the Heap provides the means of storage.
Like the Stack, the Heap doesn’t have a fixed size. The application developer must set aside sufficient Stack and Heap space so the device can operate in a stable manner regardless of the network it’s deployed into. The Stack and the Heap, on account of the dynamic nature of their information content and the speed at which this content must be accessed, suggest the use of random access memory (RAM).
• Nonvolatile memory (NVM) is a third type of storage medium employed by WSN MCUs. This storage is used to retain important information that’s specific to each device, which must be retained across power failures and/or device resets. Information such as security keys and the MCU’s media access controller (MAC) address are prime candidates for this storage area. In practice, this is usually EEPROM. Table 1 shows the primary memory areas that must be maintained and properly managed by the WSN MCUs.
Table 2 shows the upper limit of the memory partitions for what is defined as LMF WSN MCUs. An LMF device has less than 64k of program memory, along with less than 2k each of RAM and NVM storage.
Microchip and other microcontroller manufacturers offer a range of MCUs in this regard. The challenge becomes how to structure the MCU’s application firmware so it can operate within these low-memory constraints. Again, three different strategies can be applied.
NETWORK TOPOLOGIES AND ROUTING
The first strategy is to simplify the topology and routing mechanism. In wireless networking, “topology” refers to the arrangement, configuration, and relationship of the individual devices or nodes to each other on the network. It also involves the data-transmission pathways throughout that configuration.
ZigBee and other wireless protocols support two broad topology categories: star and mesh. The choice of network topology deployed can directly impact the MCU’s memory usage. Three different types of WSN devices can be used to form particular topologies:
• Coordinator: This device forms the network and enables other devices to join. It’s the central network device and provides many services, such as security, performance monitoring, and network configuration. The Coordinator is a form of a full-function device (FFD).
• Routers: The primary role of routers is to extend the network transmission range by relaying messages to other devices. They provide multiple paths to destination devices and redundancy, and they are used to extend the size of the network by supporting other child devices. They are a form of an FFD.
• End devices: These are generally of limited capability, but perform a specialized task. End devices can send and receive messages and directly communicate only with their parent. They don’t support child devices. They are a form of a reduced-function device (RFD).
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