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Article

Hardware Solutions for Low-Power Smart Edge Computing

1
CYLEONE S.A.S. Company, 34090 Montpellier, France
2
LIRMM, University Montpellier, CNRS, 34095 Montpellier, France
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
J. Low Power Electron. Appl. 2022, 12(4), 61; https://doi.org/10.3390/jlpea12040061
Submission received: 28 October 2022 / Revised: 19 November 2022 / Accepted: 21 November 2022 / Published: 25 November 2022
(This article belongs to the Special Issue Ultra-Low-Power ICs for the Internet of Things)

Abstract

:
The edge computing paradigm for Internet-of-Things brings computing closer to data sources, such as environmental sensors and cameras, using connected smart devices. Over the last few years, research in this area has been both interesting and timely. Typical services like analysis, decision, and control, can be realized by edge computing nodes executing full-fledged algorithms. Traditionally, low-power smart edge devices have been realized using resource-constrained systems executing machine learning (ML) algorithms for identifying objects or features, making decisions, etc. Initially, this paper discusses recent advances in embedded systems that are devoted to energy-efficient ML algorithm execution. A survey of the mainstream embedded computing devices for low-power IoT and edge computing is then presented. Finally, CYSmart is introduced as an innovative smart edge computing system. Two operational use cases are presented to illustrate its power efficiency.

1. Introduction

The edge computing paradigm [1] is an emerging paradigm for Internet-of-Things systems where computations are distributed across a variety of compact devices in order to bring computing capability closer to data sources, such as environmental sensors and cameras. We can mention the following advantages of edge computing over the traditional centralized computing paradigm found in cloud systems:
  • reduced communication bandwidth and power costs as a result of reduced data transfers to centralized cloud servers;
  • physical proximity of data and devices facilitates real-time data processing, such as for self-driving cars;
  • in-situ processing at the edge devices ensures privacy regarding sensitive data, and prevents their offloading to remote locations;
  • as the system is distributed, failure of some nodes can be easily overcome with a minimal impact on the global system and new devices can be added in a modular fashion to increase computing power.
Figure 1 illustrates the hierarchical layers of a typical edge computing infrastructure. Sensors in Layer 0 collect data from the environment first. In subsequent layers, the data are processed with appropriate devices based on the complexity of the processing. To meet the edge computing requirements, devices are placed close to sensors.

1.1. Machine Learning at the Edge

For edge computing nodes, implementing data analytics, particularly machine learning (ML), is a major challenge. Applications that leverage ML techniques at the edge are numerous. In essence, they deal with the inference problem. Real-time video analytics is a prominent application found in systems such as video surveillance, traffic control, and autonomous vehicles. Another notable application is feature extraction from images, for example, detecting areas and objects, identifying handwritten characters, and monitoring healthcare. To ensure people’s safety (for instance, in the event of a fire) or to minimize energy consumption by utilizing renewable resources, smart homes and cities incorporate devices that use ML techniques for sensing and controlling the environment. Amazon’s Alexa has made automatic speech recognition popular due to its success.
Based on the richness of ML techniques, three main families can be distinguished [2]. In supervised learning, classification and regression tasks are often achieved with algorithms such as support vector machines (SVM), artificial neural networks (ANNs), and linear regression. With the use of algorithms like k-means or x-means, unsupervised learning can be used for clustering and prediction tasks. Reinforcement learning focuses on decision-making through Q-learning. The majority of machine learning algorithms implemented on edge devices currently utilize inference (i.e., the process of directly solving ML problems with pre-trained ANNs) rather than training (i.e., the process of minimizing error as a function of ANN parameters, given an ML problem). One reason cited in [3] is the high bandwidth and latency costs involved in exchanging network updates across multiple edge devices (centralized model training might be more efficient since updated networks would be transferred directly to devices). Another concern in the design of edge nodes is energy and hardware costs. Structured labeled data is usually used for inference. The case is different for deep learning [4], which is used for training tasks in which precision is critical. In this process, complex multi-layer ANNs are applied along with huge amounts of raw data. Edge devices lack the computing power and data storage capacity needed for this.

1.2. Motivation and Contribution of This Study

Smart edge devices rely on embedded systems with limited resources to process sensor data. For ML workloads requiring high computing power, energy-efficient systems are necessary. In recent years, many research efforts have been focused on energy-efficient embedded system designs to solve ML problems [5,6,7,8,9]. These systems are primarily designed to make inferences. However, embedded systems, especially at edge nodes, are likely to require online learning, control, and optimization capabilities. In autonomous cars, for example, using remote cloud-based training could lead to long communication delays. Therefore, embedded training devices are preferred. However, after being trained offline, ML inference models tend to diverge once in production. The retraining of such models will therefore require online training capabilities.
To understand how low-power embedded computing devices might help fill the aforementioned demand, this paper reviews the main design approaches for energy-efficient ML algorithm execution. In the following sections, it surveys candidates that meet both smart edge computing and Internet of Things requirements for low-power devices. CYSmart is a flexible and low-power smart edge computing system that we present as an example. A few working scenarios are used to evaluate the power efficiency of CYSmart.

1.3. Outline of the Paper

First, we discuss energy-efficient computing systems dedicated to executing machine learning algorithms in Section 2. Section 3 provides a classification of low-power devices for IoT and smart edge computing on the basis of hardware resources and power dissipation constraints. This classification is then used to present a panorama of popular devices. The CYSmart low-power edge computing system is described in Section 4. Moreover, it is briefly compared with selected industrial edge computing technologies. Finally, some concluding remarks are provided in Section 5.

2. A Quick Journey in the Landscape of Energy-Efficient Compute Systems for ML Tasks

Design principle-wise, embedded machine learning can be implemented through a central processing unit (CPU), graphics processing unit (GPU), application-specific integrated circuit (ASIC), and field-programmable gate array (FPGA). There is a wide range of outcomes in terms of power and accuracy of prediction for these implementations [10].
Figure 2 roughly illustrates this tradeoff. With the ASIC implementations [11,12], execution latency is optimized, but ML model accuracy is lower as the model is customized with approximations like quantization of ANN weights (i.e., reducing numerical precision by reducing the number of bits). Contrary to ASIC chip designs, CPUs and GPUs often support high-precision numerical representations, which improve prediction accuracy at the expense of power consumption. ASICs are more energy-efficient than CPUs and GPUs since they use less computing, I/O bandwidth, and memory resources. However, their development can be time-consuming and expensive. FPGAs offer a flexible and cost-effective implementation, allowing better balancing of power consumption, response latency, and prediction accuracy, as evidenced by recent studies [13,14]. Nevertheless, this comes at the expense of programmability, e.g., when compared to CPUs and GPUs.
In Figure 3, another perspective on existing machine learning systems is summarized [7]. Inference is addressed by systems with dissipation less than 100W. Among them are Google’s Edge Tensor Processing Unit (TPUEdge) [15] and Intel’s MovidiusX processors [16], embedded GPU-based neural engines such as the Apple A12 processor [17] and Huawei Kirin 980 [18], FPGA co-processors like the Zynq-020 [19] and the Stratix-V [20] chips, as well as mobile system-on-chips (SoCs) from Nvidia like Jetson-TX2 [21] and Xavier [22]. Training systems at high performance levels consume more than 100 watts. Typically, they consist of data center chips like the Google TPU3 [23] and the Intel Nervana2 [24] and data center systems like the Nvidia DGX-2 server system [25].

2.1. Focus on ML Accelerators, GPUs and FPGAs

Mobile phone SoCs, for example, use ML acceleration to address vector and matrix operations [26]. Various neural processing units and GPUs may be combined to achieve this, as in Qualcomm Snapdragon [27], HiSilicon 600 and 900 series chips [18] and MediaTek Helio P60 [28].
A typical approach toward efficient edge devices is to design hardware accelerators for machine learning models. This is already the case for ANNs for improved energy efficiency and throughput. By minimizing data access costs across the memory hierarchy, these accelerators can enable specialized processing dataflow that better exploits the memory characteristics. In [29], authors highlight several key design specializations tailored to machine learning accelerators: instruction sets that perform linear algebra operations like matrix multiplication and convolutions; on-chip buffers and on-board high-bandwidth memory to efficiently feed data; and high-speed interconnects that enable efficient communication between multiple cores. Additional hardware specializations for inference-only designs include Winograd convolution [30] and non-digital computing [12]. Although accelerators improve the execution performance of individual ML kernels, they may have some negative impact on the overall ML model performance because of costly communications between them and the associated system-on-chip (SoC).
Embedded GPUs and FPGAs are further alternatives for accelerating ML algorithms. As shown in Figure 4, several solutions exist. We only report devices with maximum power consumption of 50W, from the exhaustive list presented in [7]. The selected devices are compared w.r.t. their performance, power consumption and computational precision levels. Accelerators generally offer better precision and power consumption tradeoff, e.g., Google’s tensor processing unit for edge computing (TPUEdge) [15] and Eyeriss [31]. On the other hand, GPUs and FPGAs globally provide better performance. The higher their computing precision, the higher their consumption, e.g., Xavier GPU [22] and ZCU102 FPGA [32].
For accelerating ML algorithms, embedded GPUs and FPGAs can also be used. Figure 4 shows several solutions. As part of the exhaustive list presented in [7], we report only devices with a maximum power consumption of 50W. We compare the selected devices in terms of performance, power consumption, and computational precision. There is generally a better tradeoff between precision and power consumption with accelerators, such as Google’s tensor processing unit for edge computing (TPUEdge) [15] and Eyeriss [31]. In contrast, GPUs and FPGAs provide better performance globally. In general, the higher the precision of the computation, the more power it consumes, as in Xavier GPU [22] and ZCU102 FPGA [32].
A comprehensive survey on hardware accelerators has been proposed very recently in [33]. The reader can refer to this survey for a full coverage of the state of the art.

2.2. From Software-Hardware Codesign to Emerging Computing Paradigms

Weight compression [34], parameter pruning, and weight quantization [35] are well-known ML optimization techniques for ANNs. Their goal is to improve energy efficiency by lowering computing complexity, data volume, and hardware resources used during the execution of the networks. Pruning involves gradually suppressing the connections between neurons in an ANN. Quantization involves reducing the number of bits in binary words. It is similar to approximate computing [36], where floating-point representations are converted to fixed-point representations. This reduces the precision of weight values in connections but speeds up execution. Another key aspect is developing compilers and runtime systems [37] that abstract away hardware details. This makes it easier to deploy and train ML models on mobile devices. The extensive software development environment made available to users by Nvidia contributes to the success of Nvidia GPUs for ML.
In the widely adopted von Neumann architectures, ML workloads based on ANNs frequently perform multiply-accumulate operations, which generate multiple data movements between memory and processors. As a result of these exchanges, there is a high execution time and power consumption, and this is known as the “memory wall”. Modern ML commodity chips combine CPUs with High-Bandwidth Memory (HBM) via efficient interconnects to address this problem. In parallel, an emerging paradigm, called near-data processing [38], has been studied to address the memory wall issue. Computing capability is built into the memory or storage, enabling data stored there to be processed. Mixed-signal circuit design and advanced memory technologies were used to accomplish this. Other near-data processing techniques include in-memory processing [39] and in-storage processing [40]. Integrated 3D technologies and emerging Non-Volatile Memory (NVM) technologies enable such realizations. In comparison to DRAM, NVMs [41,42] like Spin Torque Transfer RAM (STT-RAM) and Resistive RAM (ReRAM) have lower leakage and higher cell density. By using them, edge nodes can mitigate their idle power draw concerns. In Hybrid Memory Cube (HMC) [43], several DRAM dies are stacked above the logic layer using Through-Silicon-Vias (TSV) to address the memory access issue.

3. Classification of Low-Power Devices for IoT and Smart Edge Computing

As IoT and edge computing grow in popularity, multiple sophisticated tiny embedded computing devices have emerged over the last decade. A general and systematic way of assisting designers in choosing low power IoT and smart edge computing devices does not exist. In a recent paper, Neto et al. [5] proposed a classification for IoT devices aimed at smart cities and smart buildings. We revised this classification to better reflect a broader class of edge computing devices encountered beyond smart cities and smart buildings. This includes hardware architectures used by mobile devices such as smartphones. The enhanced classification takes into account the hardware characteristics, including both computing and memory components (which reflect the potential device performance), and the total power dissipated. The resulting classes are accompanied by some typical target algorithms that the corresponding device family can handle.
Our proposed extension of the classification from Neto et al. [5] is represented in Table 1. A total of six device classes are distinguished. The Class 0 devices are based on microcontrollers with limited memory capacity and power consumption. In general, the processed dataset is very small; for example, temperature and humidity measurements. Nevertheless, such devices can perform lightweight inference tasks using simple pre-trained models. Hence, all the subsequent device classes can be used for inference. A Class 1 device is one that can store data in addition to collecting and processing data. Such devices generally run on monocore microcontrollers or application cores with larger storage and memory capacities. Typically, such devices process only some basic statistics, such as noise reduction.
From Class 2, all devices are considered to have one or more application cores. The presence of SD card slots in the majority of these devices makes storage capacity more scalable. Devices of Class 2 are powerful enough to enable CNN inference, such as in image analysis. Their performance is good enough to execute lightweight IoT and edge workloads, as well as more intensive workloads such as training and inference.
In class 3, embedded GPUs make it possible to run lightweight training tasks. It is the first class with sufficient resources to enable real-time video analysis without any special ML accelerators.
In class 4 and class 5, we find devices that can be used in (quasi)autonomous systems such as smartphones or self-driving cars. The devices should be able to withstand environmental changes, while delivering the performance to process large datasets using high-performance accelerators, such as Nvidia GPUs found in server-class systems. A class 4 device is often intended for smartphones and is often more energy-efficient than a class 5 device, which is mostly designed for training and inference purposes.

Quick Survey of Typical Embedded Devices

Following the above classification, we now look at the most popular low-power devices used for IoT and edge computing recently. As shown in Table 2, Table 3 and Table 4, the devices identified fall into three application families:
  • the first application family includes ultra-low-power devices with limited resources suitable for lightweight IoT and edge applications. This applies to all class 0 and class 1 devices;
  • the second application family consists of the most popular devices encountered in average edge computing and IoT applications. All devices in class 2 and part of devices in class 3 are included in this class;
  • the third application family includes devices with the most powerful hardware resources for performing machine learning and inference tasks. It covers a significant portion of devices in classes 3 and 4 and 5.
We categorize each application family by its device classes, its execution unit (CPU, GPU, accelerator, etc.), the most appropriate ML task (inference vs. training), and some domain application examples. Following the three application families outlined above, we will briefly discuss an application-driven panorama of key IoT and edge devices. We rely on [6] in part for this survey.
The first application family, shown in Table 2, deals with lightweight data processing. Display devices are usually located close to data sources and are used primarily for domotics. The Arduino board is typically found in smart houses to monitor lightning [44]. Meanwhile, the most powerful devices may come with a compute accelerator integrated into them. Using its CNN accelerator, the MAX78000 device executes a light and optimized object detection algorithm on camera data [57]. Generally, the devices in Table 2 are affordable. They use programming models that are often closer to the hardware, i.e., at a low abstraction level, like assembly code.
Due to its inherent energy-efficiency, ARM technology is often adopted for embedded systems. Cortex-M microcontrollers and Cortex-A application processors are examples. In addition to ARM cores, a few designs use Intel technology, such as the Movidius Myriad 2 vision processing unit (VPU). Using them, deep neural networks (DNNs) can be run in smart cameras, for example. Also worth noting is the emerging GAP8 device, based on the RISC-V open Instruction Set Architecture (ISA) [59]. It paves the way for a new era of processor innovation sustained by a collaborative and dynamic ecosystem.
There are a number of devices that combine ARM CPUs with embedded GPUs (see Table 3 and Table 4), except for the Beaglebone Black system (see Table 2). In the powerful Jetson TX series and Tegra X1 systems, Nvidia’s Pascal and Maxwell GPUs are combined with ARM Cortex-A57 cores. In both cases, the resulting systems can consume up to 15W of power. Since GPUs are present, they provide higher computing performance at the expense of more power consumption.
GPUs such as those reported in Table 3 are capable of executing the second application family. With Raspberry Pi, this involves image processing to detect tomato disease [60] or image super-resolution [61] or image classification [62] with smartphones. Other applications involve robotics, such as robotic perception [63] implemented using the Robot Operating System (ROS). As a result, the devices used here can support higher abstraction levels of programming.
According to Table 4, all devices in the third application family combine a GPU with at least four application cores, except for the RZ/V2M board. Applications primarily focus on image and video processing. With the Jetson Nano, real-time vehicle object detection is possible [64]. Furthermore, these devices are used in smartphones, such as the Huawei P40 Pro, which is equipped with powerful video super-resolution. Additionally, they can be used with ROS in the robotics field [65] for navigation, perception, and control.
A few devices, however, combine CPUs with specific ML accelerators. ZedBoard and Coral Dev Board both integrate an FPGA-based accelerator and Google’s TPUEdge [15]. Based on the computing complexity of algorithm execution, this diversity of cores enables maximizing the efficiency of the combined processing elements.
Homogeneous multicore devices appear mostly in devices for the first application family (Table 2), and in a few cases for the second application family (Table 3). These devices dissipate only a few milliwatts or a few Watts, such as SparkFun Edge and Hello-Edge. They, however, deliver less performance than heterogeneous devices.
Lastly, it is worth noting that most of the devices reported in Table 2 and Table 3 are used for inference tasks rather than training (which is more expensive) due to their limited computing resources. Only some devices listed in Table 4 has been considered for lightweight training tasks, e.g., Odroid-XU4 board.

4. Low-Power Smart Edge Computing with CYSmart Solution

CYSmart is an edge computing system that gathers, processes, and displays locally measured data with minimal power consumption. It is capable of providing real-time feedback to domain experts. There are a number of low-power devices in this system, called CYComs, which collect data from sensors at points of interest, pre-process it, and transmit it to the CYEdge via LoRa networks, as illustrated in Figure 5.
A CYCom implements the services provided by Layers 0 and 1 in Figure 1. As a result, CYSmart is able to perform some preliminary lightweight analyses on the collected data. This analysis can be performed to filter it before sending the result to the other components of the system. The outputs CYComs can then be processed and displayed by the CYEdge component, which typically implements Layers 2 and 3 of Figure 1. Data processing algorithms can determine which device class is appropriate for implementation of a CYEdge. For energy-efficient and secure computations, the latter is deployed close to CYComs.
  • Measurement identifier and name of the point of interest
  • Type of measurement performed
  • Unit of measurement used
  • Range of measurement desired
  • Operating mode of the module (continuous measurement, on demand, sleep…)
  • Time range of system activity
  • Battery level of CYComs
  • Limit ranges of expected values
  • Alert generation
  • Transmission signal strength
Every measure is stored in the CYEdge internal memory and remains accessible at any time through:
  • a visualization tool that displays the measurement curves versus time;
  • a download of all the information stored on a USB flash drive, computer, or server. Depending on the needs of the customer or the third party software used, the file type and format are adapted.
Figure 6 presents a more detailed technical presentation of CYSmart. The CYCom and CYEdge components are detailed in the next two sections, followed by some use case scenarios.

4.1. Data Acquisition Device: CYCom

A CYCom is a device used to acquire data in the CYSmart system. Physical data can be recovered using this low-power technology running on an external battery. Data can come from digital sensors with serial communication (SPI, UART, I2C) or from analog sensors with a 16-bit Analog to Digital Converter (0–10V, 4–20 mA, or thermal resistance input). An STM32 microcontroller allows the CYCom to pre-process data (threshold detection, filtering, conversion…) before sending it by LoRa to the CYEdge. Each device can be physically configured to communicate with the CYEdge unit using one of two frequency bands (433 MHz or 866 MHz), each with four transmission channels. In the event the receiving device is not reachable, the sent frame can be stored on the receiving device’s internal flash memory (8 MB) or SD card and sent back when the connection is restored. The device can also make use of other modules, such as a micro-USB port (currently used for CYCom updates), Bluetooth module, 3-axis inertial measurement unit (IMU) module (acceleration, angular, magnetic), or GPS module directly integrated within the device.

4.2. Centralized Early Data Processing: CYEdge

Data centralisation and setup of the sensor network is achieved with the CYEdge technology. It is a box that can be connected to an external battery or to the socket. The technology consists of two parts. The first one is a Raspberry Pi 4 (Processing Unit) and the second one is a proprietary shield developed by CYleone that enables LoRa communication with CYComs. This shield is a LoRa gateway that allows the processing of AT commands sent by the Raspberry Pi. These commands are sent to setup the CYComs but also to retrieve the data frames from the CYComs. The Raspberry Pi acts mainly as a data processing and graphic display unit. It reads, processes, saves and displays data from the shield on the graphical user interface. The interface allows the configuration of the sensor network and the retrieval of all the data and configurations of each CyCom. It can be accessed by connecting via WiFi or Ethernet to the local network created by the board itself, or to an existing network. The Raspberry can also be used for other parallel tasks such as GPS measurements, digital data retrieval and synchronization of this data with that received via LoRa.

4.3. Use Case Evaluation of CYSmart

One use case application of CYSmart is to collect data from different points of interest every interval of ten minutes, in a critical environment. As shown in Figure 6, the CYComs collect data from sensors where it is difficult to take and transmit measurements, such as aeraulic measurements in basements or bunkers. Data can be stored in the CYComs and sent to the CYEdge after lightweight processing, such as threshold detection or filtering. By sending only useful and ready-to-use data, this application minimizes LoRa communication. On the CYEdge, complementary data processing can be performed. A diagram of the CYCom’s operations is shown in Figure 7.
A second use case application relies on LoRa to wirelessly transmit raw data from a CYCom directly to the CYEdge. The latter performs all data processing and displays the evolution of the values. This scenario is used to track the evolution of a process or a physical value in time, such as a refrigerator temperature in catering. Here, in most cases, the interval between two measures is around a few seconds, e.g., four seconds in our case. In addition, a CYEdge can only be paired with one to five CYComs. Figure 8 shows the corresponding flow diagram.
CYSmart devices are now ready to be evaluated according to the above use cases. As a processing unit, CYEdge utilizes a Raspberry PI 4 device with a LoRa shield. The CYCom uses a homogeneous architecture based on a STM32L496 microcontroller and ARM Cortex-M4 CPU (class 0 device hardware). The CYCom has a storage capacity of 1 MB from the STM32 and 8 MB from an external flash memory (class 0 device storage), as well as an SD card slot. There is also 320 KB memory in the STM32, corresponding to a class 0 memory.
According to the different steps of the two operational diagrams, the evaluation of CYCom is presented in Table 5. During each step, the power is measured using the generator voltage (constant 15V) and the CYCom current draw. A CYCom is connected to a generator through an ampere-meter to measure the current draw with the greatest precision between the generator value and the ampere-meter display.
From Figure 7 and Figure 8, we note two parts in the operational diagram. The first part, with steps 1 and 2, concerns the setup of a CYCom. Both steps are performed only once during the setup of the entire system and the CYComs. The latter retain their configuration in memory until they are reinitialized by the use of a hardware reset (push button inside). The second part relates to the execution routine of the device. In this routine, the device is woken up, a measurement is taken, data is processed and sent (depending on the use case), before returning to sleep until the next measurement. Steps 3 to 8 also belong to the routine and are executed in an infinite loop.
Figure 9 shows the duration distribution for the steps in one routine iteration, considering a worst-case scenario. In Case I (see Figure 9a), the worst case scenario occurs when there is 1 min between each data measure in the CYCom and there are frequent connection issues between the CYCom and CYEdge (i.e., low communication quality). In this worst-case scenario, (i) the CYCom transmits data to the CYEdge during 2 s and waits for an acknowledgement during 5 s; (ii) if no acknowledgment is received, the CYCom repeats phase (i) up to five times, otherwise it moves on to step 7. After five attempts (about 35 s), if no acknowledgement is received, the CYCom stores the data in its local memory, and proceeds to step 7.
For Case II in Figure 9b, we assume that the CYEdge and CYCom are close to each other. This minimizes the loss of communication between both components during data exchanges. It enables the CYCom to send data to the CYEdge only once during 2 s and wait for the corresponding acknowledgement during 1 s. If no acknowledgement is received, the data is stored in the local memory of the CYCom. The worst-case scenario requires 3 s to reach step 7.
Despite their high power consumption, steps 1 and 2 only consist of system setup functions executed during installation. For this reason, as shown in Figure 10a,b, various components are activated during these steps to set them up.
During the execution of the routine, the sensor power management is activated only during steps 3 and 4. Figure 10c,d show that it is the second most power consuming function after the CPU. It is activated for 10 s, as in Case I. This represents 9% of the routine execution duration in Figure 9a and only 5% in Figure 9b. This allows for stable data collection from analog sensors. To provide the most precise data, it is necessary to have so much time. However, digital sensors can take less time to acquire data than analog sensors in Case II. This explains the shorter duration in Case II. The complexity of the data processing affects the processing time, but not the power consumption.
The CYCom sends data to CYEdge in steps 5 and 6. Here, the duration depends on the communication load between both devices. The CYCom will attempt to send data to the CYEdge five times before storing it (step 8). During steps 5 and 6, Figure 10e,f shows that the LoRa communication is activated and represents up to 38% of the power consumption during the data transmission, i.e., the third most power consumer function.
Lastly, the process performs the sleep function, i.e., step 7. In Figure 10g, it is the step with the least activated functions: only the CPU is activated, resulting in the lowest power consumption. According to the use case, this function can be more or less time consuming. Figure 9a shows that in the worst-case scenario in Case I, CYCom is in this state 57% of the execution routine. Additionally, depending on the frequency of the data measurement, this step can be repeated and reach more than 95% of the process duration. In some cases, this duration can be shortened depending on the measuring frequency and the total number of CYComs deployed in the network to improve the bandwidth. With 1 s sleep duration in Case II, the routine spends 25% of its time in the worst-case scenario.

4.4. Gained Insights and Discussion

As a representative ultra-low-power device of the CYSmart system, the CYComs were the primary focus of the above use cases. There is also another component, the CYEdge, which embeds a Raspberry Pi 4 and a LoRa communication shield. Based on the power measurements of the CYEdge under normal operating conditions, it can be classified as a class 3 device, as described in Table 1. Below are some insights regarding CYSmart’s current implementation and potential improvements.
Gained insights. The CYCom component of the CYSmart system utilizes a commercial-off-the-shelf (COTS) microcontroller manufactured by STMicroelectronics. Choosing this approach reduces the cost of the component as well as the development time. A CYCom’s CPU is the primary energy consumer in the aforementioned use cases, followed by the LoRa module and the sensor power supply. Each of these three modules can be improved.
STM32 boards are based on von Neumann microarchitecture, leading to costly data movement between different hardware units. As a result, future improvements could include designing a customized solution that meets the requirements of the domain applications. This is consistent with the notion of domain-specific hardware accelerators as described in [85]. There is a lot of power consumed during one routine iteration in the second use case from the previous section without any data processing being performed. This unnecessary power consumption must be eliminated in order to improve the energy efficiency of the system. This problem may be solved by means of power gating, for example. A customized solution that incorporates such a mechanism is therefore desirable. Suitable design approaches should be considered for design space exploration by selecting high-level methodologies, e.g., [86,87,88], covering different abstraction levels: high-level analytical modeling [89,90,91,92], transaction-level modeling [93,94,95], cycle-accurate design [96,97,98], or register transfer level [99]. As surveyed in this paper, it is possible to implement the architecture using FPGA or ASIC designs at the expense of higher costly implementation efforts. As for CYComs, the CYEdge power consumption can be reduced by applying the same design methods.
Sensor power consumption is difficult to reduce since it is heavily dependent on the type of sensor being used. A wide range of digital and analog sensors can be interfaced with the CYCom. External 24 V lithium-ion batteries are currently used to power the integrated sensors. Instead of analog sensors, digital sensors with internal 3.3 V batteries could be considered here to reduce power consumption. Depending on the measurement environment, LoRa modules consume varying amounts of power. In both Cases I and II, the system can communicate across a reinforced concrete wall 90 cm thick with the initial parameters. In the case of a 15 dBm data transmission capacity and a spreading factor of 12, the maximum transmission delay is 2 s. Therefore, its maximum power consumption is 166 mW. These parameters can be adjusted according to the operational environment in order to reduce the LoRa module’s power consumption.
Comparison of CYSmart w.r.t. selected industrial solutions. As a mature low-power edge computing solution, the CYSmart system can be compared with a number of industrial technologies. For this purpose, we consider some relevant criteria, described as follows:
  • Device classes: supported device classes as defined in Table 1. This criterion implicitly suggests a range of power consumption;
  • Sensor diversity: diversity of sensor types supported by a technology, such as digital versus analog sensors, as well as sensor voltage ranges. The criterion is qualitative in nature and can be rated on three levels: high, average, and low.
  • Transmission speed: the speed of data transmission between the sensors at the edge frontier and the gateway or centralized system that is responsible for pre-processing the data. Generally, it is measured in terms of the number of samples per second (S/sec) or bits per second (b/sec);
  • Communication distance: the distance over which a technology communicates wirelessly. It is essential in critical environments, such as basements, bunkers, and nuclear power plants;
  • Number of edge layers: the number of layers considered in the hierarchical edge computing implementation, as shown in Figure 1;
  • Measurement points: the number of data measurement points (i.e., sensors) managed by a single gateway or centralized system;
  • Dimension of measurement device: the form factor of a device that incorporates sensors to collect data during the deployment of a technology;
  • Dimension of gateway/central device: the form factor of a gateway or centralized system that manages sensor data;
  • Easy deployment: the effort required for an easy deployment of a technology. This is a qualitative criterion;
  • Application diversity: it refers to the variety of applications that can be leveraged by a technology, such as smart-home, smart-industry, and smart-city. The criterion is also qualitative in nature.
In light of the aforementioned criteria, Table 6 provides a comparison of CYSmart w.r.t. the industrial edge computing technologies summarized in the sequel. The TMI-Orion company proposes a solution for the design and manufacture of high level technologies that target harsh environments. A key component of its edge computing technology is a network of smart sensors such as NanoVACQ Fullradio [100], which communicate via a 2.4 GHz radio protocol with a Radio Transceiver [101]. Using a serial protocol, the latter transmits data to a host computer that manages and displays data from a sensor network. The Gravio company develops an IoT platform that is capable of connecting several sensors. Using the ZigBee wireless protocol, these sensors communicate with the Gravio Hub [102]. Data can be viewed and managed by users.
The moneo appliance is an edge solution manufactured by IFM company [103]. It consists of a dedicated software toolbox that allows for the management of sensor parameters as well as data display. Templates are provided in the toolbox for defining network configurations. Sensors are connected to the moneo appliance via an IO-Link Master, which serves as an interface between the appliance and the gateway computer.
The Advantech company developed an IoT solution that relies on data measurement devices named WISE (e.g., WISE-4060 [104]) and an intelligent edge server (e.g., EIS-D150 [105]). By using the WiFi protocol, the WISE devices send data from the sensors to the edge server. Users are provided with a real-time dashboard for managing WISE devices. The InHand Networks company has defined a specific gateway [106], which provides data optimization in the IoT infrastructure and provides real-time response times. The gateway device can be connected to a local network. It is compatible with real-time Ethernet protocols and supports the Docker software system.
The MCM200 series components (e.g., MCM-204 [107]) are edge computing solutions designed by the Adlink company. They are standalone data acquisition devices (i.e., no host computer is required) that can monitor, analyze, and execute real-time actions. WiFi or Ethernet ports are available for communication.
Finally, the Analog Devices company offers the SmartMesh Wireless HART technology which consists of a small network manager (LTP5903-WHR [108]) that communicates with a number of sensor nodes called “motes” (e.g., LTP5900-WHR [109]). The network manager and motes must be programmed by the user. The network manager is responsible for centralizing data and communicating it to the host computer. Using analog data from sensors, the motes transmit data to the network manager.
Table 6 globally illustrates that CYSmart and Advantech technologies offer several advantages over other solutions. There are many similarities between these two technologies; however, CYSmart is capable of supporting a larger wireless communication distance than Advantech’s solution. Because of this, CYSmart is well suited to critical environments, such as nuclear power plants.

5. Summary

Embedded architectures for future edge devices likely will need to support training, control, and optimization capabilities, according to the current trends in edge computing. In this paper, we discuss recent efforts regarding energy-efficient hardware solutions for machine learning at the edge. We reviewed current design approaches and devices targeted at implementing IoT and smart edge computing with limited computing and power capabilities. Candidate low-power devices that could meet IoT and smart edge computing requirements have been surveyed. CYSmart, a flexible low-power edge computing system, was demonstrated as an interesting solution. To evaluate its power efficiency, a few working scenarios have been considered. Finally, a brief comparison of CYSmart with selected industrial edge computing technologies was presented.

Author Contributions

Conceptualization, L.M.W., J.-M.B., G.B. and A.G.; methodology, L.M.W., J.-M.B. and A.G.; software, L.M.W. and J.-M.B.; validation, L.M.W., J.-M.B. and A.G.; writing—original draft preparation, L.M.W., J.-M.B., G.B. and A.G.; writing—review and editing, L.M.W. and A.G.; supervision, J.-M.B., G.B. and A.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Acknowledgments

The authors would like to thank Guillaume Devic and Gilles Sassatelli for their feedback in early discussions on part of the current work.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
AIArtificial Intelligence
ANNArtificial Neural Network
ASICApplication-Specific Integrated Circuit
CNNConvolution Neural Network
COTSCommercial-Of-The-Shelf
CPUCentral Processing Unit
DRAMDynamic Random Access Memory
FPGAField-Programmable Gate Array
GPUGraphics Processing Unit
HBMHigh-Bandwidth Memory
HMCHybrid Memory Cube
I/OInput/Output
IMUInertial Measurement Unit
IoTInternet of Thing
ISAInstruction Set Architecture
LoRaLong Range
MLMachine Learning
NVMNon-Volatile Memory
RAMRandom Access Memory
ReRAMResistive RAM
ROSRobot Operating System
SoCSystem-on-Chip
STT-RAMSpin Transfer Torque RAM
SVMSupport Vector Machines
TPUTensor Processing Unit
TSVThrough-Silicon-Vias

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Figure 1. Hierarchical smart edge computing.
Figure 1. Hierarchical smart edge computing.
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Figure 2. Power and prediction error for four hardware designs (based adapted from [10])—higher is worse.
Figure 2. Power and prediction error for four hardware designs (based adapted from [10])—higher is worse.
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Figure 3. Computing landscape for ML: power vs. performance (adapted from [7]).
Figure 3. Computing landscape for ML: power vs. performance (adapted from [7]).
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Figure 4. Accelerators, GPUs and FPGAs for embedded ML (adapted from [7])—he darker color, the higher the metric value.
Figure 4. Accelerators, GPUs and FPGAs for embedded ML (adapted from [7])—he darker color, the higher the metric value.
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Figure 5. Overview of the CYSmart solution.
Figure 5. Overview of the CYSmart solution.
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Figure 6. Detailed view of CYSmart solution.
Figure 6. Detailed view of CYSmart solution.
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Figure 7. Operational diagram of the first application use case (Case I).
Figure 7. Operational diagram of the first application use case (Case I).
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Figure 8. Operational diagram of the second application use case (Case II).
Figure 8. Operational diagram of the second application use case (Case II).
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Figure 9. Time distribution between the different steps in the worst-case scenario, for two use cases. (a) Use case I; (b) Use case II.
Figure 9. Time distribution between the different steps in the worst-case scenario, for two use cases. (a) Use case I; (b) Use case II.
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Figure 10. Power consumption breakdown for the different steps occurring in Figure 7 and Figure 8. (a) Step 1; (b) Step 2; (c) Step 3; (d) Step 4; (e) Step 5; (f) Step 6; (g) Step 7; (h) Step 8.
Figure 10. Power consumption breakdown for the different steps occurring in Figure 7 and Figure 8. (a) Step 1; (b) Step 2; (c) Step 3; (d) Step 4; (e) Step 5; (f) Step 6; (g) Step 7; (h) Step 8.
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Table 1. Device classification for smart and low-power embedded systems (adapted with permission from [5]).
Table 1. Device classification for smart and low-power embedded systems (adapted with permission from [5]).
ClassStorageMemoryCompute
Unit Types
PowerTypical
Algorithms
0≤512 MB≤512 kBMicrocontrollers≤1 WBasic computations
(lightweight inference)
1≤4 GB≤512 MBMicrocontrollers/
Application cores
≤2 WBasic statistics
(inference)
2≥4 GB≤2 GBApplication cores≤4 WClassification/Regression
(inference)
3≥4 GB≤8 GBApplication cores≤16 WPrediction/Decision-making
(inference)
4≥4 GB≤16 GBApplication cores≥16 WDeep learning,
auto-encoders, etc.
(inference & training)
5≥4 GB≥16 GBApplication cores≥16 W
Table 2. Ultra low-power devices for IoT and edge computing.
Table 2. Ultra low-power devices for IoT and edge computing.
DeviceClassGPU/Accel.CPUML UsageApplication
Examples
Arduino
Mega
0-Microcontroller
ATmega 8-bit
@16 MHz
inference
(ANN)
domotic [44],
robotics [45]
Arduino
RP2040
0-2xARM Cortex-M0+
@133 MHz
(RP2040)
inference
(ANN)
parking traffic [46],
virtual reality [47]
MSP430G2553
LaunchPad
0-MSP430 16-Bit
RISC Architecture
@16 MHz
inference
(ANN)
activity
recognition [48]
Sony
Spresense
0-6xARM Cortex-M4F
@156 MHz
inference
(ANN)
object
detection [49]
SparkFun
Edge
0-32-bit ARM
Cortex-M4F
@48 MHz
inference
(ANN)
speech
recognition [50]
STM32F1030-ARM Cortex-M3
@72 MHz
inference
(CNN)
image
recognition [51]
STM32F765VI0-ARM Cortex-M7
@216 MHz
inference
(CNN)
image
recognition [52]
Tiny Eats0-ARM Cortex-M0+
@48 MHz
inference
(DNN)
audio
recognition [53]
Beaglebone
Black
1PowerVR
SGX530 GPU
ARM Cortex-A8
single-core @1 GHz
inference
(ANN)
robotics [54],
camera
drones [55]
Hello-Edge1-ARM Cortex-M7
(STM32F746G)
inference
(DNN)
keyword
spotting [56]
MAX780001Deep CNN
Accelerator
ARM Cortex-M4
@100 MHz
RISC-V coprocessor
@60 MHz
inference
(DNN)
object
detection [57]
ZedBoard
Dev. Board
1FPGA accel.2x ARM Cortex-A9
@667 MHz
inference
(DNN,CNN)
image
recognition [58]
Table 3. Low-power devices at the frontier of IoT and edge computing.
Table 3. Low-power devices at the frontier of IoT and edge computing.
DeviceClassGPU/Accel.CPUML UsageApplication
Examples
BeagleBone
AI
2-2x ARM Cortex-A15
@1,5 GHz
2x ARM Cortex-M4
SoC with 4 EVEs
inference
(CNN)
computer
vision [66]
Intel
Movidius
2-Myriad-2 VPUinference
(SVM)
computer
vision [67]
Raspberry
Pi 3
2400 MHz
VideoCore IV
GPU
4xARM A53
@1.2 GHz
inference
(SVM, CNN)
video
analysis [68]
medical data
processing [69]
Raspberry
Pi Z2 W
2400 MHz
VideoCore IV
GPU
4xARM Cortex-A53
@1 GHz
inference
(CNN)
object
detection [70]
RISC-V
GAP8
2-8 RISC-V 32-bit
@250 MHz +
HW ConvolutionEngine
inference
(CNN)
image, audio
processing [71]
Samsung
Galaxy S3
(Exynos 4412 SoC)
2Mali-400 MP
GPU
4xARM Cortex-A9
quad-core @1.4 GHz
inference
(CNN)
image
classif. [72]
Khadas VIM 32,34xARM Mali-G52
@800 MHz
4xARM Cortex-A73
@2.2 GHz
2xARM Cortex-A53
@1.8 GHz
inference
(CNN)
robotics [63]
Raspberry
Pi 4
2,3500 MHz
VideoCore VI
GPU
4xARM Cortex-A72
@1.5 GHz
inference
(SVM, CNN)
image
analysis [60]
Motorola
Z2 Force
(Snapdragon 835
SoC)
2, 3Qualcomm
Adreno 540 GPU
4x Kryo 280 @ 2.45 GHz
4x Kryo 280 @ 1.9 GHz
inference
(CNN)
image
classif. [62]
recognition [73]
Xiaomi
Redmi 4X
(Snapdragon 435
SoC)
2, 3Qualcomm
Adreno 505 GPU
8xARM Cortex-A53
@1.4 GHz
inference
(CNN)
image super
resolution [61]
Table 4. Powerful embedded devices for ML at the edge.
Table 4. Powerful embedded devices for ML at the edge.
DeviceClassGPU / Accel.CPUML UsageApplication
Examples
Coral
Development Board
3GC7000 Lite GPU
+
TPUEdge accel.
NXP i.MX 8M SoC
(4x ARM Cortex-A53 +
Cortex-M4F)
inference
(CNN)
image
processing
[74]
Google Pixel C
(Tegra X1 SoC)
3256-core
Maxwell GPU
4x ARM Cortex-A57
+
4x ARM Cortex-A53
inference
(SVM)
pedestrian
recognition
[75]
Jetson Nano3128-core
Maxwell GPU
4x ARM Cortex-A57inference
(CNN)
video image
recognition
[64]
Jetson TX13256-core
Maxwell GPU
4x ARM Cortex-A57
2x MB L2
inference
(CNN)
video, image
analysis [76],
robotics [77]
Odroid-XU4
(Exynos 5422 SoC)
3ARM
Mali-T628 MP6
GPU
4x ARM Cortex-A15 +
4x ARM Cortex-A7
inference/
training
(ANN)
urban flooding,
automobile
traffic [78]
RZ/V2M
Evaluation Board
3DRP-AI1x ARM Cortex-A53
@996 MHz
inference
(CNN)
image
processing [79]
Samsung
Galaxy S8
(Exynos 8895 SoC)
3ARM
Mali-G71
GPU
4x ARM Cortex-A53
@ 1.7 GHz
4x Exynos M2 @2.5 GHz
inference
(CNN)
image
recognition
[73]
Odroid-M13,44xARM Mali-G52
@ 650 MHz
4xARM Cortex-A55
@2 GHz
inference
(CNN)
video image
recognition [80]
Huawei P40 PRO
(Kirin 990)
416xARM Mali-G76
@600 MHz
2x ARM Cortex-A76
@2.86 GHz
2x ARM Cortex-A76
@2.09 GHz
4x ARM Cortex-A55
@1.86 GHz
inference
(CNN)
video super
resolution
[81]
Jetson TX24256-core
Pascal GPU
2x Denver2, 2 MB L2 +
4x ARM Cortex-A57,
2 MB L2
inference
(CNN, DNN,
SVM)
video, image
analysis [76],
robotics [82]
One Plus 9 Pro
(Snapdragon 888)
4Adreno 660 GPU1x ARM Cortex-X1
@ 2.84 GHz
3x ARM Cortex-A78
@2.42 GHz
4x ARM Cortex-A55
@1.80 GHz
inference
(CNN)
image
classification
[83]
Jetson AGX Orin52048xCUDA cores
64xTensor cores
@1.3 GHz
12xARM Cortex-A78
@2.2 GHz
inference
(CNN)
robotics [65]
Jetson AGX Xavier5512xVolta GPU8xNVIDIA Carmelinference
(CNN)
real-time
object detection
[84]
Table 5. Power consumption and duration of each steps in the use case scenarios.
Table 5. Power consumption and duration of each steps in the use case scenarios.
Step LabelsDetailed of the StepPower ConsumptionTime Duration
1Wait pairing and
synchronizing from the CYEdge
412.5 mW20 s
2Setting up measurement
parameters from the CYEdge
468 mW7 s
3Measuring digital and
analog data from sensor
357 mW10 s (Case I)
200 ms (Case II)
4Data processing
(filtering, conversion)
387 mW50 ms
5Sending stored and measured
data to the CYEdge (LoRa)
377.5 mW2–10 s
6Waiting acknowledgement
from the CYEdge
252 mW1–25 s
7Sleep until
next measuring
177 mW1 s–1 min
8Storing not sent data
in RAM memory
256.5 mW50 ms
Table 6. Comparison of CYSmart with similar edge computing technologies.
Table 6. Comparison of CYSmart with similar edge computing technologies.
CYSmartTMI
Orion
[100,101]
Gravio
[102]
Moneo
Appliance
[103]
Advantech
[104,105]
InHand
Networks
[106]
ADLINK
[107]
Smartmesh
WirelessHART
[108,109]
Device
classes
0 and 30 and 2240 and 3220 and 2
Sensor
diversity
highlowlowaveragehighlowhighaverage
Transmission
speed
0.3 S/sec10 S/sec-2500 S/sec-1000 Mb/sec256 KS/sec250 Kb/sec
Communication
distance (in meters)
80030100wired110wiredwired200
Number of
edge layers
22223222
Measurement
points
2046416-620500
Dimension of
measurement device
(in millimeters)
170 × 90 × 6531 × 129 × 7936 × 36 × 9-80 × 98 × 25--39 × 24 × 8
Dimension of
gateway/central device
(in millimeters)
245 × 110 × 85127 × 8 × 4697 × 97 × 2935 × 105 × 150260 × 140 × 54180 × 115 × 45110 × 40 × 126103 × 56 × 20
Easy
deployment
averageaveragehighhighaveragelowaveragelow
Application
diversity
highlowaveragelowhighaverageaveragehigh
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Martin Wisniewski, L.; Bec, J.-M.; Boguszewski, G.; Gamatié, A. Hardware Solutions for Low-Power Smart Edge Computing. J. Low Power Electron. Appl. 2022, 12, 61. https://doi.org/10.3390/jlpea12040061

AMA Style

Martin Wisniewski L, Bec J-M, Boguszewski G, Gamatié A. Hardware Solutions for Low-Power Smart Edge Computing. Journal of Low Power Electronics and Applications. 2022; 12(4):61. https://doi.org/10.3390/jlpea12040061

Chicago/Turabian Style

Martin Wisniewski, Lucas, Jean-Michel Bec, Guillaume Boguszewski, and Abdoulaye Gamatié. 2022. "Hardware Solutions for Low-Power Smart Edge Computing" Journal of Low Power Electronics and Applications 12, no. 4: 61. https://doi.org/10.3390/jlpea12040061

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