Implementing AIoT on MCUs

Article By : Kavita Char

Implementation of AIoT on MCUs will increase exponentially in new applications as MCUs push the boundary on performance and blur the line between MCUs and MPUs.

In my previous article, I highlighted the growing trend enabling more and more artificial intelligence (AI) in internet of (IoT) things devices to create intelligent ‘AIoT’ devices, and the various applications are benefiting from these intelligent devices. These learn from data and make autonomous decisions without human intervention, leading to products having more logical, human-like interactions with their environment.

The combination of AI and IoT has opened up new markets for MCUs (microcontrollers). It has enabled an increasing number of new applications and use cases that can use simple MCUs paired with AI acceleration to facilitate intelligent control. These AI-enabled MCUs provide a unique blend of DSP capability for compute and machine learning (ML) for inference and are now being used in applications as diverse as keyword spotting, sensor fusion, vibration analysis and voice recognition. Higher performance MCUs enable more complex applications in vision and imaging such as face recognition, fingerprint analysis and autonomous robots.

AI technologies

Here are some technologies that enable AI in IoT devices:

Machine learning (ML): Machine learning algorithms build models based on representative data, enabling devices to identify patterns automatically without human intervention. ML vendors provide algorithms, APIs and tools necessary to train models that can then be built into embedded systems. These embedded systems then use the pre-trained models to drive inferences or predictions based on new input data. Examples of applications are sensor hubs, keyword spotting, predictive maintenance and classification.

Deep learning: Deep learning is a class of machine learning that trains a system by using many layers of a neural network to extract progressively higher-level features and insights from complex input data. Deep learning works with very large, diverse and complex input data and enables systems to learn iteratively, improving the outcome with each step. Examples of applications that use deep learning are image processing, chatbots for customer service and face recognition.

Natural language processing (NLP): NLP is a branch of artificial intelligence that deals with interaction between systems and humans using natural language. NLP helps systems understand and interpret human language (text or speech) and make decisions based on that. Examples of applications are speech recognition systems, machine translation and predictive typing.

Computer vision: Machine/computer vision is a field of artificial intelligence that trains machines to gather, interpret and understand image data, and take action based on that data. Machines gather digital images/videos from cameras, use deep learning models and image analysis tools to accurately identify and classify objects, and take action based on what they “see”. Examples are fault detection on manufacturing assembly line, medical diagnostics, face recognition in retail stores and driverless car testing.

AIoT on MCUs

In times past, AI was the purview of MPUs and GPUs with powerful CPU cores, large memory resources and cloud connectivity for analytics. In recent years though, with a trend towards increased intelligence on the edge, we are starting to see MCUs being used in embedded AIoT applications. The move to the edge is being driven by latency and cost considerations and involves moving computation closer to the data. AI on MCU based IoT devices allows real-time decision making and faster response to events, and has advantages of lower bandwidth requirements, lower power, lower latency, lower costs and higher security. AIoT is enabled by higher compute capability of recent MCUs as well as availability of thin neural network (NN) frameworks that are more suited for resource constrained MCUs being used in these end devices.

A neural network is a collection of nodes, arranged in layers that receive inputs from a previous layer and generate an output that is computed from a weighted and biased sum of the inputs. This output is passed on to the next layer along all its outgoing connections. During training, the training data is fed into the first or the input layer of the network, and the output of each layer is passed on to the next. The last layer or the output layer yields the model’s predictions, which are compared to the known expected values to evaluate the model error. The training process involves refining or adjusting the weights and biases of each layer of the network at each iteration using a process called backpropagation, until the output of the network closely correlates with expected values. In other words, the network iteratively “learns” from the input data set and progressively improves the accuracy of the output prediction.

The training of the neural network requires very high compute performance and memory and is usually carried out in the cloud. After the training, this pre-trained NN model is embedded in the MCU and used as an inference engine for new incoming data based on its training.

Fig 1 - Renesas part 2 - neural network training and inference
Figure 1: Neural network training and inference

This inference generation requires much lower compute performance than the training of the model and is thus suited for an MCU. The weights of this pre-trained NN model are fixed and can be placed in flash, thus reducing the amount of SRAM required and making this suitable for more resource constrained MCUs.

Implementation on MCUs

The AIoT implementation on MCUs involves a few steps. The most common approach is to use one of the available neural network (NN) framework models like Caffe or Tensorflow Lite, suitable for MCU based end device solutions. The training of the NN model for machine learning is done in the cloud by AI specialists using tools provided by AI vendors. The optimization of the NN model and integration on the MCU is carried out using tools from the AI vendor and the MCU manufacturer. Inferencing is done on the MCU using the pre-trained NN model.

The first step in the process is done completely offline and involves capturing a large amount of data from the end device or application, which is then used to train the NN model. The topology of the model is defined by the AI developer to make best use of the available data and provide the output that is required for that application. Training of the NN model is done by passing the data sets iteratively through the model with the goal to continuously minimize the error at the output of the model. There are tools available with the NN framework that can aid in this process.

In the second step, these pre-trained models, optimized for certain functions like keyword spotting or speech recognition, are converted into a format suitable for MCUs. The first step in this process is to convert it into a flat buffer file using the AI converter tool. This can optionally be run through the quantizer, in order to reduce the size and optimize it for the MCU. This flat buffer file is then converted to C code and transferred to the target MCU as a runtime executable file.

This MCU, equipped with the pre-trained embedded AI model, can now be deployed in the end device. When new data comes in, it is run through the model and an inference is generated based on the training. When new data classes come in, the NN model can be sent back to the cloud for re-training and the new re-trained model can be programmed on the MCU, potentially via OTA (over the air) firmware upgrades.

There are two different ways that an MCU-based AI solution can be architected. For the purpose of this discussion, we are assuming the use of Arm Cortex-M cores in the target MCUs.

Fig 2 - Renesas part 2 - AI implemetation on MCUs
Figure 2: AI implementation on MCUs using offline pre-trained models.

In the first method, the converted NN model is executed on the Cortex-M CPU core and is accelerated using the CMSIS-NN libraries. This is a simple configuration that can be handled without any additional hardware acceleration and is suited for the simpler AI applications such as keyword spotting, vibration analysis and sensor hubs.

A more sophisticated and higher performance option involves including an NN accelerator or micro neural processing unit (u-NPU) hardware on the MCU. These u-NPUs accelerate machine learning in resource constrained IoT end devices and might support compression that can reduce power and size of the model. They support operators that can fully execute most of the common NN networks for audio processing, speech recognition, image classification, and object detection. The networks that are not supported by the u-NPU can fall back to the main CPU core and are accelerated by the CMSIS-NN libraries. In this method, the NN model is executed on the uNPU.

These methods show just a couple of ways to incorporate AI in MCU based devices. As MCUs push the performance boundaries to higher levels, closer to that expected from MPUs, we expect to start seeing full AI capabilities including lightweight learning algorithms and inference, being built directly on MCUs.

AI on the edge is the future

Implementation of AI on resource-constrained MCUs will increase exponentially in the future and we will continue to see new applications and use cases emerge as MCUs push the boundary on performance and blur the line between MCUs and MPUs, and more and more “thin” NN models, suitable for resource constrained devices, become available.

In the future, with an increase in MCU performance, we will likely see implementation of lightweight learning algorithms in addition to inference, being run directly on the MCU. This will open up new markets and applications for MCU manufacturers and will become an area of significant investment for them.

This article was originally published on Embedded.

Kavita Char is a senior staff product marketing manager at Renesas Electronics America. She has over 20 years of experience in software/applications engineering and product management roles. With extensive experience in IoT applications, MCUs and wireless connectivity, she is now responsible for definition and concept to launch management of next-generation Arm-based high performance MCUs and solutions at Renesas.


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