The Anatomy of Security Microcontrollers for IoT Applications

By Majeed Ahmad

Contributed By Digi-Key's North American Editors

At a time when implementing security in embedded designs seems an overwhelming task, microcontrollers (MCUs) specializing in security features are coming forward to enable security at the inception of the embedded design. That’s a good thing, because there’s no question that a new breed of embedded solutions is needed to secure the Internet of Things (IoT) applications.

A study from ABI Research estimated that less than 4% of IoT devices sold last year featured embedded security. At the same time, the market research firm forecasts that, by 2020, nearly 25% of cyberattacks will target IoT devices, making the security MCUs a hot potato.

But what is a security MCU? Many MCUs claim to offer security features, but a closer look reveals that it’s mostly lip service. This article digs deeper into the traits that define a security MCU. It will delve into the properties and features that distinguish a secure MCU from one that just makes a claim.

Let’s begin with the MCU suppliers’ quest to complement their hardware-based security solutions with an extra layer of security for bolstering defense against software vulnerabilities and network-based attacks.

Tale of two MCU collaborations

The advent of endpoint devices operating at the network edge calls for secure over-the-air (OTA) firmware updates. The RX651 microcontrollers from Renesas address this reprogramming requirement by integrating Trusted Secure IP (TSIP) and trusted flash area protection, which enables flash firmware updates in the field through secure network communications.

The TSIP offers robust key management, encrypted communication, and tampering detection to ensure strong security against external threats such as eavesdropping, tampering, and viruses (Figure 1). Likewise, the integrated dual bank flash memory makes it easier for device manufacturers to execute in-the-field firmware updates securely and reliably.

Diagram of building blocks in the Renesas RX651 microcontrollerFigure 1. A view of the security building blocks (center right) in the RX651 microcontroller. (Image source: Renesas Electronics)

The dual bank flash enables embedded system designers to realize the high root-of-trust levels through a combination of TSIP for protecting the encryption keys and encryption hardware accelerators such as AES, 3DES, RSA, SHA, and TRNG. Then there’s code flash area protection to safeguard the boot code from unauthorized reprogramming.

Next, Renesas has partnered with Secure Thingz, an embedded systems security expert, for the secure provisioning of its RX family of 32-bit microcontrollers. For that, Renesas will support the Secure Deploy architecture that Secure Thingz has created to simplify the security implementations across design and manufacturing value chains.

Another MCU supplier that has teamed up with an embedded security solution provider is STMicroelectronics. The chipmaker is collaborating with Arilou Information Security Technologies to create a multi-layer security arrangement in which hardware and software can complement each other to monitor the data streams and detect communication anomalies.

STMicro’s SPC58 Chorus series of 32-bit automotive microcontrollers embed a Hardware Security Module (HSM) that protects sensitive security information such as cryptographic keys and thus ensures protection against intrusion via communication buses in automotive body and gateway applications. HSMs provide the hardware-based root of trust to facilitate secure communications, OTA updates, and secure boot.

Now ST has added Arilou's Intrusion Detection and Prevention system (IDPS) software to its SPC58 Chorus series of automotive MCUs to detect traffic anomalies and form an extra layer of protection against cyberattacks (Figure 2). IDPS is a software solution designed to monitor the controller area network (CAN) bus and detect anomalies in the communication patterns of electronic control units (ECUs) in automotive designs.

Image of IDPS software detecting traffic and other roadside anomalies (click to enlarge)Figure 2. This is how the IDPS software detects the traffic and other roadside anomalies. (Image source: Arilou)

Specialized security MCUs

The above section has outlined MCUs that integrate security capabilities to counter physical and remote attacks. This section will explain specialized security MCUs, often called secure elements, that act as a companion chip to the main MCU while being linked over an I2C or single-wire interface.

Secure elements, which support MCUs without security features, are based on purpose-built hardware to provide a security framework against a wide array of threats. They are simple, cheap, and offload the main MCU or CPU from security-related tasks such as key storage, cryptographic acceleration, etc. That’s why they are also known as security co-processors.

In a secure element, all security building blocks work under a common boundary, which isolates authentication keys from software and thus prevents hackers from carrying out attacks like power cycling, clock glitches, and side-channel attacks. The uploading of security keys and certificates in secure elements at factories also prevents IP theft, design cloning, and product counterfeiting.

Take, for instance, Microchip’s ATECC608A secure element (Figure 3), which features a random number generator (RNG) for unique key generation while complying with the latest requirements from the National Institute of Standards and Technology (NIST). It also features cryptographic accelerators like AES-128, SHA-256, and ECC P-256 for mutual authentication.

Block diagram of the Microchip ATECC608A secure elementFigure 3. The block diagram of the ATECC608A secure element. (Image: Microchip Technology)

Finally, secure ROM for key storage facilitates an immutable environment that’s hard for hackers and spoofs to alter, and thus prevents tampering and side-channel attacks. Collectively, the ATECC608A offers services ranging from secure boot to OTA validation to secure key storage and transfer for the IoT and cloud service authentication.

Another low-cost MCU specializing in security implementations is Microchip’s SAM L11 (Figure 4), which protects power-constrained IoT nodes from threats like fault injection and side-channel attacks. It abstracts low-level security details with a modular GUI that enables developers to pick and choose relevant security features, and that’s how it simplifies the embedded security implementation.

Diagram of four use cases for Microchip SAM L11 security microcontroller (click to enlarge)Figure 4. Four security use cases employing the SAM L11 security microcontroller. (Image: Microchip Technology)

The security features that the SAM L11 abstracts include third-party provisioning services. It also incorporates Arm’s TrustZone technology that isolates secure and non-secure code within a microcontroller. Moreover, the SAM L11 streamlines IoT node’s security needs while connecting with cloud services like Amazon Web Services (AWS).

NXP’s LPC5500 microcontroller, targeted at IoT edge applications, is another example of security-based MCUs. It utilizes device-unique keys to create an immutable hardware root of trust. The keys can be locally generated by an SRAM-based physically unclonable function (PUF) that permits closed-loop transactions between the end-user and the OEM. That operation eliminates the need for third-party key handling.

Specialized security tools

While security-centric MCUs like the ATECC608A encompass security building blocks to facilitate a trusted ecosystem, they don’t address software isolation. Now, at a time when the amount of software running on MCUs is continuously growing, developers need to protect a large codebase from malicious attacks.

The IoT devices, for example, have protocol stacks for Wi-Fi, Bluetooth, TLS, etc., and the corruption of protocol stacks can impact device operation even without hackers stealing the security keys. That calls for a separation of mission-critical code from non-mission-critical code and places the critical software in a secure environment.

Arm’s TrustZone environment (Figure 5) separates mission-critical code and protocol stacks from complex operating system (OS) software and large codebases, and thus prevents firmware backdoors into security key storage areas. It creates multiple software security domains to restrict access to the specific memory, peripherals, and I/O components inside the microcontroller.

Diagram of the basic framework of TrustZone technologyFigure 5. The basic framework of TrustZone technology for isolating software into secure and non-secure zones. (Image source: Arm)

All three security MCUs, mentioned above, the ATECC608A, SAM L11, and LPC5500 (Figure 6), have incorporated TrustZone technology to separate secure and non-secure code. Furthermore, the ATECC608A secure element can work with any TrustZone-enabled microcontroller.

Diagram of TrustZone technology in the NXP LPC5500 microcontroller (click to enlarge)Figure 6. The TrustZone technology in the LPC5500 microcontroller comes bundled with the Arm Cortex M-33 processor, shown on top left part of the diagram. (Image source: NXP)

Here, it’s also worth mentioning that security MCUs and TrustZone technology complement each other in a sense that TrustZone requires hardware protection, and security MCUs such as the ATECC608A and SAM L11 facilitate that within an IoT design setting. On the other hand, TrustZone helps create a compact software environment in the MCU-centric embedded designs.


The anatomy of security MCUs shows how they simplify the embedded security implementation during the design phase and how they bypass the steep learning curve in terms of security technology expertise. These specialized MCUs also bring down the cost overhead and power consumption, two major considerations in the highly constrained IoT designs.

At a time when the inception of IoT devices is outpacing the rate at which these connected designs are securely deployed, security MCUs offer a viable path for confronting cyber threats on multiple fronts. They offer a simplified solution that comes equipped with a security design ecosystem to facilitate point-and-click development environments.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Majeed Ahmad

Majeed Ahmad is an electronics engineer with more than 20 years of experience in B2B technology media. He is former Editor-in-Chief of EE Times Asia, a sister publication of EE Times.

Majeed has authored six books on electronics. He is also a frequent contributor to electronics design publications, including All About Circuits, Electronic Products and Embedded Computing Design.

About this publisher

Digi-Key's North American Editors