IoT SoC Architectures for Edge Devices

There are two distinct SoC architectures, depending on the IoT application. Low-end IoT edge devices are characterized by small computing power and low cost. They leverage embedded flash to avoid the need for external memory. Low-end devices often run a compact Real-Time Operating System (RTOS) that requires little memory to operate or do not even run an operating system. Also, low-end devices integrate a number of peripheral and sensor interfaces that utilize the process technology’s analog features. Selecting the ideal process technology depends on cost and availability of embedded flash options. Traditionally, the SoCs for low-end devices were implemented in established processes such as 180-nm or 90-nm. Today, they are moving to 55-nm and 40-nm as embedded flash options become available, reducing additional power consumption in all modes of operation.

The Bluetooth Smart protocol is used to enable wireless connectivity in wearable devices since they require limited data transfer and low power consumption, operate on a coin-cell battery, are small in size and low in cost. Alternatively, other protocols, such as the ones based on the IEEE 802.15.4 standards are sometimes used to enable wireless connectivity in home automation and machine-to-machine applications.

A high-end device is based on a similar SoC architecture used in mobile phones, ebooks and other portable consumer electronics. High-end devices benefit from higher computing power and have higher memory requirements. They typically run Linux, Android or feature-rich OSs and include DDR memory interfaces to external DRAM memories. The SoC for high-end devices is often implemented in a range of advanced processes from 40-nm to 16/14-nm FinFET technologies. Since these devices require large amounts of data transfers, WiFi, Cellular 3G/4G and Bluetooth Smart standards are used for wireless connectivity.

Enabling Energy-Efficient IoT Edge Devices

Regardless of the SoC architecture implemented in the edge device, energy efficiency is critical because the device is expected to function on a small battery for days without needing to be recharged.

Extending battery life is critical to IoT devices and for that reason, the SoC functionality is divided into blocks. One block that needs to be always-on such as voice activation, which makes reducing dynamic power important and other blocks that are idle for extended periods of time, which makes reducing static power important.

For additional power savings, the Bluetooth Smart is used because the protocol implements a low-duty cycle of operation, requiring minimal connection and re-connection time. Designers are implementing low-power design techniques such as clock gating, power gating, smart biasing and Dynamic Voltage Frequency Scaling (DVFS).

Foundry processes like 55-nm and 40-nm make use of finer pitch features that provide higher gate density, significantly lower power, and lower manufacturing cost, which are critical considerations for IoT applications. These processes also include ultra-low-power (ULP) features that enable very low-voltage operation and reduce leakage by offering ultra-high threshold devices.

Wireless Connectivity Solutions for IoT Edge Devices

System designers have many wireless connectivity protocols to choose from, each providing unique benefits that target different applications. For wearable devices, Bluetooth Smart and, in some cases, Near-Field Communication (NFC) are ideal protocols. For machine-to-machine communications, Bluetooth Smart and protocols such as WiFi, Zigbee/Thread Group are often used. Let’s briefly describe and compare these common wireless connectivity protocols, also summarized in Table 1.

• The Bluetooth protocol is defined by the Bluetooth Special Interest Group (SIG). Bluetooth Smart, a derivative of Bluetooth Classic designed for ultra-low-power consumption is used to wirelessly connect devices over short distances in a simple way, with minimal system requirements. The Bluetooth Smart standard is defined in the latest Bluetooth 4.2 standard and is ideal for applications that require low power consumption, operate on a coin-cell battery, are small in size and low in cost. The next revision of the standard is expected to bring additional features such as mesh networking, extended range and higher data rate support that overcome some of the limitations of Bluetooth Smart in the context of home automation.
• The WiFi protocol is defined by the WiFi Alliance as a Wireless Local Area Network (WLAN) based on the IEEE 802.11 standards. The protocol is mainly used to connect devices such as personal computers, video game consoles, smartphones, tablets and digital cameras to the Internet. WiFi uses the 2.4 GHz ultra-high frequency and 5 GHz super-high frequency ISM (industrial, scientific and medical) bands. In addition to allowing Internet connection in the home, WiFi can be used for city-wide or building-wide Internet connection. The WiFi protocol features high data rates that make it suitable for content-heavy applications. The WiFi Alliance is developing a version of the standard (802.11ah) targeting low power. This version has not yet been standardized, limiting the use of WiFi in wearables and other low-power IoT applications.
• The ZigBee protocol is defined by the ZigBee Alliance and is based on the IEEE 802.15.4 standards for the PHY and MAC. It is a communication protocol that is simpler and more cost-effective than WiFi used in electrical meters, traffic management systems and other applications requiring low-power consumption and wireless data transfer at low rates. Zigbee’s adoption has been relatively modest in domains besides industrial automation. Recently other protocols based on the 802.15.4 standards, such as Thread Group, have emerged to address the limitations of ZigBee and have higher potential for adoption in the smart home.

The above wireless connectivity protocols are optimized for personal-wide and home-wide networks. Low power, wide area networks for city-wide automation make use of cellular-like protocols specialized for low power dissipation, such as the ones advocated by LoRa Alliance, SigFOX and 3GPP. In particular, 3GPP is standardizing low-power protocols based on LTE such as LTE-M, LTE-Cat 0.

Bluetooth Smart Protocol Stack

Bluetooth Smart is a layered protocol (Figure 2). At the lowest physical layer, the PHY consists of the RF transceiver and baseband features such as modulation and demodulation of the Bluetooth signal. The Link Layer establishes and manages the connections between devices, as well as other low level and security functions. Together, these layers are termed “Controller Subsystem” in the Bluetooth specification.

The application profiles are part of the “Host Subsystem” and are implemented as software running on a generic processor that may or may not be dedicated to the Bluetooth interface. Collectively, this implementation is called the Software Stack.

DesignWare Bluetooth Smart IP

Synopsys’ Bluetooth Smart IP is compliant with the latest Bluetooth Low-Energy standard and is thoroughly tested and qualified in accordance to the Bluetooth SIG procedures. The DesignWare® Bluetooth Smart IP is listed as a Qualified Design by the Bluetooth SIG.

As an Associate Member of the Bluetooth SIG, Synopsys provides SoC designers with high-quality Bluetooth Smart IP that implements the functionality defined by the latest releases of the standard, as well as the complete Bluetooth Smart protocol stack.

The DesignWare Bluetooth Smart IP complements Synopsys’ broad portfolio of silicon-proven IP solutions optimized for IoT applications. DesignWare Bluetooth Smart IP consumes very low power and operates down to sub-one volt supply, reducing power dissipation and extending battery life. The silicon-proven DesignWare Bluetooth Smart IP provides an integrated matching network for accurate signal and energy transfer between transceiver and antenna to reduce the requirement for external components, resulting in a lower bill of materials and a more compact board design. In addition to other processes, the PHY IP (Figure 3) available on 55-nm process technology, ideal for IoT SoCs, interoperates with multiple link layers and software stacks, enabling SoC designers to build complete Bluetooth solutions based on qualified IP.