Choosing the Right 8-bit MCU Communication Interface for IoT Applications

　　SiliconLabs has developed various 8-bit MCUs compatible with the 8051 core under the guiding ideology of "collection, calculation and communication" . In previous articles, we have discussed in detail the topics related to analog acquisition interfaces and 8051 computing engines. This article mainly discusses how the communication interface built into the 8051 core MCU can meet the needs of today's rapidly emerging IoT applications.

　　Introduction

　　Communication interfaces are usually divided into two categories based on the use case: machine-to-machine (M2M) and human-to-machine (HMI). There are many types of M2M interfaces, from common SPI/I2C/UART serial interfaces to more complex custom serial interfaces, crystal-free USB, and radio. HMI is commonly found in built-in interfaces in microcontrollers (MCUs), such as capacitive touch sensing, LCD, graphics drivers, gestures, and proximity sensing. M2M and HMI capabilities - and the MCUs that support them - have become key to most connected device applications in today's Internet of Things.

　　8-bit engines that provide M2M and HMI interfaces do not provide the best solution for all embedded system use cases, especially those that require intensive computing, 32-bit data processing, and large Flash space options for ARM-based MCUs. However, applications that require deterministic behavior and strict real-time control can benefit from 8-bit engines with these communication interfaces.

　　Communication Interface

　　Common Interface

　　Many 8051 MCUs have at least one UART, one I2C interface, and one SPI interface. More advanced 8-bit MCU architectures, such as those provided by Silicon Labs, can support these interfaces simultaneously and can be seamlessly assigned to external pins through the I/O Crossbar. The I/O Crossbar provides a mechanism to map any peripheral to any pin through a prioritized Crossbar. Silicon Labs' 8-bit MCUs integrate a 2% accurate internal oscillator, which allows the MCU to work properly without an external crystal while meeting the accuracy requirements of UART communication.

　　In high-speed devices, prescalers allow these peripherals to run at the appropriate rate. Advanced versions of these UARTs also integrate a baud rate generator, eliminating the need for timer-like resources and allowing support for a wider range of baud rates.

　　For many high-speed 8-bit microcontrollers, there are a number of bus interfaces that require "bit reversal". The nature of the 8051 architecture and its response time allows for external pin reversals of less than 30ns. In other cases, the interrupt hierarchy can introduce delays that make interfaces with "bit reversal" capabilities unsuitable for situations where fast bus reversals are required.

　　Crystal-free USB

　　One of the more complex communication interfaces is "crystal-less" USB, an innovation first developed and patented by Silicon Labs. This breakthrough innovation supports a full-speed USB device interface without the need for an external crystal, thus reducing BOM costs for most embedded system developers.

　　The secret to a crystal-less USB implementation lies in clock recovery techniques. Fully analog solutions using phase-locked loops (PLLs) are susceptible to leakage-induced drift, while fully digital solutions require a fast local clock to reduce output jitter and aliasing effects. The best solution uses a mixed-signal approach consisting of a digital feedback controller and an adjustable analog oscillator. This requires that the relative error between the local clock and the reference clock never increases. It is also completely data-independent (i.e., does not require any special USB communication) and has the added benefit of being more energy efficient than traditional crystal-based solutions.

　　RF Communications

　　The most complex communication interface on an 8-bit MCU is a sub-GHz transceiver with a maximum transmission rate of 256kbps and a maximum output power of 20dBm integrated on an ultra-low power 8051 core. This type of device, also known as a sub-GHz wireless MCU, provides an optimal solution for many remote sensing applications by sensing sensitive analog signals at the source and then sending them to a centralized device or node using radio. The low power consumption characteristics of 8-bit wireless MCUs make this type of device ideal for battery-powered operating environments, such as IoT sensor node applications. With its low-power processing, wireless connectivity, and remote sensing capabilities, this type of device is very suitable for the Internet of Things.

　　LIN/CAN interface

　　Two industrial standard interfaces specifically for automotive applications, LIN2.1 (master/slave) and CAN2.0, have also been integrated into 8-bit devices for various automotive applications. SiliconLabs' automotive-grade 8-bit MCU integrates an oscillator with an accuracy of ±0.5% (over the full voltage and temperature range), which enables the CAN interface to work properly without an external crystal. This performance is also unique among similar devices. Another benefit of this precision-adjustable on-chip oscillator is that it can generate high-precision PWM edge signals (on the order of 120ps), which has proven to be very practical in small motor control applications and some power control applications.
 

　　Human-machine interface

　　Many 8-bit MCUs support human machine interface functions, including low-power segment-LCD drivers, capacitive touch sensing interfaces, gestures, and proximity sensing. IoT applications require a variety of human machine interface functions because a large number of connected devices, such as security systems, smart thermostats, and lighting control systems, may have human machine interaction components.

　　Capacitive touch

　　Capacitive touch interfaces can be used almost anywhere (including under glass and plastic) and are generally very reliable and noise-resistant. Silicon Labs' capacitive touch MCUs offer sub-microampere average touch wake-up current and a 100:1 dynamic range. Since each pin conversion and detection takes about 40μs, the entire 16-pin scan can be completed in less than 700μs. This special capacitive sensing performance enables high-speed periodic scanning of active events and extended sleep intervals, thereby reducing overall power consumption. For example, Silicon Labs' ultra-low power capacitive sensing MCU can enable a remote control powered by 2 AA batteries to operate for 7 years. Capacitive sensing technology is also superior to buttons and sliders and is commonly found in devices such as white goods, kitchen appliances, and security touch panels.

　　Segment LCD

　　Segment LCD drivers can be integrated into an 8-bit MCU or as a standalone functional device. As a standalone device, LCD controllers offer the best leakage and dynamic power consumption characteristics for LCD solutions. Such devices connect to a nearby MCU via SPI or I2C. It consumes so little power that it can power itself from only one input pin and does not require a connection to VDD. In addition, the LCD driver's extremely small die size makes it ideal as a bare die or integrated in glass, rather than as a separately packaged device. (See Figure 1.)

　　Figure 1 - Standalone LCD controller example

　　Gesture, proximity and ambient lighting

　　Proximity sensing is highly desirable in many IoT end nodes and portable medical and mobile computing products that require gesture control and detection. Silicon Labs offers a range of 8-bit products that support infrared-based proximity control, as well as ambient light and UV sensing. For example, the Si114x MCU family of products enables single, dual and triple LED proximity detection with a sensing distance of up to 50 cm, multi-dimensional motion sensing, heart rate/blood oxygen and face detection. The sensor architecture can operate in direct sunlight, and the built-in light sensor can sense a maximum light intensity of 128 kLux. Light sensing technology usually requires special packaging features, such as a transparent window around the light sensor. (As shown in Figure 2, an example of a proximity sensing MCU.)

　　Figure 2 - Proximity sensing MCU with integrated advanced mixed-signal peripherals, interfaces, and drivers

　　Interface stack and drivers

　　All MCU interfaces require protocol stacks and/or drivers to enable quick integration into the system. All interfaces discussed in this article (except for very simple ones such as UART, SPI, and I2C) can be downloaded free of charge from the SiliconLabs website. For example, the full-featured USB driver for SiliconLabs' 8-bit MCU with integrated crystal-free USB is included in the USBXpress Development Kit, which provides a complete host and device software solution.
 

　　MCU Interface and IoT

　　Today's interconnected IoT ecosystem is conducive to the integration of IC devices with various interfaces. Therefore, the diverse nature of the embedded market requires these devices to support the conversion of as many types of "special interfaces" as possible.

　　Most IoT applications are "thin clients" in nature. This makes them a natural fit for 8-bit devices where the size of Flash and on-board RAM is limited. For example, most sensor applications require sensing and manipulating voltage/current and then uploading data, which is a good fit for 8-bit devices. Another example is gas and oxygen sensors in connected home applications and pressure sensors in commercial/industrial applications.

　　8-bit devices are better suited for simple control applications than 32-bit devices, especially if complex real-time I/O operations are necessary. Specifically, the 8051 architecture allows fast I/O bit operations with concurrent logic operations, which is very useful for control applications. These applications are often space-constrained and power-sensitive, which is where 8-bit devices (such as the high-speed 8051 MCUs from Silicon Labs) come in handy. It should be noted that various ARM Cortex-M series devices can also play a role in these applications, but given the system's board area, power consumption, and real-time constraints, 8-bit devices with more deterministic execution modes will perform better.

　　in conclusion

　　Today's IoT connected device applications require versatile MCUs to meet the challenges of load communication brought by multi-protocol environments. The IoT ecosystem is so diverse that the advantages of MCU interfaces and connection technologies must simply coexist on the same chip. RF integration is an excellent fusion of two basic IoT features: ultra-low power consumption and wireless communication. In addition, excellent analog performance makes it possible to create wireless sensor nodes with only minimal external support circuits.

　　Although 8-bit MCUs may not be suitable for all IoT connected device applications, they are a very good choice for cost-sensitive applications that require small package size, small memory space, high functional density, determinism and responsiveness. The high-performance 8051 8-bit architecture, as well as the many interfaces available today, is an ideal solution for most IoT applications.