Assessing 8-bit MCU communication interfaces for the Internet of Things - Embedded.com

Assessing 8-bit MCU communication interfaces for the Internet of Things

Communication interfaces can generally be divided into two use-case types: machine-to-machine (M2M) and human-machine interfaces (HMI). The M2M interfaces range from garden-variety SPI/I2C/UART serial interfaces to more exotic custom serial interfaces and crystal-less USB and radios.

The HMI capabilities frequently found in microcontrollers (MCUs) include interfaces such as capacitive touch sense, LCD, graphics drivers, and gesture and proximity sensing. M2M and HMI capabilities – and the MCUs that support them – are critical to many of today’s connected device applications that enable the Internet of Things.

An 8-bit engine offering M2M and HMI interfaces may not be the optimal solution for all embedded system use cases, especially for those systems that are computationally intensive and require 32-bit code size and large flash options supported by ARM-based MCUs. However, applications that require deterministic behavior and hard, real-time control may benefit from a high-performance 8-bit engine coupled with these communications interfaces.

Communication interfaces

Generic interfaces
Many 8051 MCUs have at least one UART and one I2C interface as well as an SPI interface. Advanced 8-bit MCU architectures, such as those offered by Silicon Labs, enable these interfaces to be used simultaneously and muxed onto external pins seamlessly through an I/O crossbar. The I/O crossbar provides a mechanism to get any peripheral to any pin through a priority crossbar mux. Silicon Labs’ 8-bit MCUs integrate an onboard two-percent-accurate internal oscillator, enabling them to work without a crystal while providing sufficient accuracy for UART traffic.

On higher speed devices, prescalers allow these peripherals to run at reasonable rates. More sophisticated versions of this UART also implement integrated baud-rate generators, thus relieving resource pressure on the timers and simultaneously allowing access to a wide range of baud rates.

Crystal-less USB
Among the more complex communication interfaces is “crystal-less” USB, an innovation first developed and patented by Silicon Labs. This breakthrough took the simple, full-speed USB device interface and removed the need for an external crystal, thus reducing the bill of materials (BOM) cost of this capability for a large number of embedded system developers.

The secret to a crystal-less USB implementation lies in the clock recovery technique. A fully analog solution using a phase-locked loop (PLL) is susceptible to leakage-induced drift, and a fully digital solution requires a fast local clock to reduce output jitter and aliasing. The optimal solution uses a hybrid mixed-signal approach consisting of a digital feedback controller and a trimmable analog oscillator. This requires that the relative error between the local and reference clocks never increase. It is also completely data-independent (i.e., does not require any special USB traffic) and has the added benefit of being relatively energy-friendly compared to traditional crystal-based solutions.

RF communications
Arguably, the most complex communications interface for 8-bit MCUs involves the integration of a sub-GHz transceiver with an ultra-low-power 8051 core with transmit data rates of up to 256 kbps and a maximum output power of 20 dbm. The device, known as a sub-GHz wireless MCU, provides an optimal solution for many remote sensing applications by enabling sensitive analog signals to be sensed at the source and transmitted via radio to an aggregation device or node. The low-energy nature of the 8-bit wireless MCU makes it ideal for operating in the battery-powered environments commonly found in IoT sensor node applications. This type of device is perfectly suited for the IoT, given its low-power processing, wireless connectivity, and remote sensing capabilities.

LIN/CAN interfaces
Two automotive-specific, industry-standard interfaces, LIN 2.1 (master/slave) and CAN 2.0, also have been implemented on various 8-bit devices targeting a wide range of automotive applications. Silicon Labs’ automotive 8-bit MCUs have a ±0.5% accuracy oscillator (across voltage and temperature that enables the CAN interface to operate without a crystal). This capability is also unique in this class of device. A side benefit of having an accurate tunable oscillator onboard is that it is possible to generate accurate PWM edge placement (on the order of 120 ps), which has proven useful in small motor control applications and some power control applications.

Generic interfaces
For many high-speed 8-bit MCUs, there are a significant number of bus interfaces that can be efficiently “bit-banged.” Given the nature of the 8051 architecture and its response time, it is possible to turn around an external pin in under 30 ns. In other modes, the interrupt hierarchy can insert delays that make it impractical to use a bit-banged interface requiring fast bus turnaround.

Human-machine interfaces
HMI capabilities supported by many 8-bit MCUs include low-power segment-LCD drivers, capacitive touch sense interfaces, and gesture and proximity sensing. IoT applications require a variety of HMI capabilities since a large number of connected devices, such as security systems, smart thermostats, and lighting control systems, may have a human interaction component.

Capacitive touch
Capacitive touch interfaces can be used almost anywhere, including under glass and plastic, and are generally robust and immune to noise. Silicon Labs’ capacitive touch MCUs offer a sub-microamp wake-on-touch average current and a 100-to-1 dynamic range. Thus, since each pin conversion and detection happens in approximately 40 µs, the entire bank of 16 pins can be scanned in under 700 µs. This exceptional capacitive sensing performance enables high-speed periodic scanning for activity as well as extended sleep intervals that reduce overall power consumption. For example, the ultra-low-power consumption of a Silicon Labs capacitive sensing MCU can enable a remote controller using this technology to operate for 7 years on 2 AA batteries. Capacitive sensing technology is also perfect for buttons and sliders, such as those found on white goods, kitchen appliances, and security touch panels.Segment LCD
A segment LCD driver can be integrated intoan 8-bit MCU or offered as a standalone, fixed-function device. As astandalone device, an LCD controller offers the best leakage and dynamicpower characteristics of any LCD solution. This device interfaces to anadjacent MCU through SPI or I2C. It consumes so little current that itis possible to power the device from an input pin and completely forgothe VDD connection. Moreover, the die is exceedingly small and is bestused as a bare die or chip-on-glass rather than as a packaged component.(See Figure 1 .)

Figure 1: Example of standalone LCD controller

Gesture, proximity and ambient lighting
Proximitysensing is highly desirable in many IoT end nodes as well as inportable medical and mobile computing products that require humangesture control and detection. Silicon Labs offers a family of 8-bitproducts supporting infrared (IR)-based proximity control as well asambient and ultraviolet (UV) light sensing. For example, the Si114x MCUfamily implements proximity detection using one, two or three LEDs with arange of up to 50 cm, multi-dimensional motion sensing, heartrate/pulse oximetry and cheek detection capabilities. This sensingarchitecture works in direct sunlight and includes a light sensorcapable of sensing light levels up to 128 kLux. Light sensing technologyoften requires special packaging features, such as a transparent windowaround the light sensors. (See Figure 2 for an example of a proximitysensing MCU.)

Figure 2: Proximity sensing MCUs integrate sophisticated mixed-signal peripherals, interfaces, and drivers

Stacks and drivers
Ofcourse, these MCU interfaces require either stacks and/or drivers toenable their quick integration into a system. The interfaces discussedin this article (except for the simple ones such as UART, SPI, and I2C)come with drivers and/or stacks available at no charge from SiliconLabs. For example, Silicon Labs’ 8-bit MCUs featuring crystal-less USBcome with a full-featured USB driver included in the USBXpressdevelopment kit that provides a complete host and device softwaresolution.

MCU interfaces and the IoT
Today’sinterconnected IoT ecosystem favors IC devices with a wide variety ofinterfaces since the heterogeneous nature of the embedded marketplacerequires these devices to be able to converse in as many “dialects” aspossible.

A significant number of IoT applications are “thinclient” in nature. This is what makes them a natural fit for an 8-bitmachine with limited flash and onboard RAM. For example, most sensorapplications where voltages/currents must be sensed and operated uponand then transmitted upstream are suited to an 8-bit machine. Examplesinclude gas and oxygen sensors in connected home applications andpressure sensors in commercial/industrial applications.

Simplecontrol applications are also better suited to 8-bit rather than 32-bitmachines, especially if complex real-time I/O manipulation is necessary.Specifically, the 8051 architecture allows fast I/O bit manipulationwith concurrent logical operations, which is useful in controlapplications. These applications are usually space- and power-sensitive,which also plays to the strengths of 8-bit devices such as thehigh-speed 8051 MCUs from Silicon Labs. Note that various ARM Cortex-Mseries devices can also play in these applications, but, given the boardarea and power and real-time limitations of the systems, an 8-bitmachine with a more deterministic execution model will perform better.

Conclusion
Today’sIoT connected device applications require versatile MCUs capable ofaddressing complex communication challenges in a multi-protocolenvironment. A preponderance of MCU interfaces and connectivitytechnologies must co-exist on the same die simply because the IoTecosystem is so diverse. RF integration in particular has done anoutstanding job of uniting two essential IoT capabilities: ultra-lowpower and wireless communication. The addition of superior analogperformance enables the creation of wireless sensor nodes requiring verylittle external support circuitry.

While 8-bit MCUs may not bethe right fit for every IoT-connected device application, they are goodchoices for cost-sensitive applications requiring small packages, smallmemory footprints, high functional density, determinism, and speed ofresponse. The high-performance 8051 8-bit architecture coupled with theplethora of interfaces available today provides an ideal solution formany IoT applications.

Thomas David is a principal design engineer for Silicon Labs’ microcontroller products. He was the lead designer for Silicon Labs’ first 32-bit MCU products, the Precision32 family based on the ARM Cortex-M3 processor. In addition, he has been involved, either as a designer or as a chip lead, with almost all of the MCUs released by Silicon Labs. Mr. David came to Silicon Labs as part of the Cygnal Integrated Products acquisition in 2003. Prior to his tenure at Cygnal, he was president of Silogix, an Austin, Texas-based silicon intellectual property company that was acquired by Cygnal Integrated Products. He holds a BSEE from Purdue University and an MSEE from Penn State University.

2 thoughts on “Assessing 8-bit MCU communication interfaces for the Internet of Things

  1. “M0's that can actually accomplish something don't tend to be small or cheap (a relative term here) and thus the package size enters into the picture. Silabs sells both and they serve two distinct markets. nnAn app that requires subtle I/O control will

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