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Assessing 8bit MCU communication interfaces for IoT

Posted: 13 Aug 2014  Print Version  Bookmark and Share

Keywords:communication interfaces  human-machine interfaces  HMI  IoT  sensor 

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 8bit 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 32bit code size and large flash options supported by ARM-based MCUs. However, applications that require deterministic behaviour and hard, real-time control may benefit from a high-performance 8bit 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 8bit 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' 8bit MCUs integrate an onboard two-per cent-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 analogue 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 analogue 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 8bit 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 analogue signals to be sensed at the source and transmitted via radio to an aggregation device or node. The low-energy nature of the 8bit 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.

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