electronica 2014 INTERNET OF THINGS Hall A5 Stand 476
“Every little bit helps”
Tom David assesses 8-bit MCU communication interfaces for the Internet of Things
C
ommunication interfaces can generally be divided into two use-case types: machine- to-machine (M2M) and human-machine interfaces (HMI). The M2M interfaces come in various flavours, ranging 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. An 8-bit engine offering M2M and HMI interfaces may not be the optimal solution for all embedded system use cases, but applications that require deterministic behaviour 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 enable these interfaces to be used simultaneously and can be 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. On higher speed devices, prescalers allow these peripherals to run at reasonable rates. Crystal-less USB : Among the more complex communication interfaces is "crystal- less" USB. This takes the simple, full-speed USB device interface and removes the need for an external crystal. 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
20 CIE electronica 2014
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
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 may have a human interaction component.
Capacitive touch interfaces can be used almost anywhere including under glass and plastic and are generally very 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
Figure 1: Example of standalone LCD controller
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.
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.
For many high-speed 8-bit MCUs, there are a significant number of bus interfaces that can be efficiently "bit-banged."
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.
A segment LCD driver can be integrated into an 8-bit MCU or offered as a standalone, fixed- function device. As a standalone device, an LCD controller offers the best leakage and dynamic power characteristics of any LCD solution. This device interfaces to an adjacent MCU through SPI or I2C. It consumes so little current that it is possible to power the device from an input pin and completely forgo the VDD connection. Moreover, the die is exceedingly small and is best used as a bare die or chip-on-glass rather than as a packaged component. (See Figure 1)
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