Types of M2M Interfaces Used in the IoT

Machine-to-machine interfaces are as old as computer automation itself. With the advent of the Internet of Things, the need for effective M2M interfaces will only grow. There are several IoT-ready 8-bit MCU applications for M2M. Let’s look at a few of them.

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 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. Silicon Labs’ 8-bit MCUs integrate an onboard two-percent-accurate internal oscillator, enabling them to work without a crystal and provide 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. 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.

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.

usb-cableLIN/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 this accurate tunable oscillator onboard is that it is possible to generate very accurate PWM edge placement (on the order of 120 ps), which has proven useful in small motor control applications and some power control applications.

In our next installment, we’ll look at HMI, or Human Machine Interfaces. Until then, visit our 8-bit MCU page to learn more.

Continue on to Part 3.

One thought on “Types of M2M Interfaces Used in the IoT

  1. Pingback: Overview of 8-bit MCUs in the IoT | Official Blog of Silicon Labs

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