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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?

LIN/CAN interfaces: Two automotive-specific, industry-standard interfaces, LIN 2.1 (master/slave) and CAN 2.0, also have been implemented on various 8bit devices targeting a wide range of automotive applications. Silicon Labs' automotive 8bit 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 8bit 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 8bit 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?s, the entire bank of 16 pins can be scanned in under 700?s. 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 into an 8bit MCU or offered as a stand-alone, fixed-function device. As a stand-alone 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 (figure 1).

Figure 1: Example of stand-alone LCD controller.

Gesture, proximity and ambient lighting: Proximity sensing is highly desirable in many IoT end nodes as well as in portable medical and mobile computing products that require human gesture control and detection. Silicon Labs offers a family of 8bit products supporting infrared (IR)-based proximity control as well as ambient and ultraviolet (UV) light sensing. For example, the Si114x MCU family implements proximity detection using one, two or three LEDs with a range of up to 50 cm, multi-dimensional motion sensing, heart rate/pulse oximetry and cheek detection capabilities. This sensing architecture works in direct sunlight and includes a light sensor capable of sensing light levels up to 128 kLux. Light sensing technology often requires special packaging features, such as a transparent window around the light sensors. Figure 2 shows an example of a proximity sensing MCU.

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

Stacks and drivers
Of course, these MCU interfaces require either stacks and/or drivers to enable their quick integration into a system. The interfaces discussed in this article (except for the simple ones such as UART, SPI, and I2C) come with drivers and/or stacks available at no charge from Silicon Labs. For example, Silicon Labs' 8bit MCUs featuring crystal-less USB come with a full-featured USB driver included in the USBXpress development kit that provides a complete host and device software solution.

MCU interfaces and the IoT
Today's interconnected IoT ecosystem favours IC devices with a wide variety of interfaces since the heterogeneous nature of the embedded marketplace requires these devices to be able to converse in as many "dialects" as possible.

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