Sometimes too much choice can be as frustrating as too little choice. Today, microcontroller users face a myriad of choices from a variety of suppliers, all competing with features, flexibility, performance and price, which can create a design dilemma. Here is some advice on how to make the best choice and on what factors should be borne in mind when considering microcontroller selection in a variety of applications to meet a design’s criteria.At first glance, MCU (microcontroller unit) users appear to be in an enviable position. Never before has such a wide choice of high-performance, feature-loaded devices been available. In addition, competition between suppliers is keeping prices down and minimizing the cost difference between 8 and 32bit devices, making the technology gap less of a price issue. This can be a significant factor in cost-sensitive designs, such as those for the consumer sector. However, such a proliferation of devices and embarrassment of choices has created its own problem: the number of valid choices available to the engineer, and the time it takes to evaluate them, has escalated.
At the same time, technology advances mean that the criteria that engineers consider when selecting a microcontroller should now be changing. Traditionally, devices were chosen on the basis of CPU performance and memory; this was because the CPU and memory occupied by far the majority of space on the microcontroller's die, and the choice of peripheral functions was therefore very limited.
However, in line with Moore’s Law, the digital circuitry in the microcontroller has been continually shrinking, and so now the CPU core generally occupies only a small area on the die. By contrast, much of the peripheral functionality is analog, and is not so readily shrunk. As a result, today the majority of die space on a microcontroller is dedicated to peripherals.
FACTORS AFFECTING CHOICE
Consequently, the choice of the best microcontroller is now more heavily affected by the number, choice and performance of on-chip peripherals than it is by the performance of the processor. Microcontrollers available on the market today offer an extraordinarily wide range of peripherals to the design engineer, including ADCs (analog-to-digital converters), DACs (digital-to-analog converters), PWMs (pulse-width modulators), as well as interfaces for I2C, Ethernet and USB. This has become a two-edged sword: there are plenty of peripherals and plenty of choices, but the constraints of integration and development times remain the same. Fortunately, a new generation of online tools is now available to help engineers find the appropriate part based on specific peripheral requirements, without having to trawl through hundreds of datasheets. This article discusses the latest trends in MCU peripheral technology and reviews online selection tools that make them accessible to engineers for review and consideration.
Communication interfaces such as I²C, UART, SPI, CAN, USB, special camera interfaces and Ethernet account for a large portion of chip area, as well as for a large number of the chip’s I/Os. The more I/Os, the bigger the package size and the greater the bill-of-materials, assembly and development cost.
Consequently, there is a substantial cost saving to be made if the design engineer can accurately specify a microcontroller with the right interfaces and eliminate unused pins. Balanced against this, however, is the need to take account of the rapid evolution of interface technology: the engineer must be aware of trends and developments in interfaces so as to future-proof designs that are required to evolve over a long product lifecycle.
UART, SPI and I²C are standard serial interfaces on most devices today, but even these can evolve. For instance, I²C is already being offered in the Fast-Mode Plus version that offers increased speeds of up to 1MHz.
USB is another case in which designers’ product choices have been quickly made to look out of date. In previous years, many design engineers have chosen to use an 8bit microcontroller because such a device could support USB 1.1 technology. However, the hardware will now be unable to support the new variants of USB, such as USB On-The-Go, which allows the device to operate as host and slave, or Hi-Speed USB in version 2.0 with data rates of 480Mbps, which needs the horsepower that a 32bit microcontroller offers.
Furthermore, Ethernet is growing in popularity in embedded designs, especially for industrial field buses. It is being increasingly used for service updates and diagnostic functions, and even in automotive applications where harsh environments and consumer expectations demand reliability. Most microcontrollers with on-board Ethernet capability only include a MAC (media-access controller). In order to implement a single-chip solution that includes an Ethernet PHY (physical interface), the design engineer will need to use a highly integrated part, for example the devices in Freescale Semiconductor's ColdFire family of microcontrollers.
DESIGNING FOR LONGETIVITY
Therefore, for engineers designing a long-life system, or for those designing a platform with multiple variants of an end product, it would be wise to choose a microcontroller that offers the flexibility to easily change and enhance peripherals. It is also desirable to choose a microcontroller that will not require major alterations to hardware.
One way to do this is to choose a microcontroller family that includes a range of pin-compatible parts, including some that use interfaces such as USB or Ethernet. Freescale’s Flexis family, for example, consists of a growing range of HCS08 (8bit) and ColdFire v1 (32bit) pin-compatible parts (Figure 1). In a similar move, NXP Semiconductors has introduced a portfolio of ARM Cortex-based microcontrollers with pin-compatible parts.
An alternative to microcontroller families is available in the form of PSoC from Cypress Semiconductor. An array of programmable mixed-signal blocks controlled by an 8bit core, PSoC devices can be programmed to provide a wide range of functions, including high-performance analog, digital and communications interfaces. PSoC’s programmability means that changes to peripheral functions can be carried out in software without the need for costly hardware re-spins.
PERIPHERAL CHOICES
Even when the design engineer is producing a stand-alone device rather than a platform, however, a bewildering selection of peripherals still affects the choice of the microcontroller. It would be very time-consuming – and is perhaps even impossible – to trawl through a complete catalogue of a microcontroller manufacturer’s parts to find exactly the right combination of peripherals for a design’s specifications. Fortunately, online selector tools will let users specify peripheral requirements and display the pin-outs of relevant parts so that device options can be considered more easily.
To help engineers come across microcontrollers with the right combination of peripherals, Freescale, Microchip and NXP have each introduced selection tools for design engineers. Freescale has introduced the Freescale Consumer and Industrial MicroSelector, Microchip has released the MAPS tool, and NXP’s contribution is titled the NXP Selector Guide. These tools support 8, 16 and 32bit microcontroller selection and are available on each supplier’s Website, with access to further information such as datasheets, reference manuals and application notes. It is worth noting that the tool cannot substitute for a designer’s evaluation of a device in the light of a particular application's requirements. Instead, the tools are intended to help an engineer find more quickly a device or devices that are suitable for engineering evaluation.
CHOOSING ONLINE
The Freescale MicroSelector requires the user to first make a high-level choice between MCUs, digital-signal controllers and microprocessors. Further specifications can then be made by choosing peripherals such as CAN, SCI/UART, ADC and PWM. Temperature range, supply-voltage range and pin count can also be specified. The results are displayed according to bit size and memory size. For software engineers, the available operating systems are also displayed for consideration. Information about each of the displayed parts can be viewed in quick-view, fact-sheet or Webpage formats.
The MAPS tool from Microchip (Figure 2) offers a selection of MCUs, analog parts and memory devices. Again, the most effective way of filtering microcontrollers is through the selection of peripherals. It is possible to specify internal oscillators, the resolution of the ADC, self-write for flash, and system-management functions.
MAPS can display results in different views to make the selection easier. These include quick-view, classic-view and side-by-side formats. The product status shown in the classic view is particularly important, as parts denoted “older” are not recommended for new designs, and parts marked as “future” are not yet available for purchase.
NXP's selection tool can be used to select not only MCUs, but also analog, datacomms, discrete, I2, SM bus, logic and USB devices. When searching for MCUs, the first view shows a table of NXP’s 8 and 32bit ARM-based MCUs, listing all features. Specifying requirements for memory, peripherals, core and package type will shrink the table. This makes it possible for engineers to concentrate on the peripherals and to review performance and memory options afterwards. The tool shows the package outline and gives easy access to more detailed product information.
A TWO-TRACK APPROACH
Manufacturers' online tools provide a fast and effective way to find appropriate microcontroller devices or device families, to ensure that the design engineer avoids mistakenly over-specifying the microcontroller and paying for functionality, pins or silicon real estate that is not required. It should be noted that, unfortunately, only officially launched devices can be selected with these tools. Very new products, or those which are to hit the market in the near future, will generally not be found using these tools.
It is in such cases that the FAEs (field-application engineers) of a large distributor can help. FAEs working in such distribution models where a broad range of companies is represented can help in all stages of the selection process. For example, in the early stages of the design process, an FAE can use knowledge of the market to often point a customer to a forthcoming device that is more suitable than the best device found by the manufacturer's online selection tool. FAEs can also provide guidance to help design engineers avoid designing-in a device that is at risk of early obsolescence.
The ideal scenario then is one where online tools and personal guidance are combined. The new generation of online tools and the technical expertise of distributors can help design engineers to navigate their way through the ever more crowded microcontroller scene.
Click here for the illustrations: Figure 1, Figure 2
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