With the widespread adoption of powerful small-form-factor microcontrollers, the cost-per-cycle has dropped low enough that it's now economical to use 16-bit and even 32-bit devices in many embedded applications. Why is it then, even as 16-bit and 32-bit MCUs deliver low-cost processing power, that 8-bit architectures remain so popular?
There are several reasons, including the advent of new consumer and medical applications that benefit from the dramatically improved feature sets and performance of the 8-bit MCU.
As designs continue to move into smaller, more compact applications with limited power resources, low-pin-count (LPC) microcontrollers with complex peripherals and power-saving features become increasingly desirable.
The capabilities available in LPC 8-bit MCUs make them excellent design choices for home appliances such as water heaters, electric pots, de-humidifiers and hair dryers.
A gradual change in control systems design, particularly for products that require sensor inputs such as medical and healthcare equipment, home appliances, automation and process control, also is providing applications for these MCUs.
As a result, 8-bit MCUs dominate the global controller market because of their relatively low prices, improved performance, and new applications. According to recent Gartner-Dataquest reports, the 8-bit MCU market totaled about $5 billion in 2008 and will continue to lead in revenue and unit shipments, contributing almost 30 percent to the total MCU market.
Demand is strong, particularly in China, because of several factors, including de-facto standard architectures, configurable implementations and licensable silicon IP.
Household appliances like air conditioners, washing machines, refrigerators, TVs, home theater systems and radio broadcast receivers, combined with sound recording and reproducing systems, are driving growth for 8-bit MCUs.
Also, the market for household appliances such as dust collectors, smart electric rice cookers, fans and remote controls will continue to grow rapidly. The emerging applications for 8-bit MCUs include the following:
Smart and Utility Metering: A smart meter can identify consumption in more detail than a conventional meter and communicate that information to the local utility for monitoring and billing purposes.
Designed specifically for the demands of utility metering/smart metering applications, 8-bit microcontrollers are the perfect combination of ultra-low-power and high-performance analog integration (for example, 16-bit to 21-bit Delta-Sigma ADC, comparators).
8-bit MCU devices can offer one- to three-phase electricity, water and gas metering, as well as radio frequency wireless interfaces for automated meter reading (AMR).
Smart Sensor Applications: LPC MCUs also help conserve energy. Battery-operated or low-power security systems such as smoke detectors, thermostats and glass breakage systems are ideal for the ultra-low-power consumption and integrated high-performance analog features these MCUs offer.
Integrated wireless networking features are being added to smart sensor applications to strengthen home automation, remote monitoring and control features. Also, 8-bit MCUs fit into 95 percent of all devices used to access the Internet.
Portable Consumer Products: New generations of portable medical products encourage manufacturers to use technologies that reduce the design complexity and time required to develop products. Manufacturers also want to provide built-in LCD controllers and touch sensor controller capabilities. Again, the 8-bit MCU is an ideal choice.
As shown in Figure 1 below Appliances benefit from the energy-saving characteristics of advanced motor control technology. Consumers want to save energy costs and reduce the environmental impact of electricity-generating facilities.
|Figure 1: Example block diagrams of household appliance applications|
Since the refrigerator is always on, it can consume up to 50 percent of the home energy budget; virtually all that electricity is used to run the compressor motor. Most refrigerators still use constant-speed on/off compressors.
However, variable-speed compressors enable more efficient cooling with precise temperature control, reducing overall energy use. In addition, manufacturers are employing more efficient brushless motors, which also reduce energy consumption.
Replacing low-end universal triac-driven motors with permanent magnet or brushless DC (BLDC) motors in washing machines increases performance and efficiency. Advanced control techniques enable shorter washing cycles, saving energy and water.
Complex systems also can incorporate an 8-bit MCU for basic functions. For instance, LPC 8-bit MCUs are easy to configure and less expensive than stocking the required discrete components. So engineers can use 6- or 8-pin MCUs to control reset circuits that need a configurable time delay in different applications or for electric lighting ballast applications. Battery chargers, electric toothbrushes, toasters, coffee machines, and toys now contain low-cost, 8-bit MCUs.
New LPC 8-bit MCUs Deliver a Variety of Features
All these applications benefit from the latest 8-bit MCUs, which offer the following features:
* High Performance: With clock speeds into the tens of megahertz and peripherals such as hardware multipliers, new 8-bit MCU architectures offer more processing power compared with previous generations.
* Integrated Features: 8-bit MCUs can incorporate a variety of sophisticated peripherals, such as internal high-speed clocks and timers, analog peripherals, smart card interfaces, touch controllers, motor control capabilities, wireless interfaces, and wired network connectivity such as integrated Controller Area Network (CAN) and Local Interconnect Network (LIN) buses.
* Integrated Non-Volatile Memory: Mask ROM has been replaced by Flash memory in most 8-bit MCUs, making them more versatile in embedded applications. Also, the range of program sizes can now include significant amounts of Flash along with E2PROM or Ferro Electric RAM like non-volatile memories, allowing more design flexibility.
* Higher Reliability: These devices have earned their reputation for high reliability by assuring continuous and correct operation. 8-bit MCUs now act as “watchdogs” or monitors for many critical applications.
* Optimized Footprints: If there is the need for basic functionality, 8-bit devices are available in a variety of small package sizes.
* Low-power Consumption: Despite their effective processing power, these MCUs provide many ways to manage power consumption so that applications consume no more power than necessary. For functions that must remain active, battery life conservation is a strong advantage since many new 8-bit Flash devices consume only nanoWatts of current and offer several types of low-power modes consuming different levels of current.
* Lower Cost: Compared with 16- and 32-bit devices, 8-bit MCUs offer significant cost savings due to smaller die size even with integrated features.
* Low Electromagnetic Interference (EMI) : In most cases, 8-bit MCUs easily meet the EMI requirements by running at relatively low speeds.
Making the right embedded design choices
Embedded system designers face a choice between 8-bit MCUs, with which they are likely to be familiar, and a 32-bit architecture, which is cost-effective and guarantees the performance required in the application. But 32-bit MCUs present a new, complicated architecture that increases the program memory requirements and code size.
System designers must consider the memory and code-size factors along with other factors, including meeting system requirements from the customer; the form factor requirements from mechanical designers; the memory size from software developers; and specifications for functions, flexibility, efficiency, and cost.
To make the best choice in 8-bit MCUs, the designer should look for an intelligent combination of on-chip peripherals. Designers also should seek controllers that are scalable and flexible enough for different applications within an appliance, such as motor control, LCD display, and external touch sensor controller interfaces using SPI/I2C for the human-machine interface (HMI).
To save space and total BOM cost, MCUs also should incorporate embedded dual-operation Flash memory for emulating E2PROM. The memory in the latest 8-bit MCUs contains two regions, one for program storage and another that can be used for data storage, acting as a substitute for E2PROM.
Integrated Flash memory is programmable across the whole operating range of the device. This allows the user to take full advantage of the application's re-programmability benefits in any environment.
In addition, the dual-operation memory can be used for Flash-based E2PROM emulation though on-chip Dual Operation Flash memory, saving the BOM cost of an external E2PROM device. Software E2PROM emulation libraries and API code for user customization are available free of charges and royalties.
Another vital requirement is integrated Flash memory that can be re-written 100,000 times with data retention guaranteed for up to 20 years. To enhance system integrity, security and reliability, MCUs integrate a wide range of hardware and software system protection and Flash security features to protect software from being read by unauthorized external access.
On-chip RC oscillating circuits and the low-voltage detection circuit are built into 8-bit MCUs to eliminate the need for an external oscillator and reset IC.
Designers also should look for programmable composite timers, which enable flexible timer configuration to pulse width modulation (PWM), pulse width counter (PWC), interval timer or input capture.
The latter measures interval times, providing the ability to handle varying system requirements. A separate watchdog timer provides a safety mechanism to monitor the flow of software, interrupt handling and execution, and the CPU clock.
Non-volatile Ferroelectric RAM. Some new LPC 8-bit MCUs integrate embedded Ferro Electric RAM (FRAM), providing high-speed access, high endurance in write mode, low power consumption, non-volatility, and excellent tamper resistance.
FRAM also suffers no performance degradation under radiation, so it is ideal for smart cards, secure data storage and battery-powered applications where high security, small memory size and low power consumption are key requirements.
Communication Interfaces . All these MCUs require a wide range of communications protocols typically used in large appliance system designs, such as UART, I2C, and LIN -UART/SPI. The LIN-UART/SPI module is designed to simplify development of LIN systems by enabling LIN break detection and synchronization without the use of an external timer channel.
Other features to look for include analog, motor control, and on-chip debug. LPC microcontrollers have key peripherals such as a high-resolution, 10-bit (or higher) analog-to-digital converter (ADC) and advanced timer modules that span performance requirements for appliances, from basic to advanced motor control.
For advanced motor control, the BLDC motor is reliable and achieves a high level of efficiency by generating the rotor magnetic flux with rotor magnets. Some 8-bit LPCs feature three independent 16-bit timer/pulse-width modulator modules. This makes them suitable to controlling a BLDC motor as a timer module, generating any PWM pattern for any single- and three-phase motor control.
BGM in-circuit emulation (ICE) provides a single-wire debugging, emulation and onboard Flash programmability interface that eliminates the need for expensive emulation tools. This capability enables developers to perform non-intrusive debugging and emulation on the fly.
The following features of 8-bit MCUs should factor in a designer's decision:
* The ability to reduce design complexity
* The capability to operate with low power
* The option for fast wake-up timing
* The integration of complex peripherals
* Reliability features such as watchdogs-single/dual
* The inherent benefits of all-Flash devices
* The associated BOM cost savings
* Application specific compliance, temperature grade levels, quality and reliability
* Easy-to-use development tools
The decision also should factor in the number of peripherals and on-chip functions that are integrated into the MCU, along with the relevance of the data and address buses.
Semiconductor vendors now address the whole cost scenario to the customer, including any savings the MCU enables in other areas. For instance, including a power-up reset circuit, on-chip RC oscillator circuit, on-chip pull-up resistors, on-chip sensors, integrated high-current drivers and other capabilities on the MCU eliminates the need for several external components, reducing costs.
Rather than focusing on processor bus widths, it's important to meet system-level demands by using the most appropriately sized data and address bus. With the added capabilities, lower power, lower cost and ultra-low-pin-count options now available for consumer, medical and household appliance applications, the 8-bit architecture continues to reinvent itself and extend its useful life.
For example, the latest trend in device design calls for advanced process technology and integration of a few precision analog peripherals, configurable general-purpose input/output (GPIO) pins, serial interfaces and fast data bus architecture. Many applications now require dependable precision analog-signal capture, conversion and conditioning, modest signal-processing capability and integrated memory, all within the minimum pad ring.
Sandhya Mallikarjun is a Staff Applications Engineer, working with the Embedded Systems Solutions Group at Fujitsu Microelectronics America. She has more than 8 years experience with the company working on embedded system design and development, including 8-bit low pin count MCUs.