Appliance Design - January 2008 - (Page 59)

ELECTRONICS External Flash/ROM External SRAM/DRAM External Bus Interface Timers CPU Core Registers Data Cache MMU/ MPU Code Cache SRAM/ DRAM Serial EEPROM/ Flash Flash Memory Cards Serial Interfaces ALU ROM/ OTP/ Flash SD/CF Interfaces Interrupt Controller OSC Note: Dash-lined blocks are optional GPIO DMA On-chip Debug EEPROM Fig. 3. Memory architectures in MCU implementations. In addition to the memory bus, serial interfaces such as I2C or SPI could be used to interface to external memory, for example, flash memory and EEPROM, for additional data storage. Fig. 3 shows an MCU with possible memory options in a typical Harvard architecture. Choosing options Sorting out various memory options for MCU offerings has never been easy. The first step for system designers is to determine their memory requirements, including size, speed, performance and power consumption, which are mainly driven by specific applications. Estimating the right memory size is one of the keys to a successful design and in keeping system costs low. Memory capacity is often a moving target in design stages as well as in product life cycles. A code change late in the design process may require more memory, as will software upgrades for added features after a product is released. For example, a new safety regulation standard, IEC60730, took effect in October 2007 in Europe for home appliance applications. This regulation requires MCUs in appliances to employ self-diagnosis mechanisms, including ROM/RAM integrity validation, CPU interrupt verification and other hardware checks, to detect faults. This may result in ongoing memory increases, both in ROM and RAM, by www.applianceDESIGN.com implementing self-test algorithms in addition to normal functions. The good news is that MCU manufacturers, such as NEC Electronics, offer lineups of function-compatible and package-compatible flash MCUs with a wide range of memory capacities, giving designers a seamless migration path to accommodate increased memory footprints without changing hardware designs. External memory (ROM or RAM) is needed when the estimated memory size exceeds onchip memory capacity. This occurs in applications demanding high-density data storage, such as an embedded graphics user interface (GUI). Next-generation home appliances are likely going to equip a touch-screen LCD graphics panel as a human interface device. In the case of a QVGA display with 16 bit-per-pixel resolution, 300 KB RAM would be required to allocate two image buffers for the LCD graphics controller alone, and this would most likely be on an external SRAM device. Applications using real-time operating systems such as Windows CE and embedded Linux will require external memory as well. A typical Windows CE or Linux application requires memory capacities ranging from a few to tens of megabytes for the realtime operating system kernel and application code. External flash memory ranging from 4 to 32 MB and paired with a similar amount of external DRAM is commonly practiced. In these cases, a higher speed MCU equipped with an MMU is often a must. Meeting speed performance requires not only application knowledge, but memory architecture understanding. One must keep in mind that flash memory access speed is limited. If the CPU runs above 30–40 MHz, access to flash memory, or program execution, will slow CPU performance. Choosing a cache or cache-like memory buffer-equipped memory architecture is one option to keep the CPU running at high speed. Another option is to choose memory architectures that allow code and data to be locked into SRAM for time-critical tasks that cannot afford to wait for flash memory access. This option is preferred in hard real-time applications, such as motor-control devices. The vector-control method has been widely used in appliance-motor drives, as it provides precise, responsive and cost-effective speed and torque control. Algorithms such as signal filters and vector controls must be processed within a predictable time to guarantee efficient and reliable motor control during fast transition operations. This real-time behavior can be better met by executing the algorithm from a higher-speed SRAM instead of cached slowerspeed flash memory. Memory types and architecture also impact power consumption. Mask ROM is the best option in terms of low power, maintaining an edge traditionally in battery-operated applications, although flash memory is closing the gap. Choosing a cache-less architecture also helps reduce power consumption. The last, but most important, factor in choosing memory options is the combination of cost, memory type, and size. In design stages or pilot runs, flash memory provides the most flexible and cost-effective solution, as designers can upgrade code any time without incurring additional cost in hardware. At high volumes, mask ROM is the better option in terms of cost, despite an initial mask charge and relatively longer lead time. With a thorough study of available memory options and application-specific requirements, designers should be able to determine the best-suited memory options to achieve the delicate balance between system performance and cost. < For more information, enter . . . . . . . . . . . . . . . . . . . . . .313 Or email: info@am .necel .com applianceDESIGN January 2008 59 http://www.appliancedesign.com

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Appliance Design - January 2008 Contents Editorial Shipments/Forecasts News Watch Makers of Commercial Refrigeration Equipment Turn to Variable-Speed Motors to Improve Energy Efficiency. New Motor Design and Control Scheme Makes Brushless Motors More Economically Feasible for a Wider Range of Applications. External Gas Molding, a Relatively New Technique, Solves Many of the Challenges Associated with the Injection Molding of Large Plastic Parts. Plastic Housings Offer Many Benefits: EMI Protection Isn't One of Them. But, New Materials and Techniques are Solving that Problem. Choosing the Optimal Strategy for Effective EMI Shielding Depends on Both Mechanical and Electrical Requirements Ethernet-Enabled Microcontroller Makes Networking Schemes Easier and More Cost Effective to Implement The Ever-Changing Array of Microcontroller Memory Options Makes it Imperative to Keep Abreast of What's Available to Determine the Optimal Choice for an Application. A Vast Range of Components and Finished Products will be Exhibited at the Country's Preeminent HVAC/R Show, AHR Expo, which is Scheduled for Jan. 22-24 in New York. Advertiser's Index Association Report: PRBA

Appliance Design - January 2008

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