Appliance Design - January 2008 - (Page 58)

ELECTRONICS ROM Mask ROM Cost Speed Programmability Volatility Power Consumption Very Low Fast No No Very Low PROM Low Fast One Time No Low EPROM Medium Medium Yes No Low EEPROM Medium Slow Yes No Medium Flash Low Medium Yes No Low SRAM High Very Fast Yes Yes Low RAM DRAM Low Fast Yes Yes Medium Fig. 2. Comparison of different memory technologies. compelling for integration in MCUs due to implementation complexity. DRAM is often used as an external memory option for economic memory expansion. Embedded DRAM (eDRAM), as opposed to external DRAM, has been used in some application-specific MCUs for high-throughput and high-density data processing. Fig. 2 shows a comparison of the various memory technologies used in MCUs. in the RAM area in addition to the ROM area to enable high-speed execution without sacrificing determinism. A second technique to improve flash memory access performance is to add banked memory between the CPU core and flash memory. It is similar to a cache in terms of buffering memory, but with a much smaller memory footprint, it usually is a multiple of the bus from one process to another, ensuring system reliability. A memory protection unit (MPU) can be used as an alternative to MMU to provide simpler memory protection without involving virtual memory translation and an associated real-time operating system. An MPU allows memory partitioning into several regions, with each region having individual protection attributes. Processes allocated in Memory architecture An optimal memory architecture plays a key role in MCU implementation for delivering high efficiency at affordable cost. Most 8-bit and 16-bit MCUs, as well as lower-end 32-bit MCUs, integrate sufficient onchip memory as a single-chip solution, while higher-end 32-bit MCUs often offer an external memory bus interface to enable off-chip memory expansion for complex applications. Flash memory offers relatively small die size and lower costs, as well as programming flexibility and security. However, access speed can be an issue. The best existing flash memory process technology can achieve a zero-waitstate access time of around 40 MHz. It is important to match the relatively slower flash memory speed with a CPU core clock if it is higher than the flash memory access limitation. Otherwise additional wait states are required during flash memory access, which slows down CPU performance. One of the techniques used to overcome the bottleneck of flash memory speed is to apply cache memory, a type of high-speed SRAM to temporarily store frequently accessed code and data for rapid access. Cache memory can be used for slower external memory to speed up access, as well. However, the side effect to this approach is that one may lose deterministic behavior if a cache miss occurs with timing-critical code and data access, as with automotive airbag controls, for example. In such applications, the memory architecture is implemented to allow code allocation directly 58 applianceDESIGN January 2008 "…a new safety regulation standard, IEC60730, just 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…" width, for example, 16 bytes or 32 bytes. This technique is only effective in a sequential program, and it imposes similar deterministic side effects the same as cache memory. Some real-time operating systems, such as Windows CE and embedded Linux, require an on-chip memory management unit (MMU) located between the cache (or memory) and CPU core to achieve virtual memory access. An MMU translates virtual memory addresses to physical addresses by page mapping and provides “larger” amounts of memory. Virtual memory also allows multiple processes to run within their own virtual memory spaces without increasing the actual memory footprint. Along with the real-time operating system, an MMU can also provide memory protection against accidental illegal access of memory physical memory regions would therefore be protected from invalid accesses. With the addition of more complexity, cost and power consumption to MCU chips, cache and MMUs are only implemented in higherend 32-bit MCUs where external memory for complex applications is often required. Most MCUs today use the Harvard architecture with separate memory buses for address and data access, while a very few still use the Von Neumann architecture that uses the same memory bus for both address and data access. The Harvard architecture is more efficient than the Von Neumann architecture because it allows the CPU to access code (in ROM) and data (in RAM) in parallel but on separate buses, therefore providing higher throughput. www.applianceDESIGN.com http://www.appliancedesign.com

Table of Contents Feed for the Digital Edition of Appliance Design - January 2008

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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