Appliance Design - January 2008 - (Page 57)

Fig. 1. An early Intel 8048-based MCU (left) with 2 KB of ultravioleterasable PROM (EPROM) and a modern MCU (right) with 128 KB of on-chip flash memory in a tiny chip, both made by NEC Electronics. ELECTRONICS Memory for Micros T oday’s 8-bit, 16-bit, and 32-bit microcontrollers (MCUs) integrate on-chip memory such as flash memory, SRAM (including cache), mask ROM, one-time-programmable (OTP) EPROM and electrically erasable PROM (EEPROM) in memory sizes ranging from a few hundred bytes to several megabytes (MB). Designers of embedded systems must make the right choices for their applications based on meeting a specific performance target or cost budget. Choosing the right memory options is one of the critical steps in the MCU selection process, as memory subsystems impact not only overall performance but also system cost. Before selecting a memory option, it’s important to understand the basics. Memory Options: There are two main types of memory available in today’s MCUs: read-only memory (ROM) and randomaccess memory (RAM). ROM refers to nonvolatile memory that is used to store code or instructions, while RAM refers to volatile memory that is used primarily to store data. ROM: Most MCU manufacturers offer mask ROM and flash memory options in product lines, with a few still offering programmable ROM (PROM) or OTP ROM. Flash memory has emerged as the preferred ROM type for MCUs, thanks to its improved performance, programmability, endurance and reliability. Over the years, it has replaced OTP memory and is slowly overtaking mask ROM. Some MCU vendors, such as NEC Electronics, have been converting the memory in their MCUs from a mixture of memory types to “all flash,” thereby enabling designers to make program changes late in the design cycle and also thereafter via in-system programming. One of the key benefits of flash memory is its self-programming capability, which enables designers to upgrade code remotely through media such as serial interfaces, wireless connections and even the Internet. Mask ROM does offer a few advantages over flash memory in certain cases, for example, it is less expensive to mass produce and has a lower leakage current when the chip is in standby operation. However, flash memory technology is gradually catching up in these areas. EEPROM is a complement to flash memory, because it offers programmability (erase and write) at the byte level instead of in blocks (hundreds of bytes). However, many MCUs now offer EEPROM emulation with onchip flash memory via hardware or software implementations, providing an economic alternative. Fig. 1 illustrates the dramatic evolution in MCU ROM technology over the three decades since the first MCU (Intel 8048) was invented in the mid-1970s. RAM: Static RAM (SRAM) is one of the primary memory types in the RAM family. The term “static” implies that SRAM retains its content as long as the power remains supplied. The other type is dynamic RAM (DRAM), which requires the memory content to be refreshed periodically to prevent data from being lost. SRAM is used in MCUs mainly for data access and occasionally for code execution because of the nature of high-speed access. The amount of SRAM in MCUs is usually less than in a counterpart ROM due to the relatively bigger cell size, and therefore higher cost. With its higher density (smaller cell size) and lower cost, DRAM offers an alternative memory option for MCUs. However, the requirement for constant refreshing and its different process technology make it less Careful study required to choose from vast array of options. by lance zheng Lance Zheng is staff technical applications engineer, Standard Solutions Strategic Business Unit, NEC Electronics America, Santa Clara, Calif. applianceDESIGN January 2008 57 www.applianceDESIGN.com 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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