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INTRODUCTION

The L18 flash memory device provides read-while-write and read-while-erase capability with density upgrades through 256-Mbit. This family of devices provides high performance at low voltage on a 16-bit data bus. Individually erasable memory blocks are sized for optimum code and data storage. Each device density contains one parameter partition and several main partitions. The flash memory array is grouped into multiple 8-Mbit partitions. By dividing the flash memory into partitions, program or erase operations can take place at the same time as read operations.

Although each partition has write, erase and burst read capabilities, simultaneous operation is limited to write or erase in one partition while other partitions are in read mode. The L18 flash memory device allows burst reads that cross partition boundaries. User application code is responsible for ensuring that burst reads don't cross into a partition that is programming or erasing. Upon initial power up or return from reset, the device defaults to asynchronous page-mode read. Configuring the Read Configuration Register enables synchronous burst-mode reads. In synchronous burst mode, output data is synchronized with a user-supplied clock signal. A WAIT signal provides easy CPU-to-flash memory synchronization. In addition to the enhanced architecture and interface, the L18 flash memory device incorporates technology that enables fast factory program and erase operations. Designed for low-voltage systems, the L18 flash memory device supports read operations with VCC at 1.8 volt, and erase and program operations with VPP at 1.8 V or 9.0 V

MEMORY

In order to enable computers to work faster, there are several types of memory available today. Within a single computer there are more than one type of memory.

TYPES OF RAM

The RAM family includes two important memory devices: static RAM (SRAM) and dynamic RAM (DRAM). The primary difference between them is the lifetime of the data they store. SRAM retains its contents as long as electrical power is applied to the chip. If the power is turned off or lost temporarily, its contents will be lost forever. DRAM, on the other hand, has an extremely short data lifetime-typically about four milliseconds. This is true even when power is applied constantly.

In short, SRAM has all the properties of the memory you think of when you hear the word RAM. Compared to that, DRAM seems useless. However, a simple piece of hardware called a DRAM controller can be used to make DRAM behave more like SRAM. The job of the DRAM controller is to periodically refresh the data stored in the DRAM. By refreshing the data before it expires, the contents of memory can be kept alive for as long as they are needed. So DRAM is also as useful as SRAM.

When deciding which type of RAM to use, a system designer must consider access time and cost. SRAM devices offer extremely fast access times (approximately four times faster than DRAM) but are much more expensive to produce. Generally, SRAM is used only where access speed is extremely important. A lower cost-per-byte makes DRAM attractive whenever large amounts of RAM are required. Many embedded systems include both types: a small block of SRAM (a few kilobytes) along a critical data path and a much larger block of dynamic random access memory (perhaps even in Megabytes) for everything else.

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