Using Nonvolatile Static RAMs

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Abstract: Vast resources have been spent by the semiconductor industry to build high-speed nonvolatile random-access read/write memory. This effort was undertaken because NV SRAM offers advantages over other memory devices such as DRAM, SRAM, EEPROM, Shadow RAM, EPROM, and ROM. NV SRAM was developed to meet specific needs of the design engineer. This article describes applications requiring this type of memory and compares this memory technology to other memory technologies available.

Vast resources have been expended by the semiconductor industry trying to build a nonvolatile random access read/write memory. The effort has been undertaken because nonvolatile RAM offers several advantages over other memory devices—DRAM, Static RAM, Shadow RAM, EEPROM, EPROM and ROM—which were developed to meet specific applications needs.



Types of Memory
Many types of memories have been devised to meet varying application needs. However, nonvolatile read/write random access memories can be substituted for all memory types independent of application, if cost is not a primary consideration.

DRAM: Dynamic Random Access Memory
A DRAM, similar to an SRAM, stores information as a 1 or a 0. In an SRAM, this information is stored in a four to six transistor flip-flop which is easy to address, but requires a relatively large memory cell. A DRAM, by comparison, stores its 1 or 0 as a charge on a small capacitor, requiring much more current then an SRAM to maintain the stored data. The net memory cell size is smaller for the DRAM than for the SRAM, so the total cost per bit of memory is less. The DRAM's capacitors must be constantly refreshed so that they retain their charge. DRAMs require more sophisticated interface circuitry.

SRAM: Static Random Access Memory
An SRAM is essentially a stable DC flip-flop requiring no clock timing or refreshing. The contents of an SRAM memory are retained as long as power is supplied. SRAMs support extremely fast access times. SRAMs also have relatively few strict timing requirements and a parallel address structure, making them particularly suited for cache and other low-density, frequent-access applications.



PSEUDO STATIC RAM: Pseudo Static Random Access Memory
The advantages of using a Static RAM are the simplicity of the interface circuitry required, and the fact that the device is by nature "static," not requiring periodic refreshing to retain its data. A DRAM, however, provides lower cost-per-bit advantages and a higher memory density. A Pseudo-static RAM combines the advantages of the SRAM and DRAM by using dynamic storage cells to retain memory, and by placing all the required refresh logic on-chip so that the device functions similarly to an SRAM.
Flash
A flash memory combines the electrical erase capability of an EEPROM with a cell that is similar to an EPROM. The result is that the modified cell may be block erased electrically instead of with UV light. This feature allows a Flash memory to accept new code updates or information while it is functioning in a system.

EEPROM: Electrically Erasable/Programmable Read Only Memory
A significant disadvantage of the EPROM memory is the fact that it cannot be reprogrammed while in a system. EPROM requires an external programming device to receive new code or data. An EEPROM eliminates this problem by providing a write function which can be used while the EEPROM is still in a circuit.