nvSRAM

nvSRAM is a type of non-volatile random-access memory (NVRAM). It is similar in operation to static random-access memory (SRAM). The current market for non-volatile memory is dominated by BBSRAMs, or battery-backed static random-access memory. However, BBSRAMs are slow and suffer from ROHS compliance issues. nvSRAMs provide 20ns or lesser access times.

nvSRAM is one of the advanced NVRAM technologies that is fast replacing the BBSRAMs, especially for applications that need battery free solutions and long term retention at SRAM speeds. nvSRAMs are used in a wide range of situations—networking, aerospace, and medical, among many others^[1] —where the preservation of data is critical and where batteries are impractical.

nvSRAM is available from 16k densities up to 8M densities from both Simtek Corporation^[2] and Cypress Semiconductor.^[3] There are other nvSRAM products from Maxim which are essentially BBSRAMs. They have a lithium battery built into the SRAM package. It is an efficient replacement for BBSRAM, EPROM or EEPROM. It is faster than EPROM and EEPROM solutions. It is better than BBSRAM solution because there is no ROHS issue associated with this type of memory. No external battery is used.

Description

Externally, nvSRAM looks like standard SRAM. However, on the inside, an nvSRAM is capable of doing more than a standard SRAM. While SRAM can read and write, nvSRAM can read, write, store and recall. The additional operations center around the non-volatile part of nvSRAM.

When reading and writing, an nvSRAM acts no differently than a standard async SRAM. The attached processor or controller sees an 8-bit SRAM interface and nothing else. The STORE operation stores data that is in a SRAM array in the non-volatile part. Cypress and Simtek nvSRAM have three ways to store data in the non-volatile area. They are:

1. Autostore,
2. Hardware store,
3. Software store.

Autostore happens automatically when the data main voltage source drops below the device's operating voltage. When this occurs, the power control is switched from Vcc to the capacitor. The capacitor will power the chip long enough to store the SRAM contents into the non-volatile part. The HSB (Hardware Store Busy) pin externally initiates a non-volatile hardware store operation. Using the HSB signal, which requests a non-volatile hardware STORE cycle, is optional. Software store is initiated by a certain sequence of operations. When the defined operations are done in sequence the software store is initiated.

Applications

Data logging is one main area where nvSRAMs are needed. POS terminals/smart terminals are now able to approve payment transactions without having to obtain approval from a remote server. Because secure data resides in the terminal, a lot of time could be saved in terms of the over-the-air verification which is slow as well as intrusion prone.

Motor vehicle crash boxes are another area where nvSRAMs could be employed effectively. The vehicle state data at the time of the crash can go a long way in validating the claims and finding the reason of the crash. This has huge financial implications in the insurance industry, and the concept of having crash boxes in passenger/commercial vehicles could become a de facto standard in near future. nvSRAMs with their fast read/write capabilities is a good fit for this application.

Similar critical applications such as medical equipment and high end servers can use nvSRAMs to store their data. In case of external power failure, or unforeseen calamities, nvSRAM can hold the data without external intervention (autostore feature). Hence it would provide the flexibility of an EEPROM but at SRAM speeds.

Applications in environments where field service is not possible/costly such as data loggers spread across geographies, routers, equipment in inhospitable conditions can use nvSRAMs, because nvSRAM does not use batteries, which have a risk of exploding/releasing harmful chemicals in harsh environments.

In short, nvSRAMs are suited for applications that need to store critical data, but no field service.

Comparisons with other types of memories

	nvSRAM	BBSRAM	Ferroelectric RAM	Magnetoresistive random-access memory
Technique	Has non-volatile elements along with high performance SRAM	Has a lithium energy source for power when external power is off	Has a ferroelectric crystal between two electrodes to form a capacitor. The moment of atoms on application of electric field is used to store data	Similar to ferroelectric RAM, but the atoms align themselves in the direction of an external magnetic force. This effect is used to store data
Data retention	20 yrs	7 yrs, dependent on battery and ambient temperature	10 yrs	20 yrs
Endurance	Unlimited	Limited	Unlimited
Store mechanism	Autostore initiated when Vcc power down is detected	Chip enable must be maintained at high logic to prevent inadvertent read/writes	Static operation. Data is stored in the non-volatile part only
Power up data restore	Non-volatile data is made available automatically in the SRAM	SRAM will switch from battery to Vcc
Substitution with SRAM	nvSRAM can be substituted for SRAM with minor board modification to add external capacitor	Provision for battery necessitates board redesign to accommodate bigger size for the battery	Some parts are pin-to-pin compatible with existing SRAMs	Pin-to-pin compatible with existing SRAMs
Soldering	Standard SMT used	Reflow solder cannot be done with battery installed as batteries may explode	Standard SMT used
Speed (best)	15–45 ns	70–100 ns	55 ns	35 ns

References

1. Computer organization. (4th ed.). [S.l.]: McGraw-Hill. ISBN 0-07-114323-8.
2. http://www.simtek.com/simtekSite.php Simtek Products
3. http://www.electronicsweekly.com/products/2008/07/15/20229/cypress-cy14b102-2mbit-and-cy14b108-8mbit-nvsrams.htm