nvSRAM

nvSRAM is a type of non-volatile random-access memory (NVRAM).[1][2] It is similar in operation to static random-access memory (SRAM).

nvSRAM is one of the advanced NVRAM technologies that is fast replacing the BBSRAMs; battery-backed static random-access memory, 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[3]—where the preservation of data is critical and where batteries are impractical.

Some products named nvSRAM are available from Cypress Semiconductor, which is a combination of SRAM and SONOS based non-volatile memory.[4] But they seem to have different internal structure from the memory cells in referenced books.[1][2]

It is faster than EPROM and EEPROM solutions. There are other nvSRAM products from Maxim Integrated, those are essentially BBSRAMs. They have a lithium battery built into the SRAM package. These are faster than EPROM and EEPROM solutions. (Refer to External links section.)

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 yrs7 yrs, dependent on battery and ambient temperature10 yrs20 yrs
Endurance UnlimitedLimitedUnlimited
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. Ma, Yanjun; Kan, Edwin (2017). Non-logic Devices in Logic Processes. Springer. ISBN 9783319483399.
  2. Xie, Yuan (2013). Emerging Memory Technologies: Design, Architecture, and Applications. Springer Science & Business Media. ISBN 9781441995513.
  3. Computer organization (4th ed.). [S.l.]: McGraw-Hill. ISBN 0-07-114323-8.
  4. http://www.electronicsweekly.com/products/2008/07/15/20229/cypress-cy14b102-2mbit-and-cy14b108-8mbit-nvsrams.htm
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