NVRAM

NVRAM

NVRAM is a type of Random Access Memory (RAM) that retains its information when power is turned off. The NVRAM is a small 24 pin DIP (Dual Inline Package) integrated circuit chip and is thus able to obtain the power needed to keep it running from the CMOS battery installed in your motherboard. It keeps track of various system parameters such as serial number, Ethernet MAC (Media Access Control) address, HOSTID, date of manufacture, etc. NVRAM is therefore a type of non-volatile memory that offers random access.The best-known form of NVRAM memory today is flash memory. Some drawbacks to flash memory include the requirement to write it in larger blocks than many computers can atomically address, and the relatively limited longevity of flash memory due to its finite number of write-erase cycles (most consumer flash products at the time of writing can only withstand around 100,000 rewrites before memory begins to deteriorate). Another drawback is the performance limitations preventing flash from matching the response times and, in some cases, the random addressability offered by traditional forms of RAM. Several newer technologies are attempting to replace flash in certain roles, and some even claim to be a truly universal memory, offering the performance of the best SRAM devices with the non-volatility of flash. To date these alternatives have not yet become mainstream.

Types of NVRAM

One type of NVRAM is SRAM that is made non-volatile by connecting it to a constant power source such as a battery. Since SRAM requires continual power supply in order to maintain its data, an NVRAM that is made from an SRAM will need to use an available power supply to make sure it continues working.

Another type of NVRAM uses EEPROM (Electrically Erasable Programmable Read-Only Memory) circuit chips to save its information when power is turned off. In this case, NVRAM is composed of a combination of SRAM and EEPROM chips incorporated into a single semi-conductor die.
Benefits of NVRAM

* NVRAM chips work like static RAM
* NVRAMs provide superior performance over other NVM products
* NVRAM's serve applications that require high-speed read/write operations with non-volatile memories such as parallel processing controllers for LANs and    antilock braking systems.
* NVRAM chips don't require much power and backup can be guaranteed for up to ten years.

Bad NVRAM

When NVRAM is failing, it generally means that your computer hardware is not retaining the necessary specialized settings that it ought to though the default BIOS settings remain. Since the BIOS relies on the settings stored in NVRAM in order to handle the particular hardware you have, performance may lack in stability. The contents of the NVRAM chip can become corrupted for a variety of reasons:

* A failure of the embedded battery. If the battery embedded in the NVRAM chip fails, then this means that your system clock will stop running and important system configuration information may not be maintained.
* A failure of the CMOS (BIOS) chip on your motherboard. If the CMOS chip is going bad or is not making proper contact with the motherboard's contacts, then the NVRAM will fail.


A huge advance in NVRAM technology was the introduction of the floating-gate transistor, which led to the introduction of erasable programmable read-only memory, or EPROM. EPROM consists of a grid of transistors whose gate terminal (the "switch") is protected by a high-quality insulator. By "pushing" electrons onto the base with the application of higher-than-normal voltage, the electrons become trapped on the far side of the insulator, thereby permanently switching the transistor "on" ("1"). EPROM can be re-set to the "base state" (all "1"s or "0"s, depending on the design) by applying ultraviolet light (UV). The UV photons have enough energy to push the electrons through the insulator and return the base to a ground state. At that point the EPROM can be re-written from scratch.

An improvement on EPROM, EEPROM, soon followed. The extra "E" stands for electrically, referring to the ability to reset EEPROM using electricity instead of UV, making the devices much easier to use in practice. The bit are re-set with the application of even higher power through the other terminals of the transistor (source and drain). This high power pulse basically sucks the electrons through the insulator, returning it to the ground state. This process has the disadvantage of mechanically degrading the chip, however, so memory systems based on floating-gate transistors generally have short write-lifetimes, on the order of 105 writes to any particular bit.

One approach to overcoming the rewrite count limitation is to have a standard SRAM where each bit is backed up by an EEPROM bit. In normal operation the chip functions as a fast SRAM and in case of power failure the content is quickly transferred to the EEPROM part, from where it gets loaded back at the next power up. Such chips were called NOVRAMs by their manufacturers.

The basis of flash memory is identical to EEPROM, and differs largely in internal layout. Flash allows its memory to be written only in blocks, which greatly simplifies the internal wiring and allows for higher densities. Memory storage density is the main determinant of cost in most computer memory systems, and due to this flash has evolved into one of the lowest cost solid-state memory devices available. Starting around 2000, demand for ever-greater quantities of flash have driven manufacturers to use only the latest fabrication systems in order to increase density as much as possible. Although fabrication limits are starting to come into play, new "multi-bit" techniques appear to be able to double or quadruple the density even at existing linewidths.

Flash and EEPROM's limited write-cycles are a serious problem for any real RAM-like role, however. Additionally, the high power needed to write the cells is a problem in low-power roles, where NVRAM is often used. The power also needs time to be "built up" in a device known as a charge pump, which makes writing dramatically slower than reading, often as much as 1,000 times. A number of new memory devices have been proposed to address these shortcomings.

To date, the only such system to enter widespread production is ferroelectric RAM, or FeRAM. FeRAM uses a ferroelectric layer in a cell that is otherwise similar to conventional DRAM, this layer holding the charge in a 1 or 0 even with the power removed. To date, FeRAM has been produced on very old fabs, and even the most advanced research samples are still twice the linewidth of most flash devices. Although this difference might be addressable under normal circumstances, as flash moves to multi-bit cells the difference in memory density appears to be growing, rather than shrinking.

Pic:- Typical Architecture Of NVRAM

Another approach to see major development effort is Magnetoresistive Random Access Memory, or MRAM, which uses magnetic elements and generally operates in a fashion similar to core, at least for the 1st generation technology. Only one MRAM chip has entered production to date: Everspin Technologies' 4 Mbit part, which is a 1st generation MRAM that utilizes cross-point field induced writing. Two 2nd generation techniques are currently in development: Thermal Assisted Switching (TAS) which is being developed by Crocus Technology, and Spin Torque Transfer (STT) on which Crocus, Hynix, IBM, and several other companies are working. STT-MRAM appears to allow for much higher densities than the 1st generation, but is lagging behind flash for the same reasons as FeRAM – enormous competitive pressures in the flash market.

Another solid-state technology to see more than purely experimental development is Phase-change RAM, or PRAM. PRAM is based on the same storage mechanism as writable CDs and DVDs, but reads them based on their changes in electrical resistance rather than changes in their optical properties. Considered a "dark horse" for some time, in 2006 Samsung announced the availability of a 512 Mb part, considerably higher capacity than either MRAM or FeRAM. The areal density of these parts appears to be even higher than modern flash devices, the lower overall storage being due to the lack of multi-bit encoding. This announcement was followed by one from Intel and STMicroelectronics, who demonstrated their own PRAM devices at the 2006 Intel Developer Forum in October. One of the most attended sessions in the IEDM December 2006 was the presentation by IBM of their PRAM technology.

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