Non-volatile random-access memory

Last updated

Non-volatile random-access memory (NVRAM) is random-access memory that retains data without applied power. This is in contrast to dynamic random-access memory (DRAM) and static random-access memory (SRAM), which both maintain data only for as long as power is applied, or forms of sequential-access memory such as magnetic tape, which cannot be randomly accessed but which retains data indefinitely without electric power.

Contents

Read-only memory devices can be used to store system firmware in embedded systems such as an automotive ignition system control or home appliance. They are also used to hold the initial processor instructions required to bootstrap a computer system. Read-write memory such as NVRAM can be used to store calibration constants, passwords, or setup information, and may be integrated into a microcontroller.

If the main memory of a computer system were non-volatile, it would greatly reduce the time required to start a system after a power interruption. Current existing types of semiconductor non-volatile memory have limitations in memory size, power consumption, or operating life that make them impractical for main memory. Development is going on for the use of non-volatile memory chips as a system's main memory, as persistent memory. A standard for persistent memory known as NVDIMM-P has been published in 2021. [1] [2] [3]

Early NVRAMs

Some early computers used magnetic drum which was non-volatile as a byproduct of its construction. The industry moved to magnetic-core memory in the later 1950s, which stored data in the polarity of small magnets. Since the magnets held their state even with the power removed, core memory was also non-volatile. Other memory types required constant power to retain data, such as vacuum tube or solid-state flip-flops, Williams tube, and semiconductor memory (static or dynamic RAM).

Advances in semiconductor fabrication in the 1970s led to a new generation of solid state memories that magnetic-core memory could not match on cost or density. Today dynamic RAM forms the vast majority of a typical computer's main memory. Many systems require at least some non-volatile memory. Desktop computers require permanent storage of the instructions required to load the operating system. Embedded systems, such as an engine control computer for a car, must retain their instructions when power is removed. Many systems used a combination of RAM and some form of ROM for these roles.

Custom ROM integrated circuits were one solution. The memory contents were stored as a pattern of the last mask used for manufacturing the integrated circuit, and so could not be modified once completed.

PROM improved on this design, allowing the chip to be written electrically by the end-user. PROM consists of a series of diodes that are initially all set to a single value, 1 for instance. By applying higher power than normal, a selected diode can be burned out (like a fuse), thereby permanently setting that bit to 0. PROM facilitated prototyping and small-volume manufacturing. Many semiconductor manufacturers provided a PROM version of their mask ROM part so that development firmware could be tested before ordering a mask ROM.

Currently, the best-known form of both NV-RAM and EEPROM memory is flash memory. Some drawbacks to flash memory include the requirement to write it in larger blocks than many computers can automatically address, and the relatively limited longevity of flash memory due to its finite number of write-erase cycles (as of January 2010 most consumer flash products can withstand only around 100,000 rewrites before memory begins to deteriorate)[ citation needed ]. 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. As of June 2018 these alternatives have not yet become mainstream.

Those who required real RAM-like performance and non-volatility typically have had to use conventional RAM devices and a battery backup. For example, IBM PC's and successors beginning with the IBM PC AT used nonvolatile BIOS memory, often called CMOS RAM or parameter RAM, and this was a common solution in other early microcomputer systems like the original Apple Macintosh, which used a small amount of memory powered by a battery for storing basic setup information like the selected boot volume. (The original IBM PC and PC XT instead used DIP switches to represent up to 24 bits of system configuration data; DIP or similar switches are another, primitive type of programmable ROM device that was widely used in the 1970s and 1980s for very small amounts of data—typically no more than 8 bytes.) Before industry standardization on the IBM PC architecture, some other microcomputer models used battery-backed RAM more extensively: for example, in the TRS-80 Model 100/Tandy 102, all of the main memory (8 KB minimum, 32 KB maximum) is battery-backed SRAM. Also, in the 1990s many video game software cartridges (e.g. for consoles such as the Sega Genesis) included battery-backed RAM to retain saved games, high scores, and similar data. Also, some arcade video game cabinets contain CPU modules that include battery-backed RAM containing keys for on-the-fly game software decryption. Much larger battery-backed memories are still used today as caches for high-speed databases that require a performance level newer NVRAM devices have not yet managed to meet.

Floating-gate MOSFET

A huge advance in NVRAM technology was the introduction of the floating-gate MOSFET 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 reset to the base state (all 1s or 0s, 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 bits are re-set with the application of even higher power through the other terminals of the transistor (source and drain). This high-power pulse, in effect, 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 in general 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 [4] 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 line widths.

Commercialized Alternatives

Flash and EEPROM's limited write-cycles are a serious problem for any real RAM-like role. In addition, 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.

Ferroelectric RAM

To date, the only such system to enter widespread production is ferroelectric RAM, or F-RAM (sometimes referred to as FeRAM). F-RAM is a random-access memory similar in construction to DRAM but (instead of a dielectric layer like in DRAM) contains a thin ferroelectric film of lead zirconate titanate [Pb(Zr,Ti)O3], commonly referred to as PZT. The Zr/Ti atoms in the PZT change polarity in an electric field, thereby producing a binary switch. Unlike RAM devices, F-RAM retains its data memory when power is shut off or interrupted, due to the PZT crystal maintaining polarity. Due to this crystal structure and how it is influenced, F-RAM offers distinct properties from other nonvolatile memory options, including extremely high endurance (exceeding 1016 access cycles for 3.3 V devices), ultra low power consumption (since F-RAM does not require a charge pump like other non-volatile memories), single-cycle write speeds, and gamma radiation tolerance. [5] Ramtron International has developed, produced, and licensed ferroelectric RAM (F-RAM), and other companies that have licensed and produced F-RAM technology include Texas Instruments, Rohm, and Fujitsu.

Magnetoresistive RAM

Another approach to see major development effort is magnetoresistive random-access memory, or MRAM, which uses magnetic elements and in general operates in a fashion similar to core, at least for the first-generation technology. Only one MRAM chip has entered production to date: Everspin Technologies' 4 Mbit part, which is a first-generation MRAM that utilizes cross-point field induced writing. [6] Two second-generation techniques are currently in development: Thermal Assisted Switching (TAS), [7] which is being developed by Crocus Technology, and spin-transfer torque (STT) on which Crocus, Hynix, IBM, and several other companies are working. [8] STT-MRAM appears to allow for much higher densities than those of the first generation, but is lagging behind flash for the same reasons as FeRAM – enormous competitive pressures in the flash market.

Phase-change RAM

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 Mbit 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.

Intel and Micron Technology had a joint venture to sell PRAM devices under the names 3D XPoint, Optane and QuantX, which was discontinued in July 2022. [9] [10]

STMicroelectronics manufactures phase-change memory devices for automotive applications.

Researched Alternatives

Millipede memory

Perhaps one of the more innovative solutions is millipede memory, developed by IBM. Millipede is, in essence, a punched card rendered using nanotechnology in order to dramatically increase areal density. Although it was planned to introduce Millipede as early as 2003, unexpected problems in development delayed this until 2005, by which point it was no longer competitive with flash. In theory the technology offers storage densities on the order of 1 Tbit/in² (≈155 Gbit/cm2), greater than even the best hard drive technologies currently in use (perpendicular recording offers 636 Gbit/in² (≈98.6 Gbit/cm2) as of Dec. 2011 [11] ), but future heat-assisted magnetic recording and patterned media together could support densities of 10 Tbit/in² [12] (≈1.55 Tbit/cm2). However, slow read and write times for memories this large seem to limit this technology to hard drive replacements as opposed to high-speed RAM-like uses, although to a very large degree the same is true of flash as well.

FeFET memory

An alternative application of (hafnium oxide based) ferroelectrics is Fe FET based memory, which utilises a ferroelectric between the gate and device of a field-effect transistor. Such devices are claimed to have the advantage that they utilise the same technology as HKMG (high-L metal gate) based lithography, and scale to the same size as a conventional FET at a given process node. As of 2017 32Mbit devices have been demonstrated at 22 nm.

See also

Related Research Articles

<span class="mw-page-title-main">Computer memory</span> Computer component that stores information for immediate use

Computer memory stores information, such as data and programs, for immediate use in the computer. The term memory is often synonymous with the term primary storage or main memory. An archaic synonym for memory is store.

<span class="mw-page-title-main">Microcontroller</span> Small computer on a single integrated circuit

A microcontroller or microcontroller unit (MCU) is a small computer on a single integrated circuit. A microcontroller contains one or more CPUs along with memory and programmable input/output peripherals. Program memory in the form of ferroelectric RAM, NOR flash or OTP ROM is also often included on chip, as well as a small amount of RAM. Microcontrollers are designed for embedded applications, in contrast to the microprocessors used in personal computers or other general purpose applications consisting of various discrete chips.

<span class="mw-page-title-main">EEPROM</span> Computer memory used for small quantities of data

EEPROM or E2PROM (electrically erasable programmable read-only memory) is a type of non-volatile memory. It is used in computers, usually integrated in microcontrollers such as smart cards and remote keyless systems, or as a separate chip device, to store relatively small amounts of data by allowing individual bytes to be erased and reprogrammed.

<span class="mw-page-title-main">Static random-access memory</span> Type of computer memory

Static random-access memory is a type of random-access memory (RAM) that uses latching circuitry (flip-flop) to store each bit. SRAM is volatile memory; data is lost when power is removed.

<span class="mw-page-title-main">Programmable logic device</span> Reconfigurable digital circuit element

A programmable logic device (PLD) is an electronic component used to build reconfigurable digital circuits. Unlike digital logic constructed using discrete logic gates with fixed functions, the function of a PLD is undefined at the time of manufacture. Before the PLD can be used in a circuit it must be programmed to implement the desired function. Compared to fixed logic devices, programmable logic devices simplify the design of complex logic and may offer superior performance. Unlike for microprocessors, programming a PLD changes the connections made between the gates in the device.

Magnetoresistive random-access memory (MRAM) is a type of non-volatile random-access memory which stores data in magnetic domains. Developed in the mid-1980s, proponents have argued that magnetoresistive RAM will eventually surpass competing technologies to become a dominant or even universal memory. Currently, memory technologies in use such as flash RAM and DRAM have practical advantages that have so far kept MRAM in a niche role in the market.

Non-volatile memory (NVM) or non-volatile storage is a type of computer memory that can retain stored information even after power is removed. In contrast, volatile memory needs constant power in order to retain data.

Nano-RAM is a proprietary computer memory technology from the company Nantero. It is a type of nonvolatile random-access memory based on the position of carbon nanotubes deposited on a chip-like substrate. In theory, the small size of the nanotubes allows for very high density memories. Nantero also refers to it as NRAM.

<span class="mw-page-title-main">Magnetic storage</span> Recording of data on a magnetizable medium

Magnetic storage or magnetic recording is the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetisation in a magnetizable material to store data and is a form of non-volatile memory. The information is accessed using one or more read/write heads.

<span class="mw-page-title-main">Programmer (hardware)</span> Device that configures programmable integrated circuits

A programmer, device programmer, chip programmer, device burner, or PROM writer is a piece of electronic equipment that arranges written software or firmware to configure programmable non-volatile integrated circuits, called programmable devices. The target devices include PROM, EPROM, EEPROM, Flash memory, eMMC, MRAM, FeRAM, NVRAM, PLDs, PLAs, PALs, GALs, CPLDs, FPGAs, and microcontrollers.

Semiconductor memory is a digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to devices in which data is stored within metal–oxide–semiconductor (MOS) memory cells on a silicon integrated circuit memory chip. There are numerous different types using different semiconductor technologies. The two main types of random-access memory (RAM) are static RAM (SRAM), which uses several transistors per memory cell, and dynamic RAM (DRAM), which uses a transistor and a MOS capacitor per cell. Non-volatile memory uses floating-gate memory cells, which consist of a single floating-gate transistor per cell.

<span class="mw-page-title-main">Ferroelectric RAM</span> Novel type of computer memory

Ferroelectric RAM is a random-access memory similar in construction to DRAM but using a ferroelectric layer instead of a dielectric layer to achieve non-volatility. FeRAM is one of a growing number of alternative non-volatile random-access memory technologies that offer the same functionality as flash memory. An FeRAM chip contains a thin film of ferroelectric material, often lead zirconate titanate, commonly referred to as PZT. The atoms in the PZT layer change polarity in an electric field, thereby producing a power-efficient binary switch. However, the most important aspect of the PZT is that it is not affected by power disruption or magnetic interference, making FeRAM a reliable nonvolatile memory.

<span class="mw-page-title-main">Spin-transfer torque</span> Physical magnetic effect

Spin-transfer torque (STT) is an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve can be modified using a spin-polarized current.

nvSRAM is a type of non-volatile random-access memory (NVRAM). nvSRAM extends the functionality of basic SRAM by adding non-volatile storage such as an EEPROM to the SRAM chip. In operation, data is written to and read from the SRAM portion with high-speed access; the data in SRAM can then be stored into or retrieved from the non-volatile storage at lower speeds when needed.

Racetrack memory or domain-wall memory (DWM) is an experimental non-volatile memory device under development at IBM's Almaden Research Center by a team led by physicist Stuart Parkin. In early 2008, a 3-bit version was successfully demonstrated. If it were to be developed successfully, racetrack memory would offer storage density higher than comparable solid-state memory devices like flash memory.

<span class="mw-page-title-main">Read-only memory</span> Electronic memory that cannot be changed

Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing software that is rarely changed during the life of the system, also known as firmware. Software applications for programmable devices can be distributed as plug-in cartridges containing ROM.

<span class="mw-page-title-main">Random-access memory</span> Form of computer data storage

Random-access memory is a form of electronic computer memory that can be read and changed in any order, typically used to store working data and machine code. A random-access memory device allows data items to be read or written in almost the same amount of time irrespective of the physical location of data inside the memory, in contrast with other direct-access data storage media, where the time required to read and write data items varies significantly depending on their physical locations on the recording medium, due to mechanical limitations such as media rotation speeds and arm movement.

Everspin Technologies is a public semiconductor company headquartered in Chandler, Arizona, United States. It develops and manufactures discrete magnetoresistive RAM or magnetoresistive random-access memory (MRAM) products, including Toggle MRAM and Spin-Transfer Torque MRAM (STT-MRAM) product families. It also licenses its technology for use in embedded MRAM (eMRAM) applications, magnetic sensor applications as well as performs backend foundry services for eMRAM.

A NVDIMM or non-volatile DIMM is a type of persistent random-access memory for computers using widely used DIMM form-factors. Non-volatile memory is memory that retains its contents even when electrical power is removed, for example from an unexpected power loss, system crash, or normal shutdown. Properly used, NVDIMMs can improve application performance and system crash recovery time. They enhance solid-state drive (SSD) endurance and reliability.

<span class="mw-page-title-main">Memory cell (computing)</span> Part of computer memory

The memory cell is the fundamental building block of computer memory. The memory cell is an electronic circuit that stores one bit of binary information and it must be set to store a logic 1 and reset to store a logic 0. Its value is maintained/stored until it is changed by the set/reset process. The value in the memory cell can be accessed by reading it.

References

  1. ↑ "JEDEC DDR5 & NVDIMM-P Standards Under Development" (Press release). JEDEC. 2017-03-30.
  2. ↑ "JEDEC to Hold Workshops for DDR5, LPDDR5 & NVDIMM-P Standards" (Press release). JEDEC. 2019-09-05.
  3. ↑ "JEDEC Publishes DDR4 NVDIMM-P Bus Protocol Standard" (Press release). JEDEC. 2021-02-17.
  4. ↑ Chan, Peter (2005-04-21). "X4C105 NOVRAM Features and Applications" (PDF). Intersil. Archived from the original (PDF) on 2007-06-14.
  5. ↑ "F-RAM Memory Technology". Ramtron. Archived from the original on 2012-04-18. Retrieved 2012-06-08.
  6. ↑ "Technology". Everspin. Archived from the original on June 10, 2009.
  7. ↑ Hoberman, Barry. "The Emergence of Practical MRAM" (PDF). Crocus Technology. Archived from the original (PDF) on 2011-04-27. Retrieved 2009-07-20.
  8. ↑ LaPedus, Mark (2009-06-18). "Tower invests in Crocus, tips MRAM foundry deal". EE Times . Retrieved 2020-01-09.
  9. ↑ Mann, Tobias (2022-07-29). "Why Intel killed its Optane memory business". The Register. Situation Publishing. Retrieved 2022-11-18.
  10. ↑ Allyn Malventano (June 2, 2017). "HOW 3D XPOINT PHASE-CHANGE MEMORY WORKS". PC Perspective.
  11. ↑ "Hitachi GST Ships One Terabyte Per Platter Hard Drives" (Press release). Hitachi Global Storage Technologies. 2011-08-03. Archived from the original on 2011-10-26. Retrieved 2011-12-17.
  12. ↑ Johnston, Casey (2011-05-07). "New hard drive write method packs in one terabit per inch". Ars Technica. Retrieved 2011-12-17.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.