MRAM (MagnetoResistive Random Access Memory) By : Dhruv Dani 200601163 Shitij Kumar 200601084 Team - N
Flow of Presentation Current Memory Technologies Riddles Introduction Principle, Structure and Working Working Modes Schematic Overview MRAM v/s Other Memory Elements Applications in Embedded Systems Case Studies Supported Microcontrollers and Companies Constraints References
Current Memory Technologies Volatile When the power is switched off the information is lost. Restarting: programs and data need to be reloaded resulting in increase of idle time. Non -Volatile Can retain stored information permanently Stores information that does not require frequent changing. Read/Write/Erase cycles consume a lot of time. Commonly Known Memories Volatile – Static RAM (SRAM), Dynamic RAM (DRAM) Non – Volatile – Flash, EEPROM
Riddle - 1 A car component manufacturing company ‘X’ has to built Air Bag systems for a range of cars. The requisites of building such a system are that it has to interact with the various sensors which detect and record passenger weight and are employed in other safety devices on the vehicle which perform various crucial tasks like detecting the impact of the possible collision. Such a real time system requires the memory to be susceptible to continuous reads, writes and overwrites in each clocked interval. As an embedded engineer for this company X which kind of memory would you use to implement such a system?
Riddle - 2 The Defense Research and Development Organization of a nation ‘C’ has to build a system which can be employed by them for their military and aerospace applications. These systems at present require constant power supply to maintain various kinds databases consisting of confidential information. The battery generally acts as the power supply and is entrusted to keep the information accessible and safe at all times. But it has been experienced day in and day out that systems which use battery back-up have an inherent reliability problem. As an embedded engineer what modifications would you do to such a system?
What might be the solution???
MRAM Magneto-resistive RAM, or Magnetic RAM is a form of non-volatile RAM memory technology that uses magnetic charges to store data instead of electric charges. In the early 1990’s the company Honeywell conceived a new class of Magnetoresistence memory devices which offered promise for high density, random access and non – volatile memory. Commercially, the first mass production of MRAMS was started by Freescale – Everspin.
Principle on which MRAM works!! Principle MRAM works on the principle Magnetoresistance. The change in resistance with the magnetic state of the device is an effect known as Magnetoresistance. The resistance with which the magnetic state is associated is measured on the basis of the magnetic moment of the inherent layers of the MRAM.
Structure of the MRAM MRAM is constituted of various storage elements called Magnetic Tunnel Junctions (MTJ) integrated with CMOS processing. Each MTJ is composed of two layers (ferromagnetic plates), fixed and free separated by a thin dielectric material. Fixed layer : Magnetic polarity is fixed Free Layer : Magnetic polarity is subject to change in accordance with the magnetic field which is the resultant of the applied current.
Working of MRAM When a bias is applied to the MTJ, electrons that are spin polarized by the magnetic layers traverse the dielectric barrier through a process known as tunneling. The MTJ device has a low resistance when the magnetic moment of the free layer is parallel to the fixed layer and a high resistance when the free layer moment is oriented anti-parallel to the fixed layer moment. The data is stored as a magnetic state rather than a charge, and sensed by measuring the resistance without disturbing the magnetic state. The magnetic polarization does not leak away with time like charge does, so the information is stored even when the power is turned off. Switching the magnetic polarization between the two states does not involve actual movement of electrons or atoms, and thus no known wear-out mechanism exists.
Working Modes Write Mode During the write operation, current pulses are passed through a digit line and a bit line, writing only the bit at the cross point of those two lines. Read Mode During the read operation, the target bit’s isolation transistor is turned on to bias the MTJ, and the resulting current is compared to a reference to determine if the resistance state is low or high using the sense line.
Schematic Overview (1/2)
Schematic Overview (2/2)
MRAM V/s Other Existing Memory Elements (1/4) Flash - Continuous writes can wear out some flash memories in 10 days. Meanwhile, MRAM can endure infinite write cycles because no charging or discharging is involved. Rotation of magnetic polarities during , is a non-destructive and non - degrading operation. MRAM uses current that creates a magnetic field to program the free layer. Furthermore, flash performs a write or erase operation on a large block of the memory array. MRAM perform writes on individual addresses.
MRAM V/s Other Existing Memory Elements (2/4) SRAM - It requires power to retain memory contents. MRAM memory contents are held in the polarity of its free magnetic layer. Since it is magnetic, this layer retains its state even without power. The leakage effect comes into the picture in the case of SRAMs technologies shrink. Given MRAM's non-volatility, power down techniques can be used in the system for zero current leakage. Battery – Backed SRAM - This consists of an SRAM unit and an accompanying battery in the same package to make it non - volatile. Meanwhile, MRAM performs read/ write at a speed faster than battery-backed SRAM. This improves reliability and dismisses environmental issues linked with battery disposal.
MRAM V/s Other Existing Memory Elements (3/4) EEPROM: This standalone memory has much slower programming speeds compared with MRAM, and limited write-cycling capability. DRAM: Memory has to be frequently refreshed to retain data because capacitors are used to store the data and there is a continuous charge leakage. So more power is used. FRAM: Ferroelectric RAM (FRAM) is non - volatile has typical small array sizes ranging from 4Kbit to 1Mbit. The array sizes are small because this technology has limited scalability. Without such scalability limitations, MRAM can offer larger memory arrays. Moreover, MRAM can be programmed faster than FRAM.
MRAM V/s Other Existing Memory Elements (4/4)
Applications in Embedded Systems
Some more Applications Data Storage Industrial Automation Gaming Energy Management Communication Transportation
Case Study 1 Automotive applications using sensors can benefit from MRAM. Since sensors write data continuously, flash memories have difficulty keeping up with such data flow. New airbag systems also have sensors to detect and record passenger weight, interactions with other safety devices on the vehicle and the impact of collision. Other automotive systems such as odometers, tire pressure log and ABS require frequent writes to memory that easily exceed the write-erase capabilities of flash and wear out its memory. MRAM's unlimited write-cycling capability ensures a more reliable system for mission-critical devices such as airbags and ABS.
Case Study 2 The use of MRAM in the military is also gaining wide acceptance. Many systems use battery-backed SRAM and have inherent reliability issues with battery use. We have already seen that MRAMS are faster than battery backed SRAMs and are more environment friendly. On similar grounds we feel that in future years MRAM can be integrated with a GPS to replace its inherent battery – backed memory.
Supported Microcontroller and Companies Rad – Hard MCS8051 instruction-compatible microcontrollers (MCUs) that can directly interface with Honeywell's new HXNV-0100 64k x 16 radiation-hardened Magnetic RAM (mram). Most readily available MRAM – Freescale’s MR2A16A. 3.3 V – Single Power Supply 35 ns – Access Time 8 bit – 16 bit – Flexible data bus. IBM, Motorola, Infineon Technologies, Sony, HP , NVE Corporation are some other manufacturers. Renesas Technologies Plans to Launch MCU with Embedded MRAM in 2008, use MRAM to provide unified memory for MCUs
Constraints Power Efficiency Writing in to MRAM requires substantial current. Changing data in magnetic fields requires a large power. Size Half select Problem: Induced field overlaps adjacent cells over a small area. Manufacturing As chips get smaller individual circuits hold less charge. It has been found experimentally that the resistance of the magnetic device varies exponentially with the thickness. Cost It is comparatively higher than the other memories
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