The following text is taken from a Texas Instruments publication
entitled TIB0203 Magnetic-Bubble Memory and Associated Circuits
dated November 1978.
The publication also contains datasheets for the following parts:
If anyone has any of these chips or demonstration kits, please drop me a line!
Magnetic bubble memory technology has advanced considerably since the concept was introduced by Bell Telephone Laboratories in 1967. Research indicated that small cylindrical magnetic domains, which are called magnetic bubbles, can be formed in single-crystal thin films of synthetic ferrites or garnets when an external magnetic field is applied perpendicularly to the surface of the film. These bubbles can be moved laterally through the film by using a varying magnetic field. These characteristics of magnetic bubles make them ideally suited for serial storage of data bits; the presence or absence of a bubble in a bit position is used to define the logic state. Since the diameter of a bubble is so small (as little as a tenth of a micrometer), many thousands of data bits can be stored in a signle bubble-memory chip. In the spring of 1977 Texas Instruments was the first to market a 92,304-bit bubble memory. This bubble memory is much like magnetic tape or magnetic disc memory storage in that it is nonvolatile meaning that the data is retained even when power is no longer applied to the chip. Since bubble memories are a product of solid-state technology (there are no moving parts), they have higher reliability than tape or disc storage and do not require any preventive maintenance. In addition, the bubble memory is small and lightweight and is, therefore, an excellent choice for compact designs and portable applications.
functional operation of bubble memories
The basic bubble-memory package contains the bubble-memory chip, magnetic field coils, and permanent magnets as shown in Figure 1. A rotating magnetic field created by two mutually perpendicular coils causes the data in the form of magnetic bubbles to move serially through the magnetic field in a manner similar to data in a semiconductor shift register. Two permanent magnets provide nonvolatility and allow for the stable existence of magnetic-bubble domains. Interfacing circuits that are compatible with standard TTL devices complete the memory module to allow a convenient building-block concept for the nonvolatile memory system.
The chip is composed of a nonmagnetic crystaline substrate upon which a thin crystalline magnetic epitaxial film is grown. Only certain materials exhibit the properties necessary to form magnetic bubbles and these include orthoferrites, hexagonal ferrites, synthetic garnets, and amorphous metal films. Among these, the synthetic garnets have the best combination of the desired properties. Synthetic garnets support the formation of small magnetic bubbles that allow high-density data storage. The bubbles are highly mobile and are stable over a fairly wide range of temperatures.
The material chosen for the substrate depends on several factors. The crystalline structure should be compatible with that of the magnetic film, it should have nearly the same coefficient of expansion, and it should be nonmagnetic. The most-used garnet substrate with these properties is gadolinium gallium garnet (GGG). The magnetic film grown on this substrate has a crystalline structure that will allow the formation of magnetic domains (bubbles) in a plane perpendicular to the substrate.
Without the influence of an external magnetic field, these magnetic domains form random serpentine patterns of equal area, minimizing the total magnetic energy of the magnetic film (see Figure 2). The magnetic field of the serpentine domains tends to line up primarily along a single axis (the "easy" axis) that is perpendicular to the plane of the film. If an external magnetic field is applied, its energy tends to expand domains polarized in the direction of the field and to shrink those polarized opposite to the field until they become small cylinders embedded in a background of opposite magnetization. Viewed on end, these cylinders have the appearance of small circles or bubbles with diameters from 2 to 30 micrometers. Increasing the field further causes the bubble to collapse or to be "annihilated". The external field provides a bias that makes the bubbles stable. This bias, being a static field, can be readily provided by permanent magnets with no expenditure of power.
Before bubbles can be shifted through the magnetic film, they must be generated in accordance with input data. Bubbles are generated by locally altering the bias field with a magnetic field produced by a pulse of current through a microscopic one-turn metallized loop. This loop is located on a secondary layer immediately above the magnetic film on the surface of the chip. Given a current of the correct amplitude and polarity through the one-turn loop, a localized vertical magnetic field opposite to that of the permanent magnets is produced. This localized field establishes a domain wall inversion in the magnetic film resulting in bubble creation.
For these reasons TI has chosen the major-minor loop architecture, which offers a dramatic improvement in access time. As shown in Figure 5, during a write operation (data entry), data is generated one bit at a time in the major loop. The data is then transferred in parallel to the minor loops where it circulates until the next time data is to be read out of the memory.
During a write operation data are introduced into the major loop by pulses of current through the hairpin loop of the generator. The major loop is essentially a unidirectional circular shift register from which data can be transferred in parallel to the minor loops. Thus a block of data is entered in the major loop and shifted until the first data bit is aligned with the most remote minor loop. At that time, each parallel transfer element receives a current pulse that produces a localized magnetic field causing the transfer of all the bubbles in the major loop to the top bit position of the corresponding minor loop. Once data is written into the magnetic bubble memory, new data may be written only by first removing the old data by doing a destructive read. In this operation bubbles are transferred from the minor loops and annihilated by running them into the Permalloy guard rail that usually surrounds bubble devices.
During a read operation the data block to be accessed in the minor loops is rotated until it is adjacent to the major loop. At this time the data block is transferred in parallel to the major loop. The block of data is them serially shifted to the replicator where the data stream is duplicated. The duplicated data takes the path to the magneto-resistive detector element. The presence of a bubble in the detector lowers the resistance resulting in a corresponding increase in detector current, which can be detected via a sense amplifier. The original data stream remaining in the major loop is rotated and transferred back into the minor loops thus saving the data for further operations.
The magnetic-bubble-memory devices are fabricated using fine geometries that make the manufacture of perfect devices a difficult task. In order to increase production yields and achieve correspondingly lower costs, redundant minor loops on the bubble-memory chip allow some loops to be defective. Defective loops are determined at final test and a map of these loops is supplied to the end user so that the defective loops can be avoided in the final memory system. This redundancy of minor loops can be handled in several ways. The map could be written into a software program that would direct data to be stored only to the perfect minor loops, but this would require a unique software package for each memory system. Alternatively, the map could be stored in the MBM (magnetic-bubble memory) itself with some risk of being written over with new data. The recommended approach is to store the map in a programmable read-only memory (PROM). Each bit in a page of data would then be written to the MBM or read from it in accordance with the contents of the PROM, thus preventing data bits from the defective minor loops from mingling with valid data. Of course all this requires control circuitry in addition to that necessary for the timing and control of the alternating current in the field coils, the transfer of data to and from the minor loops, and the replication and detection of the magnetic bubbles.
interfacing with bubble memories
Figures 6 and 7.
The time at which a particular data bit is detected in the MBM may not exactly match the time at which it is needed in the system. The sense amplifier not only increases the voltage level of the detected data, but also provides temporary storage of the data bits in a circuit called a D-type flip-flop. The sense amplifier receives a control input from the function timing generator to transfer the detected data into the internal flip-flop. In addition, the function-timing generator provides the control signals necessary to put the existing data in a known position during a power shut down. When the system is turned on again, the stored data can then be accurately located and retrieved.
In a typical system the major computing and data processing is done by a microprocessor. To provide a convenient interface from the microprocessor to the MBM system, a custom controller is needed for the read, write, and memory-addressing operations. The TMS5502/TMS9916 MBM controller responds to commands from the microprocessor system and sends control signals to the function timing generator necessary to access a page (or pages) of data. The controller maintains page-position information, handles serial-parallel data conversion between the bubble memory and the microprocessor, and generates the control signals to the function-timing generator to perform read and write operations while handling the redundancy of the minor loops.
advantages of bubble memories
The future growth of distributed process systems will be greatly impacted by magnetic-bubble memories. These microprocessor-based systems demand high-density mass storage at low cost. Magnetic-bubble memories satisfy all of these requirements with definite advantages over the existing magnetic storage technologies. MBM's advantages over moving-head disks or floppy disks are low access time (the time ncessary to retrieve the desired data), small physical size, low user entry cost, no maintenance, and higher reliability.
The advantages of MBM's over random-access memories (RAM's) are nonvolatility, potentially lower price per bit, and more bits per chip. The RAM has the advantage of much better access time, higher transfer rate, and simpler interfacing.
In summary, the main MBM advantages are the low wnetry price versus disks for the low-end user, nonvolatility versus semiconductor memories, and high-density storage in a small physical space. Because magnetic bubble memories are a solid-state, nonvolatile technology, they are ideally suited for portable applications as well as providing memory for traditional processing systems. Industrial applications include memory for numerical control machines and various types of process control. Solid-state bubble memories are more reliable in harsh environments; they are affected much less by shock, vibration, dirt, and dust than electromechanical magnetic memories. Innovative new products include data terminals, calculators, word processing, voice storage, and measurement equipment.
A typical bubble memory circuit, from a datasheet later in the publication.
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Updated April 10, 2004