Transcript
White Paper 08/2015
Flash Memory in Industry What is important for use in an industrial environment?
https://support.industry.siemens.com/cs/ww/en/view/109737064
Warranty and liability
Warranty and liability Note
The Application Examples are not binding and do not claim to be complete regarding the circuits shown, equipping and any eventuality. The Application Examples do not represent customer-specific solutions. They are only intended to provide support for typical applications. You are responsible for ensuring that the described products are used correctly. These Application Examples do not relieve you of the responsibility to use safe practices in application, installation, operation and maintenance. When using these Application Examples, you recognize that we cannot be made liable for any damage/claims beyond the liability clause described. We reserve the right to make changes to these Application Examples at any time without prior notice. If there are any deviations between the recommendations provided in these Application Examples and other Siemens publications – e.g. Catalogs – the contents of the other documents have priority.
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Siemens provides products and solutions with industrial security functions that support the secure operation of plants, systems, machines and networks. In order to protect plants, systems, machines and networks against cyber threats, it is necessary to implement – and continuously maintain – a holistic, state-of-the-art industrial security concept. Siemens’ products and solutions only form one element of such a concept. Customer is responsible to prevent unauthorized access to its plants, systems, machines and networks. Systems, machines and components should only be connected to the enterprise network or the internet if and to the extent necessary and with appropriate security measures (e.g. use of firewalls and network segmentation) in place. Additionally, Siemens’ guidance on appropriate security measures should be taken into account. For more information about industrial security, please visit http://www.siemens.com/industrialsecurity. Siemens’ products and solutions undergo continuous development to make them more secure. Siemens strongly recommends to apply product updates as soon as available and to always use the latest product versions. Use of product versions that are no longer supported, and failure to apply latest updates may increase customer’s exposure to cyber threats. To stay informed about product updates, subscribe to the Siemens Industrial Security RSS Feed under http://www.siemens.com/industrialsecurity.
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Table of contents
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1 Introduction
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Introduction The first flash memories were developed in the 80s. Since then, they have been improved and developed continuously. Today, flash memories of different capacities are available for countless applications, for example USB sticks, SD memory cards and SSD memory cards. What is important when using flash memories in an industrial environment? This question will be dealt with on the next pages.
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Flash Memory Flash memories are a good alternative to conventional hard disks when it comes to storing data permanently. Flash memories are non-volatile semiconductor memories. Once stored, data are preserved even without an external voltage source.
Flash memory vs. hard disk drive
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Flash memories and hard disks are based on fundamentally different technologies. Both have advantages and disadvantages. Which of the two should be used to store data depends on the planned application scenario and on the available budget. The following table compares the two storage technologies. Siemens AG 2015 All rights reserved
Table 2-1 Flash memory • • •
Short access times for reading and writing data No moving consumables, making it resistant to vibration and shock Lower power consumption than hard disks
Hard disk • •
Very high number of writing cycles possible Lower costs per gigabyte data storage
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Hard disk = data storage medium based on rotating disks coated with magnetic material, also called hard disk drive (HDD).
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2 Flash Memory
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Flash storage types If you have decided in favor of flash memories, we have to distinguish between removable and permanently installed storage media in a first step. Their suitability for the resulting requirements depends on the intended application.
General structure Figure 2-1 shows the general structure of a flash memory. It consists of one or more flash modules on which the actual data are stored. A controller manages the data internally and distributes it among the individual flash modules. Additionally, the controller features an interface for exchanging data between the connected device and the flash memory. Figure 2-1 structure of a flash storage medium
Industrial PC
Flash Storage Medium
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Interface
Controller
Control Data I/O
Flash Module
Flash Module
Flash Module
Flash Module
Flash Module
Flash Module
Flash Module
Flash Module
This basic structure is the same for all subsequent memory types. CF CF stands for Compact Flash. This is a flash memory in the form of removable 2 memory cards. The data are exchanged through an IDE interface . CFast CFast is also a storage medium in the form of a removable memory card, but it 3 uses the faster SATA protocol to exchange data. SSD The solid state disk (SSD) is another form of flash memory. Usually, SSDs are permanently installed in computers. They can be designed in the same form factor as hard disks or as a memory chip with mSATA interface.
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IDE = Integrated Disc Electronic, an interface with parallel data transfer SATA = Serial Advanced Technology Attachment, an interface with serial data transmission
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2 Flash Memory
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Memory cell types Flash memories work by storing multiple electrons in one memory cell and by interpreting the filling level (full/empty) of this cell as binary value 0 or 1. This is accomplished by measuring the voltage level at the cell, which allows the smallest information unit, exactly 1 bit, to be stored. Such memory cells are called singlelevel cell (SLC). To store more information units in one memory cell, it has to be possible to distinguish additional filling levels or voltage levels. Memory cells capable of storing more than 1 bit are called multi-level cell (MLC). Historically, the first cells of this type were capable of storing exactly two bits. Now it is possible to store 3 bits in a single memory cell. This requires distinguishing additional voltage levels. Storage cells of this kind are called triple-level cell (TLC). The following table gives an overview of these three memory cell types. Table 2-2 Number of storable bits per memory cell SLC
MLC
TLC
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So far, we have looked at a two-dimensional arrangement of cells. A threedimensional layout of memory cells in several layers as in the so-called V-NAND further boosts the storage capacity of flash modules.
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Reliability
Write endurance The number of write and erase cycles of a single flash memory cell is limited. The question how often the operating system and the applications running on it will presumably write data to the flash memory is hence crucial when selecting the suitable storage medium. The write capacity of a flash memory is also referred to as write endurance. In flash memories of the CF type, the quality of this property is frequently specified as the maximum number of write cycles of a single memory cell. For CFast or SSD flash memories in contrast, the terabyte (TB) capacity of writing data to the flash memory is quoted more often. Wear leveling Since the number of possible write cycles of one memory cell is limited, write accesses have to be distributed as evenly as possible among all available memory cells. This is achieved by special wear leveling algorithms in the memory controller. Error management Nevertheless, read or write errors are bound to occur sooner or later due to memory cells wear. Such cells can be compensated for to a certain degree by mechanisms such as bad block management, spare block management and error correction codes (ECC).
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3 Conclusion
Data retention w/o power Over time, even the best memory cells will lose some of their stored electrons. But of course the goal is to preserve the data stored on the storage medium also during longer periods of inactivity, for example during storage or transport. The maximum duration of guaranteed data retention of a flash memory without power supply is indicated by the “Data retention w/o power” specification.
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Industrial requirements
Power loss protection If there is a voltage loss during the write process, data may be written incompletely to the flash memory. This not only affects the user data but also the internal storage management data which the controller needs to allocated the user data to different flash modules. If the controller loses its “memory” in this manner, the flash memory will fail completely as a result. Flash memories for industrial applications should be protected against such failure. S.M.A.R.T. Siemens AG 2015 All rights reserved
To diagnose and detect possible failures at an early stage, the self-monitoring, analysis and reporting technology (S.M.A.R.T.) was developed for storage media. It can be used to determine the condition of the storage medium by reading values such as the temperature of the data storage medium, the volume of data written or the number of defective memory areas. If the storage medium is equipped with this feature, the corresponding diagnostics software can automatically send a message to replace the storage medium before it fails for good.
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Conclusion In an industrial environment, resistance to vibration and shock is a decisive advantage. Flash memories are hence particularly well suited to be used in the vicinity of machines if they provide enough storage capacity for the application. At present, conventional hard disk drives offer more capacity and longevity to store large data volumes or frequently updating data such as complex databases. In industrial areas, a vibration damping suspension of the hard disk drives is crucial. To prevent damages to the flash memory during power loss, they should be fitted with power loss protection. S.M.A.R.T. assures early recognition of pending data carrier failure. This function should also be available for CF or CFast. Therefore, it is worth taking a good look at the respective flash memory specification.
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