Flash memory is a non-volatile form of computer memory that remembers all information even when the device is turned off. In this, the user can electronically erase or reprogram the data as and when required. Although it was first introduced in the 1980s, its use has witnessed a rapid increase in recent years. This is because the production of portable and technologically advanced devices has grown enormously and almost all of these devices are equipped with flash memory.
Nowadays we can find the real application of flash memory in so many devices around us. This includes USB memory sticks, digital cameras, MP3 players, smartphones and so on. As we can see around us, memory cards are the most common forms of flash memory available in the mass market. These have become the latest means used to store and transmit data between high-tech devices. There are different categories of these cards depending on their applications and capacity.
Here we will talk about the basic concept of Flash memory along with all the important details like its functioning, types and life cycle.
What does Flash memory mean?
There are two types of computer memory: RAM (random access memory) and ROM (read-only memory). The first remembers everything as long as the computer is turned on, while the second is already stored with the necessary information and therefore does not forget any information. A computer needs a combination of both, which is why hard drives are used. These units use a combination of RAM and ROM chips, thus storing information that is remembered for an indefinite period of time, where the user can read and write the data.
Likewise, small and portable devices, such as MP3 players and digital cameras, are equipped with flash memory. These memory cards reflect a mix of RAM and ROM features in such a way that they can store data for an indefinite period of time and the data can be rewritten or erased as per need.
Types of Flash memory
There are basically 2 types of Flash memory, depending on the applications. The technology used in both categories is similar, but the approach to reading and writing data is different. Let's take a look at these -:
• NAND Flash Memory
The NAND flash architecture was introduced by Toshiba in 1989 to meet the need for lower cost per bit and higher performance. It offers high cell densities, which implies large storage capacity along with fast write and erase rates. Furthermore, the system interface is very consistent, which makes updates easier; however, it also has special requirements.
• NOR Flash Memory
Neither flash chip was introduced by Intel in 1988 which proved to be a suitable replacement for ROM chips. With NOR, users can run an application directly from flash instead of using application code in system RAM. A higher level of software for NOR is offered by several popular vendors such as Microsoft and Intel. It is highly economical for lower capacities i.e. 1-4 Mbytes and offers high read performance but lacks speed in write and delete functions.
Flash memory operation
To understand how Flash memory works, let's first check how a transistor works. A normal transistor is similar to a tube through which electricity flows like water. Its two ends are called the source and drain and there is a gate between which blocks the electricity. When the power is on, the gate is open and electricity flows freely, storing a. But when the power is turned off, the gate also closes and blocks the flow of electricity, storing zero. Now, the next time the power is turned on, the gate will remain closed and therefore cannot retain the information.
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In flash memory, the transistor has two ports, namely: control and float. The two ends, i.e. source and drain, are electron rich but are separated by a p-type material which is electron deficient. So, to allow the electrons to flow freely between the two ports, a positive voltage is applied to the contact points (word line and bit line) which pulls the electrons quickly. During this, some electrons get trapped between the control and the floating gate. Now, even when the power is turned off, the leaked electrons remain there for an indefinite period of time. Hence, it is capable of retaining all data whether the power is on or off.
Flash Memory Programming
Previously, programming Flash memory was quite difficult due to the limited number of program erase cycles. They used to perform a few hundred cycles mainly because of the destructive breakdown of the gate's thin oxide layer. However, over time, improvements were made to improve the quality of the oxide layer, which in turn helped increase the lifespan of flash memory. Chips are now programmed without worrying about their useful life or the number of program cycles.
Flash memory wear mechanism
As mentioned above, the lifespan of flash memory is a major concern to ensure that stored data is not lost. This wear occurs mainly because the oxide layer begins to degrade with use over a long period of time. The processes used to program and erase the Flash memory cycle are called hot electron injection and tunneling, respectively. This not only degrades the erasure characteristics but also leads to the closing of the memory cell boundary window.
Dealing with Flash memory wear
To solve the problem of limited erase cycles and limited lifespan, there is a procedure called wear leveling. The main purpose of this is to track the used blocks and then distribute the program and erase cycles evenly across memory. This helps prevent overuse of blocks and their premature failure. There are three ways to deal with this problem, described below:
• No wear leveling
In this, the logical addresses of the operating systems of the operating systems to the physical address of the memory. When the location changes, the content of that block is erased and reprogrammed. This is very time consuming and doesn't do much to solve the wear problem.
• Dynamic Wear Leveling
In this approach, a map is used to link the operating system's logical addresses to the physical location of flash memory. When new data is written, the original block is marked as invalid and the map is linked to a new block. It is called dynamic because it only recycles the changed data while the old blocks are marked as invalid and are not used.
• Static wear leveling
It is the most effective approach among the two mentioned above. It is similar to the dynamic method except that it moves the unchanged i.e. static data periodically and therefore evens out cell usage across memory.
Flash memory applications
Flash memory can be found in almost every portable device around us. Basically, there are three forms that are quite common today. These applications are described below -:
• USB flash drive
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These devices are popularly known as pen drives and contain flash memory with an integrated USB interface. The data contained in such devices is removable and rewritable. The maximum storage capacity carried by a USB drive is up to 2TB and is being planned. They replaced floppy disks because they have more storage capacity and transfer data at a much faster rate.
• Memory card
Memory card is a storage device used in almost all devices nowadays, including mp3 players, cell phones, laptops, digital cameras and video game consoles. This form of flash memory evolved from compact flash cards in 1994 to postage stamp-sized Secure Digital cards in 2001. Nowadays we can find the latest miniSD and microSD cards that are equipped with greater storage capacities and work faster.
• SSD card
SSD, also known as Solid State Drive/Disc, is the latest form of flash memory that has replaced hard drives in computers. Since there are no moving parts involved, the chances of mechanical failure are negligible. They are much smaller in size, consume less power, run cooler and still respond at a much faster rate. The demand for SSDs is driven by the need for higher I/O performance. They have lower random access and read latency, which makes them suitable for heavy workloads.
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