USB Memory Stick: an ideal memory device 2

Published:2011/8/8 2:30:00 Author:Amy From:SeekIC

By Harry Baggen

Changing the drive letter

The block diagram gives a fair impression of the memory chip complexity — a lot of intelli­gence lurks inside the memory to make sure all processes run properly.

One of the main points to note with any Flash memory is reli­ability. Flash memory cells do not have eternal life an the odd cell may malfunction dur­ing operation. That’s why an extensive internal error check­ing system is available (ECC, error correction code). With all operations, (read, write, but also erasure), the data integrity is checked. In case an error is found, it is corrected and the contents of the entire ’block’ containing the error is copied to another, free, block in the memory.

The manufacturer claims a life­time of 100,000 erasure and write cycles to the device. Data is held in the memory cells for at least 10 years.

How does a Flash memory work?

The basic construction of Flash memory is derived from the EPROM. Each memory cell consists of MOSFET with two gates (Figure 2). A floating gate is ’inserted’ between the control gate and the substrate of the FET. This floating gate employs an extremely thin layer of silicon dioxide hence forms a nearly perfect isola­tion, preventing electronics injected into the gate from leaking away when the supply voltage is switched off. The number of electrons present on the floating gate determines if a logic High or Low level is being stored. Cell program­ming and erasure is affected by applying a relatively strong field between gate and sub­strate, allowing electrons to travel across the isolation bar­rier. This process is called Fowler-Nordheim tunnelling. In the mean time, millions of USB memory sticks and cards have proved the reliability of Flash-based memory devices — they can be relied upon to keep your data secure for many years.

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