Flash Memory

FLASH MEMORY

DEFINITION:-

Flash memory is used for easy and fast information storage in such devices as digital cameras and home video game consoles. It is used more as a hard drive than as RAM. In fact, Flash memory is considered a solid state storage device. Solid state means that there are no moving parts -- everything is electronic instead of mechanical. Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed.

TYPES:-

1.NAND

2.NOR

NOR memory:-

Reading from NOR flash is similar to reading from random-access memory, provided the address and data bus are mapped correctly. Because of this, most microprocessors can use NOR flash memory as execute in place (XIP) memory, meaning that programs stored in NOR flash can be executed directly without the need to copy them into RAM.

NAND memories:-

NAND flash memories cannot provide execute in place due to their different construction principles. These memories are accessed much like block devices such as hard disks or memory cards. The pages are typically 512 or 2,048 bytes in size. Associated with each page are a few bytes (typically 12–16 bytes) that should be used for storage of an error detection and correction checksum.

Distinction between NOR and NAND flash

NOR and NAND flash differ in two important ways:

• the connections of the individual memory cells are different

• the interface provided for reading and writing the memory is different (NOR allows random-access for reading, NAND allows only block access)

CONSTRUCTION:-

Flash memory is a type of EEPROM chip. It has a grid of columns and rows with a cell that has two transistors at each intersection The two transistors are separated from each other by a thin oxide layer. One of the transistors is known as a floating gate, and the other one is the control gate. The floating gate's only link to the row, or wordline, is through the control gate. As long as this link is in place, the cell has a value of 1. To change the value to a 0 requires a curious process called Fowler-Nordheim tunneling. Next, we'll talk about tunneling.

Principles of operation:-

Flash memory stores information in an array of floating-gate transistors, called "cells". In traditional single-level cell (SLC) devices, each cell stores only one bit of information. Some newer flash memory, known as multi-level cell (MLC) devices, can store more than one bit per cell by choosing between multiple levels of electrical charge to apply to the floating gates of its cells.

How Flash Memory Works:-

Electronic memory comes in a variety of forms to serve a variety of purposes. Flash memory is used for easy and fast information storage in such devices as digital cameras and home video game consoles. It is used more as a hard drive than as RAM. In fact, Flash memory is considered a solid state storage device. Solid state means that there are no moving parts -- everything is electronic instead of mechanical.

TUNNELING :-

Tunneling is used to alter the placement of electrons in the floating gate. An electrical charge, usually 10 to 13 volts, is applied to the floating gate. The charge comes from the column, or bitline, enters the floating gate and drains to a ground.

This charge causes the floating-gate transistor to act like an electron gun. The excited electrons are pushed through and trapped on other side of the thin oxide layer, giving it a negative charge. These negatively charged electrons act as a barrier between the control gate and the floating gate. A special device called a cell sensor monitors the level of the charge passing through the floating gate. If the flow through the gate is greater than 50 percent of the charge, it has a value of 1. When the charge passing through drops below the 50-percent threshold, the value changes to 0. A blank EEPROM has all of the gates fully open, giving each cell a value of 1.

The electrons in the cells of a Flash-memory chip can be returned to normal ("1") by the application of an electric field, a higher-voltage charge. Flash memory uses in-circuit wiring to apply the electric field either to the entire chip or to predetermined sections known as blocks. This erases the targeted area of the chip,