Flash memory basics and its interface to a processor:
Flash memory or a flash RAM is a type of nonvolatile semiconductor memory device where stored data exists even when memory device is not electrically powered. It's an improved version of electrically erasable programmable read-only memory (EEPROM). The difference between Flash Memory and EEPROM are, EEPROM erases and rewrite its content one byte at a time or in other words, at byte level. Where as Flash memory erases or writes its data in entire blocks, which makes it a very fast memory compared to EEPROM. Flash memory can't replace DRAM and SRAM because the speed at which the DRAM/SRAM can access data and also their ability to address at byte level can't be matched by Flash.
The flash memory is also termed as Solid-state Storage Device (SSD) due to the absence of moving parts in comparison to traditional computer hard disk drive.
Flash memory types:
The two main types of flash memory are the NOR Flash & NAND Flash. Intel is the first company to introduce commercial (NOR type) flash chip in 1988 and Toshiba released world's first NAND-flash in 1989.
NOR-flash is slower in erase-operation and write-operation compared to NAND-flash. That means the NAND-flash has faster erase and write times. More over NAND has smaller erase units. So fewer erases are needed. NOR-flash can read data slightly faster than NAND.
NOR offers complete address and data buses to randomly access any of its memory location (addressable to every byte). This makes it a suitable replacement for older ROM BIOS/firmware chips, which rarely needs to be updated. Its endurance is 10,000 to 1,000,000 erase cycles. NOR is highly suitable for storing code in embedded systems. Most of the today's microcontrollers comes with built in flash memory.
NAND-flash occupies smaller chip area per cell. This maker NAND available in greater storage densities and at lower costs per bit than NOR-flash. It also has up to ten times the endurance of NOR-flash. NAND is more fit as storage media for large files including video and audio. The USB thumb drives, SD cards and MMC cards are of NAND type.
NAND-flash does not provide a random-access external address bus so the data must be read on a block-wise basis (also known as page access), where each block holds hundreds to thousands of bits, resembling to a kind of sequential data access. This is one of the main reasons why the NAND-flash is unsuitable to replace the ROM, because most of the microprocessors and microcontrollers require byte-level random access.
A write operation in any type of flash device can only be performed on an empty or erased unit. So in most cases write operation must be preceded by an erase operation. The erase operation is fairly straightforward in the case of NAND-flash devices. But for a NOR-flash, it is mandatory that all bytes in the target block should be written with zeros before they can be erased.
The size of an erase-block in NOR-flash ranges from 64 to 128 Kbytes. Here a write/erase operation can take up to 5 s. But the NAND-flash has erase blocks 8 to 32 Kbytes in size. So it is obvious that the NAND performs the identical operation in a lesser time duration.
INOR-flash interface resembles closely to a SRAM memory interface, which has enough address pins to map its entire media, allowing for easy access to every byte contained in it, where as the NAND-flash go for serially accessed complicated I/O mapped interface. Here the same pins are used for control, address & data.
In traditional single-level cell flash devices, each cell stores only one bit of information. Later, many developers have developed a new form of flash memory known as multi-level cell flash that can store/hold more than one bits rather than a single bit in each memory cell, thus doubling the capacity of memory.
Flash memory cell structure:
Flash memory stores data in an array of memory cells. The memory cells are made from floating-gate MOSFETS (known as FGMOS). These FG MOSFETs (or FGMOS in short) have the ability to store an electrical charge for extended periods of time (2 to 10 years) even without a connecting to a power supply.
The FGMOS is actually fabricated by electrically isolating the gate of a standard MOS transistor, so that there are no resistive connections to this gate (floating gate) (see Fig 1). A secondary gate (more than one in the case of multiple gate transistor) known as control gate is then deposited above this floating gate and is electrically isolated from it using an insulator like Si02. There will be only capacitive connection between the new inputs (control gates) and the floating gate, because the floating gate is completely surrounded by highly resistive material (SiO2). So, in terms of its DC operating point, the FG is a floating node.
Each cell (FGMOS) of a NOR-flash memory resembles a standard MOSFET, except the FGMOS has two gates instead of one (see fig 2). On top is the control gate, as in ordinary MOS transistors. Below this control gate, situates the new gate called floating gate, which is insulated all around by the oxide layer (SiO2). The floating gate is interposed between the control gate and the MOSFET channel. Because the floating gate is electrically isolated by the oxide layer, any electrons placed on it are trapped there and, under normal conditions, will not discharge for many years.
The names, NOR-flash & NAND-flash came from the structure used for the interconnections between memory cells (see fig 3).
Cells in NOR-flash are connected in parallel to the bit lines so that each cell can be read/write/erase individually. This parallel connection of cells closely resembles to the parallel connection of transistors in a CMOS NOR gate, that's how it derives the name as NOR flash. In NAND-flash, cells are connected in series resembling a NAND gate, and so the name. The series connection prevents the cells from being programmed individually. These cells must be read in series.
A typical flash-array has a grid of columns and rows of FGMOS-transistor cells as shown in the Fig 4. The word line WL is the horizontal line and bit line BL is the vertical line (shown in Fig 4). The Control gates of the FGMOS cells are connected to the word-line WL. The decoded address is actually applied to this word-line. The bit line BL connects drains of the FGMOS cells together and represent data bus. The Source-line SL connects sources of the FGMOS to common ground. The voltage combinations applied to WL and BL define an operation, whether it is read, erase or program.
Flash stores the data by removing or putting electrons on its floating gate (see fig 5). Charge on floating gate affects the threshold of the memory element. When electrons are present on the floating gate, no current flows through the transistor, indicating a logic-0. When electrons are removed from the floating gate, the transistor starts conducting, indicating a logic-1. This is achieved by applying voltages between the control gate and source or drain.
Fowler-Nordheim (F-N) Tunneling and hot-electron injection are some of the process by which these operations are carried out in the flash cell.
Tunneling is a process where electrons are transported through a barrier. Here the barrier is considered as the thickness of the Si02 insulator layer surrounding the floating gate. The tunneling process in oxide was first reported by Fowler and Nordheim, so the name.
Let us now try to know how a NOR-flash cell operates. In NOR-flash program (or the memory write) is carried out via "hot electron injection" and erase via quantum tunneling.
1. Erase operation:
The raw state of flash memory cells (A single-level NOR flash cell) will be bit 1's, (at default state) because floating gates carry no negative charges. Erasing a flash-memory cell (resetting to a logical 1) is achieved by applying a voltage across the source and control gate (word line). The voltage can be in the range of -9V to -12V. And also apply around 6V to the source. The electrons in the floating gate are pulled off and transferred to the source by quantum tunneling (a tunnel current). In other words, electrons tunnel from the floating gate to the source and substrate.
2. Write (program) operation:
A NOR flash cell can be programmed, or set to a binary "0" value, by the following procedure.
While writing a high voltage of around 12V is applied to the control gate (word line). If high voltage around 7V is applied to Bit Line (Drain terminal), bit 0 is stored in the cell. The channel is now turned on, so electrons can flow from the source to the drain. Through the thin oxide layer electrons move to the floating gate. The source-drain current is sufficiently high to cause some high-energy electrons to jump through the insulating layer onto the floating gate, via a process called hot-electron injection.
Due to applied voltage at floating-gate the excited electrons are forced through and trapped on other side of the thin oxide layer, giving it a negative charge on the floating gate. These negatively charged electrons act as a barrier between the control gate and the floating gate.
If low voltage is applied to the drain via the bit line, the amount of electrons on the floating gate remains the same, and logic state doesn't change, storing the bit 1. Since floating gate is insulated by oxide, the charge accumulated on the floating gate will not leak out, even if the power is turned off.
A device called a cell sensor watches the level of the charge passing through the floating gate. If the flow through the gate crosses 50 percent threshold, it has a value of 1. When the charge passing through decline to below 50-percent threshold, than the value changes to 0.
Because of the very good insulation properties of SiO2, the charge on the floating gate leaks away very slowly.
3. Read operation:
Apply a voltage around 5V to the control gate and around 1V to the drain. The state of the memory cell is distinguished by the current flowing between the drain and the source.
To read the data, a voltage is applied to the control gate, and the MOSFET channel will be either conducting or remain insulating, based on the threshold voltage of the cell, which is in turn controlled by charge on the floating gate. The current flow through the MOSFET channel is sensed and forms a binary code, reproducing the stored data.
Flash Memory Interfacing:
Flash memory interface is same as SRAM interface, except that the flash memory requires a 12V/5V programming voltage to erase and write new data.
The above figure is the schematic circuit diagram of a NOR flash IC 28F400 from Intel interfaced to a 16-bit (data) processor or a microcontroller. The 28F400 can be configured as 512K x 8 memory device or as a 256K x 16 memory device. Here in the above case its 512K x 8 configuration. The control connections pins CE, OE & WE are similar to SRAM interface.
The A0 - A17 are address pins and DQ0 to DQ15 are data pins.
The function of each control pins are,
OE (OUTPUT ENABLE): Enables the device's outputs through the data buffers during a read cycle. OE is active low.
WE (WRITE ENABLE): Controls writes to the command register and array blocks. WE is active low. Addresses and data are latched on the rising edge of the WE pulse.
CE (CHIP ENABLE): Activates the device's control logic, input buffers, decoders and sense amplifiers. CE is active low. CE high de-selects the memory device and reduces power consumption to standby levels. If CE and RP are high, but not at a CMOS high level, the standby current will increase due to current flow through the CE and RP input stages.
BYTE: Configures whether the device operates in byte-wide mode (x8) or word-wide mode (x16). This pin must be set at power-up or return from deep power-down and not changed during device operation. BYTE pin must be controlled at CMOS levels to meet the CMOS current specification in standby mode.
When BYTE is at logic low, the byte-wide mode is enabled, where data is read and programmed on DQ0-DQ7 and DQ15/A-1 becomes the lowest order address that decodes between the upper and lower byte. DQ8-DQ14 are tri-stated during the byte-wide mode.
When BYTE is at logic high, the word-wide mode is enabled, where data is read and programmed on DQ0-DQ15.
Vpp (PROGRAM/ERASE POWER SUPPLY): For erasing memory array blocks or programming data in each block, a voltage either of 5 V ± 10% or 12 V ± 5% must be applied to this pin. When VPP < VPPLK all blocks are locked and protected against Program and Erase commands.
RP/PWD (RESET/DEEP POWER-DOWN): Uses three voltage levels (VIL, VIH, and VHH) to control two different functions: reset/deep power-down mode and boot block unlocking. It is backward compatible with the BX/BL/BV products. When RP is at logic low, the device is in reset/deep power-down mode, which puts the outputs at High-Z, resets the Write State Machine, and draws minimum current.
When RP is at logic high, the device is in standard operation. When RP transitions from logic-low to logic-high, the device defaults to the read array mode.
When RP is at VHH, the boot block is unlocked and can be programmed or erased. This overrides any control from the WP input.
The decoder IC 74LS139 is employed in the above circuit for selecting the flash memory through A19 and IO/M as inputs.
To get some more idea please read this application note available at,
To know some of the nor flash memory IC chips available in the market click on the article below
To know some of the latest flash memory IC replacing non -volatile memory chips such as FRAM PRAM and FRAM available in the market click on the article below
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