EEPROM
From Wikipedia, the free encyclopedia
An EEPROM (also called an E2PROM) or Electrically Erasable Programmable Read-Only Memory, is a non-volatile storage chip used in computers and other devices to store small amounts of volatile (configuration) data. When larger amounts of more static data are to be stored (such as in USB flash drives) other memory types like flash memory are more economic. SEEPROM, meaning Serial EEPROM, is an EEPROM chip that uses a serial interface to the circuit board.
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[edit] Overview
Serial EEPROMs (SEEPROMs) come in a range of capacities from a few bytes to over 128 kilobytes and are typically used to store configuration parameters, and in modern computers they replace the hitherto CMOS nonvolatile BIOS memory. For example, in personal computers these chips are used to store the BIOS code and related system settings.
The main advantage of EEPROMs over EPROMs is that they are erased electrically instead of by ultraviolet light; this is faster and can be done in-circuit.
While RAM has no limitations on rewrites to memory, EEPROMs are limited in that repeated write and erase cycles eventually damage the thin insulating layer, a process called 'wear out'. Some early EEPROMs could only perform about 100 erase-write cycles but new models specify 1,000,000 (2006) erase-write cycles or more. Flash memory is cheaper than EEPROM but will wear out faster, typically after 10,000 erase-write cycles. Another reason why EEPROM is more effective for storing configuration data is that Flash memory has to erase multiple memory locations at a time. Changing a single byte is only possible by rewriting a whole block, which causes the flash memory to wear out more quickly than one might otherwise expect.
RAM is generally much faster to write than EEPROM (typically a few nanoseconds as opposed to a few microseconds), and most types of RAM are volatile (they lose their contents when power is removed). EEPROM retains its data after powerdown.
[edit] Operation
There are different types of electrical interfaces to EEPROM devices. Main categories of these interface types are :
How the device is operated depends on the electrical interface.
[edit] Serial bus devices
Most common serial interface types are SPI, I²C and 1-Wire.
The serial EEPROM typically operates in three phases: OP-Code Phase, Address Phase and Data Phase. The OP-Code is usually the first 8-bits input to the serial input pin of the EEPROM device, followed by 16 to 24 bits of addressing depending on the depth of the device.
Each EEPROM device typically has its own set of OP-Code instructions to map to different functions. Some of the common operations on SPI EEPROM devices are:
- Write Enable (WREN)
- Write Disable (WRDI)
- Read Status Register (RDSR)
- Write Status Register (WRSR)
- Read Data (READ)
- Write Data (WRITE)
Other operations supported by some EEPROM devices are:
- Program
- Sector Erase
- Chip Erase commands
The final phase of operation is typically the data phase. In case of WRITE, the controller needs to supply data until the chip select is low. In case of read, the controller needs to sample the data every serial clock edge until the chip select is low.
[edit] Parallel bus devices
Parallel EEPROM devices typically has an 8-bit data bus and an address bus wide enough to cover the complete memory. Most devices have chip select and write protect pins. Some microcontrollers also have integrated parallel EEPROM.
Operation of a parallel EEPROM is simple and fast when compared to serial EEPROM.
[edit] Related types
Flash memory is a later form of EEPROM. In the industry, there is a convention to reserve the term EEPROM to byte-wise writable memories compared to block-wise writable flash memories. EEPROM takes more die area than flash memory for the same capacity because each cell usually needs both a read, write and erase transistor where in flash memory the erase circuits are shared by blocks of many (often 512×8) cells.
Newer technologies such as FeRAM and MRAM are slowly replacing EEPROMs in some applications.
[edit] Comparison with EPROM
EPROM has two significant limitations:
- it has to be taken out of the circuit and put under an ultraviolet light source to erase it
- higher (non-TTL) voltages are generally required to program it. These voltages are normally not available in regular computer circuit boards.
EEPROM can be programmed and erased electrically.
The difference between EPROM and EEPROM lies in the thickness of the insulating layer — in an EPROM it is about 3 nanometres thick whereas in an EEPROM it is much thinner — typically around 1 nanometre. This thinner insulating layer allows for lower voltages to be used for programming.
[edit] EEPROM manufacturers
- Aplus Flash Technology
- Atmel
- Hitachi
- Maxwell Technologies
- Microchip Technology
- Philips
- Renesas Technology
- Samsung Electronics
- STMicroelectronics
[edit] See also
- NVRAM
- Flash memory
- DataFlash. Provides higher capacity compared to serial EEPROM chips, however, it requires more complex interface source code.
