Posted on May 26, 2009 in Knowledge Base

EEPROM (also written E2PROM and pronounced “e-e-prom,” “double-e prom,” “e-squared,” or simply “e-prom”) stands for Electrically ErasableProgrammable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration tables or device configuration.

When larger amounts of static data are to be stored (such as in USB flash drives) a specific type of EEPROM such as flash memory is more economical than traditional EEPROM devices. EEPROMs are realized as arrays of floating-gate transistors.

EEPROM is user-modifiable read-only memory (ROM) that can be erased and reprogrammed (written to) repeatedly through the application of higher than normal electrical voltage generated externally or internally in the case of modern EEPROMs. EPROM usually must be removed from the device for erasing and programming, whereas EEPROMs can be programmed and erased in-circuit. Originally, EEPROMs were limited to single byte operations which made them slower, but modern EEPROMs allow multi-byte page operations. It also has a limited life – that is, the number of times it could be reprogrammed was limited to tens or hundreds of thousands of times. That limitation has been extended to a million write operations in modern EEPROMs. In an EEPROM that is frequently reprogrammed while the computer is in use, the life of the EEPROM can be an important design consideration. It is for this reason that EEPROMs were used for configuration information, rather than random access memory.

Functions of EEPROM

There are different types of electrical interfaces to EEPROM devices. Main categories of these interface types are:

  • Serial bus
  • Parallel bus

How the device is operated depends on the electrical interface.

Serial bus devices

Most common serial interface types are SPI, I²C, Microwire, UNI/O, and 1-Wire. These interfaces require between 1 and 4 control signals for operation, resulting in a memory device in an 8 pin (or less) package.

The serial EEPROM (or SEEPROM) 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 (or with most I²C devices, is implicit); followed by 8 to 24 bits of addressing depending on the depth of the device, then data to be read or written.

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 (WRENAL)
  • 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

Parallel bus devices

Parallel EEPROM devices typically have 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, but these devices are larger due to the higher pin count (28 pins or more) and have been decreasing in popularity in favor of serial EEPROM or Flash.

Other devices

EEPROM memory is used to enable features in other types of products that are not strictly memory products. Products such as real-time clocks, digital potentiometers, digital temperature sensors, among others, may have small amounts of EEPROM to store calibration information or other data that needs to be available in the event of power loss.

Failure modes

There are two limitations of stored information; endurance, and data retention.

During rewrites, the gate oxide in the floating-gate transistors gradually accumulates trapped electrons. The electric field of the trapped electrons adds to the electrons in the floating gate, lowering the window between threshold voltages for zeros vs ones. After sufficient number of rewrite cycles, the difference becomes too small to be recognizable, the cell is stuck in programmed state, and endurance failure occurs. The manufacturers usually specify the maximum number of rewrites being 1 million or more.

During storage, the electrons injected into the floating gate may drift through the insulator, especially at increased temperature, and cause charge loss, reverting the cell into erased state. The manufacturers usually guarantee data retention of 10 years or more.

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 erasable memories compared to block-wise erasable 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, while in flash memory the erase circuits are shared by large blocks of cells (often 512×8).

Newer non-volatile memory technologies such as FeRAM and MRAM are slowly replacing EEPROMs in some applications, but are expected to remain a small fraction of the EEPROM market for the foreseeable future.

Comparison with EPROM and EEPROM/Flash

The difference between EPROM and EEPROM lies in the way that the memory programs and erases. EEPROM can be programmed and erased electrically using field electron emission (more commonly known in the industry as “Fowler–Nordheim tunneling”).

EPROMs can’t be erased electrically, and are programmed via hot carrier injection onto the floating gate. Erase is via an ultraviolet light source, although in practice many EPROMs are encapsulated in plastic that is opaque to UV light, making them “one-time programmable”.

Most NOR Flash memory is a hybrid style—programming is through hot carrier injection and erase is throughFowler–Nordheim tunneling.