*** Should probably be called non-volatile long term read/write memory

EEPROM - electrically erasable programmable ROM.

  Both setting and erasing done with voltage higher than logic range.

  Early versions required fairly high (27V?) voltage to burn.

  Later versions use a lower voltage (12V) available on the motherboard 
    or have internal boosting circuits called charge-pumps.

  More expensive to manufacture than ROM, PROM, EPROM, but much more convenient
    to work with.

  Each byte individually set and erased.  
    Requires byte to be erased before writing. Process slow.
      (Too expensive to address at bit level.)
     
  Newer EEPROMs provide for parallel block level erasure before byte writes.

  Two forms :
    Serial EEPROMs uses very small number of pins to interface with system.
      Clock, 1-bit data in, 1-bit data out, and some control lines.
 
        Control lines : (example Samsung s524a40x10 8-pin 1k bits)  
          Clock
          Write Protect (hard wire read-only - blocks write command)
          A0,A1,A2 (chip select address pins. not used for internal memory)
        
          (with data i/o, +power, ground = 8 pins)
 
      Contains a micro-decoder (can execute a small set of instructions)

      Access done by providing instruction, then target address, 
        and data is either written or read.

      Serial EEPROMs are slow to access.

      Small pin-out, 8 pin common.

        Smart cards
        Phone sim cards
        Automotive devices
        Home appliances such as microwaves with preset programs.

        Or devices that read ROM contents into other memory on power up.

   Parallel EEPROM have separate address, data, and control lines.  
     No instruction decoder.

     Much faster to work with. But require large pin-out >= 28 common.

     Reads as fast as DRAM.

  After repeated changes, gate loose its ability to change. 
    Early models 10s thousands of times, 
    modern designs in the range of 1 million.

  A set gate may "leak" and eventually return to zero (> 10 Yrs).

  Modern flash memory is an advanced form of EEPROM.