An EEPROM is an Electrically Erasable Promgrammable Read Only
Memory.  Comparing it with other memories...

RAM     Random Access Memory (generally read/write)

ROM     Read Only Memory (generally mask programmed during
manufacture)

PROM    Programmable Read Only Memory, often progammed by "blowing
fuses" on the chip.

EPROM   Erasable Programmable Read Only Memory.  Programmed in the
field (memory is generally stored by holding charge on the floating gates
of field effect transistors).  Can be erased by exposure to ultraviolet
light.

OTPROM  One Time Programmable Read Only Memory.  This is an EPROM
in a case that does not have a window for erasing with ultraviolet light.
 This is a lower cost unit than EPROM and is used when the contents will
not have to be changed but volume is not sufficient to warrant masked
ROM.

EEPROM  Electrically Erasable Programmable Read Only Memory.
Similar to EPROM, but can be erased electrically instead of with
ultraviolet light.  This can be used to hold code that can be updated in
circuit, hold calibration or other user data.  EEPROMs have a limited
number of write cycles they will survive (no limit on reads), and have a
relatively slow write cycle time (1 to 10mS).  My favorite EEPROM is the
Xicor 2816 which looks to the circuit like a 2Kx8 static RAM that is slow
on the write side.  After writing to the chip, you cannot access it for
10mS.  No other external circuitry is required.  You can either use a
timer (or software loop) to determine when the EEPROM is ready, or go
into a loop reading the address that was just written.  Valid data will
appear when the EEPROM has finished the write cycle.

EEPROMs nonvolatility (remembering without power) makes it valuable,
again, for code or calibration data.  A RAM with a battery backup can
also be used for the same purpose.  The battery backed RAM has the
advantage that it's faster (no write delay), larger sizes are available,
and (especially for larger sizes) is generally cheaper.  The battery
backed RAM DOES suffer from memory loss when the battery goes dead.

To cut down on the number of pins (at the expense of speed), many EEPROMs
are available in 8 pin packages with serial access.  The smaller pin
count makes the package smaller and saves pins on the microcontroller
talking to it.



Harold
.

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