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DS01019A-page 2 © 2005 Microchip Technology Inc.
As will be described in more detail later, if the designer
has any control over certain environmental or operating
conditions, he should observe the following basic
guidelines:
• Keep the application temperature as low as
possible.
• Keep the application voltage (or the V
CC voltage
on the EEPROM) as low as possible.
• Write as few bytes as possible.
• Use page write feature whenever possible
• Write data as infrequently as possible.
With these basic guidelines applied to the fullest extent,
the endurance of EEPROM-based devices can be
extended well beyond the specified minimum
endurance. Under certain very specific conditions,
Microchip Serial EEPROMs have been shown to last
for over 100 million cycles, and 10-million cycle
applications are common.
WRITE MODES IN EEPROMS
There are three ways that EEPROM-based devices
can have the entire array data contents changed.
These are: Byte mode, Page mode and Block (or Bulk)
mode. Some types of devices support all three modes,
others may only support one or two modes. The mode
that is used to write an EEPROM-based device may
affect the long term endurance of the product. Byte
mode writing is used when the contents of the array are
changed one byte at a time. For some devices this is
the only user-accessible write mode available. To
change the entire contents of a Serial EEPROM in this
way would take up to 6 minutes (using 5 ms per byte on
a 512K Serial EEPROM). Page mode writing is a
popular feature on many designs of EEPROM memory
products. Again using 512K serial EEPROM as an
example, this feature allows up to 128 bytes of data to
be written to the memory in the same time that one byte
would normally take. In this mode, the write time for a
512K Serial EEPROM can be cut from 6 minutes to 3
seconds. Block cycle is generally a test mode used by
EEPROM manufacturers to make it easier to test the
products. Some types of EEPROM-based products
have these modes as user options (such as the ERAL
and WRAL mode in 93LXXX products), but generally
this mode is not user accessible. A block write can be
done in as little as 1 ms, allowing millions of write cycles
to be completed in a few hours.
A general rule to follow in choosing write modes is that
the larger the number of bytes being written in a single
instruction, the longer the device will last. For example,
in Byte mode, a device might start to fail after 300,000
cycles under a particular set of conditions. But the
device may last 600,000 cycles in Page mode under
the same conditions. In Block mode, the device might
last 1 million cycles under the same conditions.
The reason for this is related to the internal design of
any EEPROM-based product. In these devices, an
internal “charge-pump” takes the applied V
CC voltage
(typically 1.8V to 5.5V) and increases it to 15V to 20V.
This voltage is required to induce “Fowler-Nordhiem
Tunneling” that is used to program and erase
EEPROM-based devices.
The charge pump voltage is used to program however
many EEPROM-cells are being programmed. For
example, in Byte mode, all the cells in a byte (8 or 16)
are biased with the charge pump voltage. In Block
mode, all the cells in the array (up to 1M depending on
the device) are biased with the charge pump voltage.
The charge pump is like a current source during
conditions of high load, so the voltage put out by the
charge pump will be reduced slightly if more bytes are
being written. If the whole array is being programmed
then the charge pump voltage will be significantly
reduced.
Generally, the lower the charge pump voltage the
better the endurance (there is a limit since the charge
pump voltage needs to be high enough to program the
cell) and so the best endurance is generally achieved
by using Block mode cycling. Page mode is worse than
Block mode, but better than Byte mode. Block mode is
generally not a very useful cycling mode to the end
user, since the data contents in the whole array will be
changed to the same value (generally 00 or FF). When
Microchip tests EEPROM-based products, we use Byte
mode cycling on devices which do not have a Page
mode, and Page mode cycling for those that do. We
encourage our customers to use Page mode writing on
all products that have Page mode in order to get the
highest endurance.
ENDURANCE TESTING
METHODOLOGIES
Different manufacturers use different ways to both
cycle and test EEPROM-based products. There is no
standard for endurance cycling or for testing devices
after cycling.
There are two groups of testing that Microchip performs
on all products: qualification and production. Qualifica-
tion testing is done for all new products and major
changes to a product or manufacturing process.
Production testing is done on all devices shipped to our
customers.
Qualification testing at Microchip is used to ensure that
the device is reliable. A great deal of testing is done,
including endurance testing on all EEPROM-based
products. Endurance cycling is generally done at 85°C.
After the rated number of cycles, the sample is tested
to a full production test program. After endurance, the
units are subject to “data retention” to ensure that the
required 200 years of data retention will be achieved
after the maximum number of cycles has been
completed.