1995 Microchip Technology Inc. DS00601A-page 1
INTRODUCTION
The endurance of an EEPROM-based device will be
quoted by a manufacturer in terms of the minimum
number of erase/write cycles (write cycles) that the
device is capable of sustaining before failure. A write
cycle is generally considered to be the operation that
changes data in a device from one value to the next.
There are several EEPROM-based devices available
on the market. Microchip Technology Incorporated
makes three general types of EEPROM-based
product: Serial EEPROMs, Parallel EEPROMs, and
EEPROM-based Microcontrollers. As a manufacturer
of many EEPROM products, Microchip is concerned
with endurance and continues to try to educate its
customers on the importance of this topic.
There are many differences in the interpretation of
“endurance” that can result in misleading or inaccurate
information being used in design decisions. This paper
hopes to clear up any questions that the customer ma y
have in the subject of endurance, without becoming so
technical that the information given is not helpful.
There is no widely used standard for any type of
endurance test. Each manufacturer will use their own
endurance testing methodology. This report will
describe all the testing options, and which tests
Microchip performs on its EEPROM-based products.
The MIL-STD cycling test (Method 1033) has not been
updated since 1977 and is well out of date as applied
to EEPROM non-v olatile memories. The standard does
not distinguish the difference between block cycling
and byte cycling, and gives a ver y poor failure criteria.
Microchip does not use this standard.
A uthor: David Wilkie
Reliability Engineer
BASIC TERMS
The definition of “endurance” (as applied to EEPROMs)
in the first part of this introduction contains various words
and phrases that require clear definition and under-
standing. As shown in the f ollowing par agraphs, diff erent
manufacturers will use different standards.
“Endurance cycling” is a test performed by all
manufacturers (and some customers) to determine
how man y “write cycles” the product will achie v e bef ore
failing.
The “minimum number of write cycles” is the least
number of times that you can expect to subject the
product to a “write cycle” before it fails.
“Failure” is a somewhat arbitrary definition, since a
device only truly fails when it no longer meets the
customer expectation, and does not operate in his
system. A failure can be defined in this, the loosest of
definitions, or the most stringent of definitions (whereby
a device would fail if it did not meet any of the data
sheet parameters), as well as a wide range in betw een.
For example, if the device did not correctly store data
into a particular address that the customer was not
using, then the device would work correctly for the
customer but would fail a functional test set by the
manufacturer. Likewise, if the device drew more
current than the data sheet specified after some time,
but the customer application could supply the current
needed, the device would work in the customer
application but would fail a parametric test set by the
manufacturer.
Microchip uses the most stringent definition:
A failure
occurs when the device fails to meet any data sheet
condition under any guaranteed operating
condition of temperature and voltage
.
The number of devices that can fail before a particular
endurance criteria is not met is also somewhat flexible.
Even the most quality conscious manufacturer will
occasionally hav e a failure, so a f ailure level is defined.
The industry standard conditions for many types of
reliability tests are set by JEDEC (the Joint Electronic
De vice Engineering Council). JEDEC defines that if 5%
or less of a given sample fails at a given endurance
goal, then that goal has been met. For example, if a
sample of 100 units are endurance cycled to 1 million
cycles and 3 hav e failed at 100,000 cycles and a further
7 hav e failed at 1 million cycles, then the sample would
have an endurance of 100,000 cycles.
AN601
EEPROM Endurance Tutorial
Thi d t t d ith F M k 4 0 4