M68HC11
REFERENCE MANUAL
ON-CHIP MEMORY
MOTOROLA
4-21
cause the learned priorities would remain in effect through power-off periods, the sys-
tem would begin with these learned priorities. As system demands change, priorities
would change to match system requirements.
4.4.7 Software Methods to Extend Life Expectancy
EEPROM memory is subject to a long-term wear-out mechanism. Though the detailed
mechanics of the failure mechanism are still the subject of much research, the current
understanding is that charge is trapped in the thin oxide layers isolating the floating
gate of the EEPROM storage transistor. The charge is trapped during tunneling, which
only occurs while programming or erasing an EEPROM bit. The life expectancy of an
EEPROM bit is expressed as a number of write-erase cycles (such as 10,000 write-
erase cycles). Changing a bit from one to zero (write) followed by a change from zero
back to one (erase) is considered one write-erase cycle. As a bit accumulates trapped
charge and approaches wear-out, the time required to program or erase the location
gets longer until the allotted time is no longer enough to program or erase the location,
which implies that some extension of life expectancy might be achieved by using long-
er programming and erase times. However, the same wear-out mechanism that caus-
es longer programming time also causes reduced retention capability. Limited data
has been collected to support the use of as much as 20 ms for program and erase
times rather than the 10 ms suggested in the data sheets. A new MC68HC11A8 oper-
ating at 5 V and 2 MHz can typically program an EEPROM location in about 2 ms.
Many factors affect the useful life expectancy of an EEPROM location. Programming
or erasing an EEPROM location at high ambient temperature accelerates wear-out.
The Motorola reliability figures are based on all program and erase operations occur-
ring at worst-case ambient temperature, but no realistic application would experience
such harsh conditions. Temperature has a dramatic effect on write-erase endurance.
An EEPROM having a life expectancy of 5,000 write-erase cycles at 125
°
C typically
has a life expectancy of 100,000 write-erase cycles at 55
°
C. Motorola publishes a
quarterly reliability report which includes the latest life-expectancy data for this rapidly
changing technology. The quality of the thin oxides (processing) is maintained at a
very high level, but there is still some lot-to-lot variation affecting write-erase endur-
ance. The belief is that charge is more easily trapped at sites where the oxide lattice
structure is imperfect.
The method recommended in Motorola data sheets for changing an EEPROM byte
from one data value (other than $FF) to another is to erase the location before pro-
gramming the new value. In this manual, this method will be called ‘erase-before-
write’. When the new data value contains no ones where there is currently a zero in
the EEPROM location (no bits need to be erased), there are two additional methods
of arriving at the desired value without first erasing the EEPROM location. The first of
these methods is called ‘program-more-zeros’. To program more zeros, the new value
would be programmed into the EEPROM location. Bits previously not programmed will
be programmed to zero, and bits already zero remain programmed. The second meth-
od, which does not involve erasure of the location before reprogramming, is called ‘se-
lective-write’. In the selective-write method, a value is calculated that contains zeros
in bits needing to change from one to zero and contains ones in all other bits. This cal-