MOTOROLA
4-26
ON-CHIP MEMORY
M68HC11
REFERENCE MANUAL
floating-gate transistor to become conductive. It may be that the shunting path does
not develop until the bits have already finished programming. If two or more new bits
are being programmed and one were to become programmed (conductive) before the
other(s), V
PP
might become shunted before the other bit(s) could finish being pro-
grammed. One possible reason this unequal programming problem does not arise is
because the bits in a byte are so physically close to each other that they should have
nearly identical properties.
Figure 4-11 Composite Programming Method
The selective-write method may result in soft programming. The shunt path may de-
velop so late in the programming process that the bits are programmed well enough
to be read back as ones but not well enough to provide reliable data retention. On the
other hand, soft programming might be beneficial by limiting the stress on the thin ox-
ides. The selective-write strategy should be viewed with skepticism until additional
study can prove it has merit.
In most cases, EEPROM locations are only exposed to a few write-erase cycles in the
lifetime of a product. In some applications, a few variables need to endure several hun-
dred thousand write-erase cycles (e.g., the odometer reading in an automobile). Since
only a few variables require these extended write-erase cycle lifetimes, it is practical
to consider solutions involving the use of multiple EEPROM locations for the storage
of each such variable. Using an EEPROM location as an ordinary binary counter is
perhaps the worst case for EEPROM wear-out because the least significant bit toggles
at every count; thus, the EEPROM location must be erased and reprogrammed at
each count and is exposed to one write-erase cycle for every two counts.
A count value could be encoded so that an EEPROM location could be programmed
eight times by the program-more-zeros method before it has to be erased. In such a
scheme, the EEPROM location would only experience one write-erase cycle every
eight counts. This scheme of bit-position coding would only be needed for the low-or-
der bits of a counter since the high-order bits change much less frequently.
To extend the write-erase lifetime of a variable even further, using multiple EEPROM
locations would allow switching to a different location when the current location ap-
proachedwear-out.Theproblemistodecidewhenalocationisapproachingwear-out.
ARRAY GROUND
(NOT DRIVEN)
7
VPP
6
5
4
3
2
1
0
VSS
0
0
1
1
0
1
VDD
VDD
VPP
VDD
VPP
VDD
0
0
VDD
VDD