Phytec miniMODUL-537, Hardware Manual

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RAM-Write-Protection and Battery Back-up
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PHYTEC Meßtechnik GmbH 1993 L-006e_2
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7.2 Hints for the Utilization of the Battery Buffering
The safety of a circuit with battery buffering for volatile memories
with write-protection is generally overestimated. A battery buffered
RAM never replaces an EPROM. It is not advisable to use a battery
buffered RAM for long term storage of programs. Already a program
crash stimulated externally, might overwrite critical partitions of the
RAM. The battery buffered RAM should be used only for data and
parameter storage.
A most reliable circuit for write-protection during the application and
removing of the supply voltage, must include the power supply and
the voltage regulator, which is not incorporated in this case.
The RAM should be used as program memory only if the program can
be down loaded easily or if the program is redundant and can be
loaded from an EPROM.
You minimize problems with the RAM write-protection, if you
observe the followings advices:
Provide a well defined, bounce-free decrease and increase of the
supply voltage VCC. The unplugging of the module in the operational
status and the turn on/off procedure of the regulated voltage VCC with
mechanical contacts might overcharge the write-protection.
Wire the supply voltage before the voltage regulator and provide
sufficient capacitors to achieve a defined voltage transition.
Provide, if required, checksum or other redundancy to recognize
memory errors and means to correct them, independently if you use
the RAM as data- or program- memory.
The application as program-memory makes only sense, if the
programs can be reloaded easily. In case of doubt, use an EPROM as
program-memory.
The probability of a memory-error is negligible, but cannot be
excluded. Please consider this fact for your application.