On Mon, 7 Dec 1998 13:00:32 CST Kelly Schauf <k3jsch@HOTMAIL.COM> writes:

>
>All I am trying to do is come up with some rudimentary way of
>communicating and reading/writing/testing one of this type of part at
>a
>time.  The reason is this:  There are a number of these that are being
>used in a weather monitoring application (used in a Motorola
>68000/68010, 10MHz microprocessor card).  There are 16 of these used
>per
>board.  After being in the field for a while, some as short as a few
>months in the field, some as long as two years, there is a failure
>when
>an attempt is made to boot up the system.  The machine displays a "RAM
>failure" during the boot sequence, mentioning an address where the
>failure occurred.  The failure rate is relatively low, but the
>customer
>is concerned nonetheless.

OK, you have 16 256Kx1 RAMS going to a 16-bit processor, forming a
256Kx16 array.  If one of the chips fails, it could affect any address
(since all 16 chips are active at all 256K addresses), but the *bit*
affected will be unique to the faulty chip.  The memory testing routine
should report both the faulty bit and the address in order to make it
possible to identify the defective chip.

DRAMs don't tend to fail often but you may have gotten into a run of bad
ones.  Replacing them all with ones of a higher speed grade and maybe
from a different manufacturer should take care of marginal situations.



>I am wondering if I could use the 16F84 to write to
>this part, do the refreshes, and then somehow do some sort of readback
>or other technique to see if any of the data has changed.

You can buy a DRAM tester or look at several magazine articles on how to
build one.  Most of these designs used logic array chips to get the
necessary speed to operate the DRAM at its minimum access time.  Though
it is possible to operate a DRAM from a PIC, it won't be much of a test
since the operating speed will be much below the maximum.

As a really cheap alternative, a weather computer board with 15
known-good DRAMS could be used as a tester.  Plug the suspect chip in as
the 16th DRAM and see if a failure is reported.  It would be good to test
the DRAMs under stress such as turning up and down the voltage and
heating or cooling the chip under test.

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