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Re: Component selection



The on-chip EPROM was never reprogrammed by high energy particles. It survives
intact. We exposed the EPROM to about 300 MeV energy and could not reprogram any
bits.

Now, total dose is different. When exposed to an X-ray source, at about 20 Krads,
the EEPROM was programmed to all zeros. This was due to the radiation building up a
charge in the dielectric layer of the chip. In a LEO orbit, you shouldn't have this
problem for a long while. The amount of time is determined by the actual altitude
and inclination.

If you are considering FLASH memory, the AMD NOR-FLASH devices do work well in
space. It has been known that typical FLASH memory can get bit flips. The AMD
NOR-FLASH seems to work fine. It is flying as an experimental payload on the ARGOS
satellite for about 3 years. I believe it is flying at about 750 Km in a polar
orbit.

If you look at the OPAL satellite. That uses all commercial  components. It has a
Motorola MC68302 processor, and what ever memory that came with the CPU board. They
are also flying MAXIM devices such current sensors. For the six months that the
satellite has been flying, I have been told it has seen abot a half dozen upsets and
no degredation in the MAXIM components. The processor is considered quite soft. I
have heard from other designers that they will never touch the processor. So, if
OPAL can run so well for this long at Solar Max, the environment is not that hostile
for LEO. In fact from some of the SEU and latchup data I've seen, the South Atlantic
Anomymally is worse than the poles.



Ivan

Ken Tentarelli wrote:

> Ivan wrote:
>
> > We are using commercial memory. It is the memory on the PIC.
> > If a latchup occurs, the current jumps. We have circuitry to detect the latchup
> > which will cycle power to the controller board.  ............
>
> Ivan,
> Thanks for the explanation.    The problem of most concern to me is the
> integrity of program memory.   As I recall PICs use on-chip EPROM
> for program memory.   If any bit in program memory gets corrupted it
> needs to be restored to the proper value.  I assume you are using
> on-chip EPROM for program memory and off-chip EEPROM for
> data memory. Does the statement:
>
>         "The memory does not get changed. Just the output
>          buffer of the EEPROM. This  means a high
>           energy particle can change the bit pattern for a CPU
>           instruction  being read."
>
> mean that you saw no permanently changed bits in either
> EPROM or EEPROM ?    This is vitally important to me
> as I would like to avoid having to error correct the program
> memory.  I've seen claims that EPROM doesn't get corrupted
> but I've not heard of real test results to that effect  -- and I
> don't have facilities to do the test here.
>
> Your observation about errors during reads agrees with actual
> satellite data where telemetry has shown errors and then subsequently
> corrected itself.
>
> Thanks again,
>
> Ken AC1H

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