[eagle] Re: CAN-Do noise abatement plan and questions
eagle at howardlong.com
Thu Jun 14 13:58:27 PDT 2007
> OK. The PIC MCU that I'm considering should be safe. It's
> rated for 40 years data retention in an automotive
> temperature range. Most parts guarantee only 10 year data
> retention or say nothing on the data sheet.
On ESEO, we have avoided PLLs because of the requirement to program them at
power on, although I have used them often before for programming PLLs on the
ground: those PIC10F SOT23 packages are really neat! ESEO has a default mode
of a traditional analog linear transponder. PICs have not faired
particularly well in radiation tests, and I'd suggest that it may be wise to
consider both SELs/SEBs as well as total dose.
You can shield against TID to a large extent by depending on tantalum as
previously described, as well as the spacecraft's aluminum structure and
enclosures, but this will not prevent SEEs from happening. After shielding,
SEEs that are predominently caused by high energy protons. You may well need
to consider other mitigation efforts against these high energy inner Van
Allen (proton) belt effects.
On designs with PICs, I have implemented over current protection using
INA200 devices that cut the power for about 30s as soon as a preset current
is passed. This is an attempt to reduce the chance of an SEB, when a FET
effectively turns into an SCR clamping VSS and VDD.
Care should be taken when setting the current trip - not only do you not
want to power down the device if it genuinely wants extra current, but
equally TID (total ionising dose) means that over time the FETs will take
gradually more current anyway.
Achim Volhardt DH2VA already did some radiation tests on a PIC12F device
programming an SPI DDS (AD9834) at CERN when the CANDo's were done also. I
attach his notes. As well as the CREME96 simultations that Achim describes,
there is also SPENVIS: unlike CREME96, SPENVIS can include both the electron
and the proton belts in its analyses. I cannot stress enough the effect that
the orbit has on the amount of ionizing radiation that a spacecraft is
subjected to! Indeed, the AO-10 IHU's early demise is due to its low
inclination HEO orbit, spending inordinate amounts of unplanned time within
the Van Allen belts. By increasing the inclination, significantly reducing
the time spent in the Van Allen belts during an elliptic HEO orbit, ionising
radiation can be reduced by orders of magnitude.
Although there is some science in this, it's a fairly subjective risk
analysis that guides the tradeoff between the complexity of mitigation
circuitry against the KISS principal. After all, if the planned propulsion
fails, you're left in a near equatorial orbit and a big dose of radiation!
73, Howard G6LVB
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