Since flash is not so heat resistant why not use OTP or better CERDIP
EPROM based PICs with no onboard A/D or other analog devices and with
external clock from a heat-rated can oscillator (or diy). Once upon a time
I made some tests and I came to the conclusion that one can make parts
work a little beyond the rated temperature by tampering with the supply
voltage. The main effects of temperature are to shift the leakage and
thresholds (increases with CMOS) and decrease the gain. Thus higher
voltage and shifted IO logic levels should work for CMOS, with a derated
(?) clock. My experiments were with NMOS parts where things behave
differently. I suspect that a CERDIP 16C54A/XT (I still have those) could
be run at 150deg C with 500kHz clock at 5.5V or so. The chips may have to
be reprogrammed every half a year or so since the EPROM is also erased
faster when hot.

Peter

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