Do Popes say Mass?
Do chickens have wings?
Do snakes have teeth?
Do PICS have ports?
Do engineers have bloodshot eyes from staying up too late?
(in other words) yup


>      A while back, there was a thread on PICs with UARTs etc,
>      getting into a lock-up state on brown-out, that a MCLR couldn't
>      clear.
>
>      Has anybody had a similar problem on PIC12C508/9s?

Big time.  ANY micro can be susceptible to brownouts, and if your
product can fail in an unsafe way with a stuck micro, IT WILL.

>      Or would I probably be ok if I use a MAX811 or similar Micro
>      supervisor to put the PIC into reset when the supply is below
>      2.65V.?

Any good design incorporates something like this, especially if it is
mass produced and could fail in an unsafe way.
>
>      (The supply is a big cap that probably won't go to 0 volts
>      in-between power-ups)

Don't count on it.  Power lines have a standard "circuit breaker "
called a recloser.  A recloser will sense an overload, and interrupt
the power for a few milliseconds.  It then reconnects.  If there is
still an overload, it interrupts for a few seconds.  Then if there is
an overload again, it will interrupt power a third time, sometimes
ten seconds.  Can your cap ride through a ten second brownout?  Don't
count on it.
Best Regards,

Lawrence Lile