>> How about using (p-channel?) FETs for switching between +12 and +5?
>> The 'on' resistance might be low enough. The 'on' resistance of a
>> 4066 CMOS switch might even be ok.
>>
>> Anybody care to beat this idea down?

>I have seen 4066 switches used in similar applications; unfortunately, I
am not aware of anyone making a 74HCT4066--that would be a really useful
part [for those uninitiated, a 4066 or 74HC4066 is a digital switch which
may be used to switch any signal or voltage between its Vss (ground) and
VDD (supply) rails.  Since the supply rail may be up to +15v, the device
may be used to conveniently switch programming voltage and other such
signals.  Unfortunately, the switching threshhold for "standard" CMOS or
74HC parts is VDD/2; if VDD is 12 volts this means the switching thresh-
hold is at least 6 volts--not something a normal port can deal with.  The
74HCT series of parts switches at around two volts independent of supply
and would be ideal in this application.]

>Actually, this gets back to a question I was asking before: what is the
maximum allowable voltage on the RA4 pin (16C84 etc.)?  The documentation
says there's no diode to VDD, but it doesn't grand permission to go above
VDD even so.


As I wrote in another reply, the 'absolute maximum ratings' state 14V on RA4,
 but as you write there's no 'design ratings'.