I need to sample a 12v signal with a PIC...

Jim Granville wrote:
> CBFalconer wrote:
> <snip>
>>
>> Also watch out for the capacitance of the input pin.  Using the 22k
>> suggested above (22 x 10e3) and a capacitance of 5 pf (5 x 10e-9)
>> the time constant is 100e-6, or 100 uS.  For a 12 V signal to rise
>> to 1/3 will take about 1/3 the TC, or about 30 uS.  Also, slow rise
>> times on input pins play havoc with internal logic and power
>> dissipation.  A divider is probably better controlled.
> 
> Is getting ones Pico and Nano farads confused, as bad, or worse, than
> top-posting ? ;)

Worse.  That is my second mistake this decade.  Must be getting
old.  At least the correct calculation shows that the effect is
trivial.  It's like the federal budget, 10e3 here, 10e3 there, and
pretty soon you are talking about real effects.

-- 
 <http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt>
 <http://www.securityfocus.com/columnists/423>
 
 "A man who is right every time is not likely to do very much."
                           -- Francis Crick, co-discover of DNA
 "There is nothing more amazing than stupidity in action."
                                             -- Thomas Matthews