I do a 12K per year job with a 12c508 and the internal diodes work fine.
I use 2 x 4m7 in series and a small cap (470p) on the pin. You have to
watch the voltage rating of the resistors they are at best 200v and the
peak voltage is 1.707 x 240 (350 ish) particularly on 0805 smd parts.

Regards Steve...

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Olin Lathrop
Sent: 26 October 2003 16:29
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: PIC <-> 240VAC

Mohit Mahajan wrote:
> Refering to Microchip's AN521, it shows a PIC connected to AC supply
> line (of "several hundred volts") using just a 5 megaohm resistor in
> series in between. Safety issues apart, it looks too simple to be
true.
> Is this enough to detect a zero-crossing (eg. for a PIC based triac
> dimmer)or   should I put two clamping diodes on the pin detecting the
> zero-crossing? I'm using a PIC16F870. I've read the manual and I
believe
> it doesn't have protection diodes in its i/o pins.
>
>                               --->|---o Vdd
>                   R           |
>  240VAC o-------/\/\/\---o----o---------o PIC i/p
>                  5 M     |    |
>                          |    ---|<---o Vss
>                          \
>                      R   /
>                     100K \
>                          |
>                          o
>                         Gnd

Actually PICs do have protection diodes on most pins.  The exceptions
are
MCLR because is needs to tolerate 13V during high voltage programming,
and
RA4 when it is open drain.  Aside from those pins, you don't need
external
diodes in the circuit above.

However, while the protection diodes are there and they work, I would
try
not to rely on them unless this is a very high volume product where
every
penny matters.  I've also seen strange A/D readings on one channel when
another had a little current going thru its protection diode.  I don't
know how universally the issue applies.

Barring very high volume design, I would do what you showed above, but
then add a 10Kohm resistor in series with the PIC pin.  Diode forward
voltages can vary, and if your clamping diodes clip at a little higher
voltage than those in the PIC, you can still get some current thru the
PIC
diodes.  With the 10Kohm resistor in series, the difference in diode
voltage will be divided by 10Kohms to make the PIC diode clamp current.
That will be so small you can ignore it.

After that, there are many other refinements.  Whether they make sense
depends on price, risk of failure, noise immunity, etc.  For example,
some
hysteresis will be useful in some applications.  Other applications
might
benefit from filtering high frequency noise.  You might also consider
splitting the 5Mohm resistor into two separate resistors to increase
creapage distance and immunity to dirt and humidity.


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