[time-nuts] Clocking a PIC16F628A from a Rubidium Standard

[Orin Eman]

On Fri, Nov 25, 2011 at 1:21 PM, Robert Atkinson <robert8rpi at yahoo.co.uk>wrote:

>
>
> Hi,
>
> Microchip cerainly condone using input protection diodes of PIC devices as
> clamps. There are application notes for zero-crossing detection which
> connect the input to the 115V AC line via a resistor. Note that these are
> intentional protection diodes, not unavoidable parasitic junctions. Typical
> Absolute max clamp current (inc. 16F628) is +-20mA.



Yes, absolute max - that below which the part doesn't let the magic smoke
out...  but note the comment below the absolute max ratings (I'm taking
this from a PIC16F88 datasheet since I happen to be using this part):
"This is a stress rating only and functional operation of the device at
those or any other conditions above those indicated in the operation
listings of this specification is not implied. Exposure to maximum rating
conditions for extended periods may affect device reliability."

Note also the max voltage on any pin is something like Vdd+0.3V.  One
wonders how much conducting their diodes do with 0.3V across them.

Personally, I don't let the protection diodes conduct in normal operation.

Orin.