Protecting PIC I/O Lines

[Geoff]

Hi,

I was just wondering what the best way to protect a PIC I/O in an automotive
application. I want to be able to detect a positive input ranging from
5-40V. The PIC is running at 5V. I have thought of a zener regulator on
the input but at over such a wide voltage range means large parts to
dissipate the power which I am trying to avoid. Is it safe to trust the
clamp diodes in the PIC or will this lead to damage over the long term? I
have been told to avoid this method.

Any feedback appreciated,


Geoff

 

[Andy Wood]

Hi,

I was just wondering what the best way to protect a PIC I/O in an automotive
application. I want to be able to detect a positive input ranging from
5-40V. The PIC is running at 5V. I have thought of a zener regulator on
the input but at over such a wide voltage range means large parts to
dissipate the power which I am trying to avoid. Is it safe to trust the
clamp diodes in the PIC or will this lead to damage over the long term? I
have been told to avoid this method.

Any feedback appreciated,


Geoff

Microchip have an Application Note for a lamp dimmer using a 12C508.
It features a zero crossing detector with the mains (120V) connected
to an I/O pin via nothing more than a 20M resistor (well, OK, there is
also a varistor across the mains, and a bit of a disclaimer about
transients). If they can do that I'm sure you don't need anything
elaborate to handle +40V.



Andy Wood
[email protected]

 

[The Real Andy]

Microchip have an Application Note for a lamp dimmer using a 12C508.
It features a zero crossing detector with the mains (120V) connected
to an I/O pin via nothing more than a 20M resistor (well, OK, there is
also a varistor across the mains, and a bit of a disclaimer about
transients). If they can do that I'm sure you don't need anything
elaborate to handle +40V.

All depends on what is is connected to and what speed you want from
the pins. A simple resistor + zener may do the trick. If you a real
worried, a TVS is good, but beware of the capacitance if its a high
speed application.

 

[Caliban]

There's some good advice in this:

http://www.intel.com/design/auto/mcs96/applnots/21031302.PDF