Hi Paul,

You seem to be concerned about the devil that is lurking round the corner !

Circuit protections is a topic by itself and will warrant a heavy discussion if it is to cover the whole gamut of things.

It depends on so many factors depending on the environment of the end product. When designing professional equipments, guys have to contend with High Voltage strikes, lightning bolts, snapped ground links and wiered things like 230V ac lines shorting with communication bus.

Take the case of a simple data acquistion card that sits snuggly in the warmth of the PC motherboard - most of the things are already taken care off as it feeds on the +5, +12 and -12 supplies of the SMPS. The only point to take care is the I/O entry points and external grounding. Thus optically isolating the digital signals and using a zener clamp in series with a low rated fuse for each of the analog voltage channels will provide adequate protection. And nowadays you get linear opto-isolaters for analog channels - though I have not tried one due to reasons of availability here.

Coming to the MCU, most damages that I have seen happen are when interfacing the MCU pcb with another pcb that has a separate power supply. Ground links are not properly established and you have every kind of problem. The line drivers like 74244 / 74245 do provide some kind of protection but then I would reckon thier purpose is to enhance the bus driving capability of the MCU and not protection per se. A 74245 has a rating of (max) 225mA of short circuit output current. Good for a start, but then it also has a limitation that it can withstand this for 1 second and one pin at a time. Any suggestions to ensure this ?

In case you are planning to use the MCU for generating control signals for high voltage SCRs or DIACs then I would suggest that you get the help of someone knowledgable to start with. More than protection of MCU, these circuits are potential killers as they handle high voltage at pcb level. Not to mention the mess-up they create with switching spikes unless properly designed.

So unless you come back with specifics, providing help is difficult. If you want generalized information suggest contact Prof. Google at www.google.com

Hope that helps.

Raghu