[SPAM bayesian] - Protecting Microcontrollers (was Re:[sdiy] converting a 10v p to p toa 0-5 volt signal) - Bayesian Filter detected spam

[Scott Nordlund]

I'm combining several messages here...

> I don't mean to sound like a jerk, as I'm honestly curious (and don't know the answers myself)......
> Isn't this something that everyone who works w/ micros has had to deal w/ since day one? (well, everyone who interfaces micros w/ anything analog in the real world) Aren't there standards for doing this type of stuff?

>From what I've seen in schematics, etc. it's typically some variation on what's already been mentioned here- 2 diodes and a resistor or something like that.  I'd suspect in most cases it's something that doesn't really get much attention, and is applied simply based on the designer's preference.  I'm certainly open to investigating it a little more, but I don't know of it being discussed in depth anywhere (not that I've looked!).

>I don't think you understand the problem you are trying to solve.   
>Its not the voltage that is the problem - its the current that flows  
>into/out of the pin when the applied voltage is large enough to turn  
>on the internal ESD protection diodes.

As far as I understand (and I'm certainly open to being corrected), you don't want to forward-bias these diodes when the device is powered.  I guess it would depend on the specific design, but in general these diodes are only parasitic structures that exist as by-products of the CMOS process.  Sending any current through them while power is applied risks switching on parasitic thyristors that could short the power supply rails until power is removed (latchup...).

I really understand this only in theory- I'm not familiar with any practical implementations, so I don't know if it's standard practice to circumvent this by isolating the inputs and outputs from the rest of the substrate, etc.  I wouldn't want to risk it, though, if I can satisfy my paranoia with a couple extra components.

Any VLSI designers here who can tell me otherwise?

>A good protection technique uses a current limit resistor followed by two
>diodes one the + voltage the other to the - voltage Followed by a Second
>current limit resistor. The voltage at the Diodes can run from ~ .6 volts
>above and below the power rails but the second resistor limits the current
>so that the on chip input protection can clamp the .6 volts at very low
>current without being damaged.

This is the best approach that I know of.  If you use Schottky diodes (for lower voltage drop and fast response time), you'd probably be nearly bulletproof (though I haven't tested this...).

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