AVR-Chat

Hi,

    This thread is getting more and more interesting.... And I am thinking 
more and more how much overkill I do in many of my projects... At least they 
do not blow on my face frequently :-)

    What about using a "spark gap" on the PCB right where the signal get's 
inside the box ? I use it at all my DB-9 and DB-25's.. This guarantees that 
the voltages inside the box will never be over 2KV.. This way we could make 
a input circuit with just passive components that is really idiot proof 
unless someone plugs the inputs to 127 volts continually ! We have also to 
be carefull with the clearance over the resistors, 2 in series would 
probably be a better option.. The "spark gap" is just a semi-circle on the 
PCB without solder mask placed very close to the DB-25 pins and connect to 
ground.

    That would be something like this:

 input -+- 47K ---------- 47K -+-- 1K -- AVR pin
        |                      |
     Spark Gap                10nF
with 0.18 mm Clearance         |
        |                      |
       -+----------------------+

     Is there any "normal" situation in a automotive environment that would 
blow the AVR with this input circuit ???

Best regards,
Alexandre Guimaraes


> There have been a number of good responses here. However, most are
> overkill IMO. You don't really need external diodes, for instance, as
> long as you can limit the input current into the internal diodes to less
> than 1-10mA or so (see the data sheet).
>
> You can easily simulate various ESD scenarios with a SPICE program. I've
> done so, and found that the R/C network that I initially wanted to use
> wasn't safe enough. Problem is that when the C charges up to where the
> input protection diodes want to start conducting, you can have
> additional energy that has to go somewhere. And it goes through the
> diodes. Adding a second R between the R/C junction and the input pin
> then lets you limit the current into the input diodes to something safe,
> even with a multi-KV input.
>
> Here's an example of an input protection circuit that I've used for a
> 0V-to-Vcc input. It has a rolloff around 160 Hz.
>
> The input impedance seen by the AVR pin is effectively just the 1K,
> because the 10nF is so much bigger than the internal s/h capacitor.
>
>
> input --- 100K -+-- 1K -- AVR pin
>                 |
>                10nF
>                 |
>       ----------+