AVR-Chat

If data rates will allow, simply add a resistor between the AVR port and
the transient protector. I think (but may be wrong) that communication
with Atmel has established a limit of 0.1ma forward current in the
internal over-voltage protection diodes inside an AVR. Thus, if the
transient protector will practically limit the input voltage to 7.5V, then
a series resistor of (7.5V-5V)/0.1ma = 25K would be appropriate; I would
use 27K or 33K.

The thing to watch out for is rise/fall time at the port pin. A typical
CMOS input has 5-7pf. Lets assume 5pf for the small geometry recent
generation chips. So, in combination with 27K, the time constant would be
around 140ns and the rise/fall time would be about 300ns. So, as long as
the data rate on any protected input is less than 1bit/300ns, you would be
OK.

The transient protector is still important in your environment, especially
with the possibility of lightning. It will limit the input voltage to
something under 12V instead of 100s of volts.

Jim

> Hmmm.   It's beginning to sound like a crap-shoot.    The device I'm
> specifically looking at protecting is an AVR uC, its I/O lines
> specifically.
> How would you do it, if I may ask?   The environment is a boat one,
> basically an automobile one (engines, pumps, motors) with the added
> features
> of local radar, VHF and HF radio transmissions, as well as wireless
> computer
> and instrument transmissions.  Lightning too.   Shielded cables are a
> given,
> as is spike suppression on inductive loads.   Thanks, Steve
>
>
>
> From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
> Of wagnerj@proaxis.com
> Sent: Thursday, January 06, 2011 8:36 AM
> To: AVR-Chat@yahoogroups.com
> Subject: RE: [AVR-Chat] AVX Transguard
>
>
>
>
>
> You need to look at the tolerance specs. And, it depends, greatly, on what
> the connected electronics will tolerate.
>
> I just looked up the AVX parts, and they are spec'd a bit differently from
> some of the others. The 3.3V parts are rated for a 5.0V +/-20% breakdown.
> That means anywhere from 4.0V to 6.0V. with a "clamping" voltage of 12V at
> 1A.
>
> If you look at the V-I chart in the spec sheet, it shows that the 3.3V
> ones have a forward current of (about) 100ua at 3.3V, 1ma at 5V, 10ma at
> about 6V, 100ma at maybe 7V, and 1A at around 8V.
>
> So, they are NOT really "open" at 3.3V and not really breaking down at 5V.
> So, your other circuitry had better be able to handle more than 6V!
>
> The thing you will be relying on is that ESD has a pretty large source
> impedance (some K ohms for human body model) and a source capacitance of
> some 1000s of pf. Thus, the ability to source high currents is very
> limited. This is what saves the circuit, in the end.
>
> Jim
>
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>
>
> ://www.eset.com
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