AVR-Chat

On Fri, 23 Sep 2005 21:08:19 -0000 you wrote:

>--- In AVR-Chat@yahoogroups.com, Ralph Hilton <ralph@r...> wrote:
>
>> Connect a 10k resistor from the sensor output to the ADC input and 
>10k from the ADC input
>> to ground. that will halve the voltage on the ADC input pin. For 
>safety I would add a
>> protection diode from the ADC input to the +5 volt line.
>
>...if you look at Atmel's Application Note #182 you can see that the 
>built-in protection diodes of AVRs are able to handle currents up to 
>1mA. If you limit the current from/to the pins with your external 
>resistor to that (at max. input Voltage including some healthy safety 
>margin), you won't need the extra diodes.
>
>HTH,
>Stefan

True, I read that note some time back.. However it is very easy for people new to
electronics to exceed that. On a test board playing around it is better to be safe that
sorry.

Personally I take my chances and if I blow the chip then I just plug in another but if
someone is on a limited budget and new to the scene then a protection diode might save
them buying a new AVR.

I blew out 2 chips this month due to errors on my part. But I have the budget to cover it.
Students don't. So if someone sounds like they are learning then I give them advice which
seems appropriate. I remember my student days when I would have to add up the cost of an
OC71 and every resistor in my project. But that was more than 30 years ago. A diode is
cheap and will probably short out saving the AVR if there is a problem.


--
Ralph Hilton
http://www.ralphhilton.org
C-Meter: http://www.cmeter.org
FZAOINT http://www.fzaoint.net