AVR-Chat

On Wed, Aug 12, 2009 at 12:37 PM, Don Kinzer<dkinzer@easystreet.net> wrote:
> --- In AVR-Chat@yahoogroups.com, "Dave McLaughlin" <dave_mclaughlin@...> wrote:
>> I would put a 5.1V zener diode across the input to the AVR ADC
>
> If you do so, you must be careful to select a size for the upper resistor in the divider so that the current is limited to a safe level for the zener.
>
> Most modern AVRs do have clamping diodes to Vcc and ground on all I/O pins but you can add an external clamping diode (instead of the zener).  The external diode should have a forward voltage lower than 0.5V since that appears to be the forward voltage of the internal clamping diodes.  Here, again, the upper resistor in the divider must be large enough to limit the current to a safe level for the clamping diodes.

If you want to be ultra safe, calculate the divider to top out at the
zener voltage, and then add a second divider to take the zener voltage
down to VCC.
That way you can't ever exceed VCC.