i/o pin protection

[dude]

what is the best way of protecting a i/o from overvoltage? zener? Transient voltage suppression diode?
anything else u can use?


and how do you protect the io pins against reverse polarity whitout effekting the voltage that the pin reads?

i mean if you have a schottky then 0.7 volt will go over the schottky and less will arrive to the pin i guess. and then maybe you read '0' when it should be '1' at the pin. is there some way of doing this whitout effecting the i/o pin.

 

[ericgibbs]

hi,
If you are talking about PIC pins as Input, all the pins have internal protection diodes, one to +Vs and one to 0V.

When driving an input pin from an external voltage which is greater than +Vs, [including moderate transients] you should use a series resistor to limit the input current to less than 20mA.

 

[dude]

hi

no i was actually talking about atmega uC =)
sorry. maybe i should have writen this

 

[misterT]

Atmega controllers also have internal protection diodes. The input pins can tolerate voltages from -0.5V to VCC+0.5V (with respect to ground).

Guaranteed logic voltage levels:

VCC 2.4 to 5.5V
Input low voltage (max): 0.3*Vcc
Input high voltage (min): 0.6*Vcc

VCC 1.8 to 2.4 V
Input low voltage (max): 0.2*Vcc
Input high voltage (min): 0.7*Vcc

I/O Pin pull-up resistor is 20 - 50 kOhm.

Above values are from Atmega48P/88P/168P/328P datasheet.

Additional protection is application specific, but zener with resistor works well for over voltage protection. A single resistor to limit pin current is a good basic protection also. If you have some extreme conditions then you need to tell us more about them.

 

[dude]

okej, thanks for the fast answer.


what i need is a i/o like this:

digital i/o: 0 = 0-2V, 1 = 4-40V 0-50Hz (-3dB) protected against wrong polarity over voltage(50 V).

its for a school project. i use a atmega ATMEGA324PV-10AU with vcc 5 volt.
what i was thinking was a schottky diode of 0,5 volt voltage drop.

if i get in a 2 volt (0 logical) then the schottky will have a drop with 0.5 volt=>1.5 volt
(0.3*5v=1.5) witch will give a logical zero.

if i get in a 4 volt (1 logical) then the schottky will have a drop with 0.5 volt=>3.5 volt
(0.6*5v=3.5) witch will give a logical one.

then i will have a filter for the 0-50hz.

i was thinking of having a zener and a resistor for the overvoltage protection...i was thinking of having the avalanche voltage at 5 volt. and then some resistor. i have to think about how to calculate the resistance..hmm..

what do you think about my approach?