I/O protection diode rating

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2 I/O pins on a mega128 (PA0 and PB5) have 10K series resistors. They are configured as inputs. Due to an external wiring error, 14V DC was applied to these pins (via the 10K series resistors). This caused the mega128 to blow. This happened on 2 different boards. Prior to the wiring error both boards worked perfectly. I am surprised that 0.9 mA could damage the chip. I have looked in the data sheet for the rating of the protection diodes on the I/O pins but can't find this info. Does anyone know the rating ?

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I have just realised that the 0.9 mA did not cause the chip to blow. An external volts free contact connects the inputs (mentioned above) to +5V and when it closed it shorted +14V to the +5V rail which blew the chip. However I would still like to know the rating of the protection diodes if anyone can help.

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Personally, I would not rely on the internal clamp diodes, even with the harmless 10K input series resistance, to avoid any possible ESD causing latchups or further damage. These are parasitic wafer diodes and are not meant to be there for I/O protection.
The input zeners, if the extra components and the extra input capacitance are not issues, seem to be safe enough. Another workaround is the use of ESD diode packs, that clamp the excessive input and reverse currents to Vcc and GND, respectively. But again, those might raise an unprotected Vcc, causing mass failures.
For difficult enviroments, I suggest the single zener per input solution.

George.

I hope for nothing; I fear nothing; I am free. (Nikos Kazantzakis)

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From https://www.avrfreaks.net/index.p...

Quote:
For help anyone here is the response of Atmel AVR Technical Support

Quote:
If you apply more than Vcc + 0.5V to the input, you'll apply power to the AVR through the I/O pin and the diode to Vcc. Now Vcc will raise to the input voltage on the I/O pin - 0.5V.

Note that it's not recommended that the clamping diodes are conducting more than maximum 1 mA.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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thanks/ efharisto guys for the information

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Try the folowing:

connect a diode (1N4148 or better a Shottky (e.g.BAT54)) with cathode to VCC and anode directly to the pin. Then use a input protection resistor (10k, 1k or so)...
And then use protection Diodes (TVS for example) for the VCC line (every 5V chip should survive shortly 6.3V)
Use big electrolytics on Vcc to absorb voltage transients

If you're sure there would only be positive signals, try the following:
Put a resitor (10k , 1k or so) from Vcc to the input pin. Then use a diode with the anode to the input pin and the cathode to the measurement point.
The Following happens: if the external input voltage is hi (higher than the port trigger level), the resistor supply the input voltage for the AVR pin, no current will flow between the external signal and the AVR. (The external voltage can be as high as the diode break reverse voltage (20V++).
If the external signal is low, it will draw current through the rsistor and the voltage at the AVR input will be about the external voltage plus the diode drop....
The perfect way to protect the port pins. But keep in mind, that it will not work with high frquencys (resistance to hi and diode too slow)
But until some 10kHz this is a good solution !

Andreas

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Thanks Andreas for the useful tips.