I have a circuit containing an Atmega328PB operating at 5V. Due to changes in the operational environment of the circuit, an I/O pin is now exposed to 6.1V from another part of the circuit.
The 6.1V is passing through a 68,000Ohm resistor prior to proceeding to the I/O port.
The 328PB datasheet lists an Absolute Maximum Rating on I/O pins of +0.5V over VCC.
The datasheet also states in section "17.1 I/O Ports - Overview" that all I/O pins have protections diodes.
There is no other mention of the protection diodes in the datasheet and no specifications are given for them concerning current or forward voltage.
The circuit has operated 24/7 for months without incident.
My Thoughts, Assumptions, & Speculation:
Since no additional information on the protection diodes are given in the datasheet, I'm left with only assumptions. I assume the protection diodes will have a forward voltage drop around 0.7V. This leaves the designers with a safety margin of +0.2V within their specification of +0.5V I/O pin tolerance in the datasheet. Working with this assumption, the 6.1V voltage yields 0.4V across the diode, and even if the diode conducts at +0.5V over VCC as a worst case scenario, then this would present a maximum of 0.6V. Considering that the voltage is passing through a 68KOhm resistor, the current through the internal protection diode is between 6 to 9uA (microamps).
With humble speculation and possibly defunct intuition, I don't see this minimal current as thermally problematic and not likely to cause long-term harm. However, I may not be considering all of the possibilities or other adverse effects caused within the micro controller.
Let me know your thoughts and thank you for your time!
The 6.1V is passing through a 68,000Ohm resistor prior to proceeding to the I/O port.
No, it's not. Voltage does not pass through anything. Voltage is a difference in potential between 2 points. Current passes through.
The voltage you are concerned with is 1.1V, that's 6.1V - 5V. Actually, it's probably less than that as the pins can tolerate a bit above Vcc, but lets stick with 1.1V.
You then apply Ohm's Law and calculate the current into the pin, given that you have 1.1V and a 68k Ohm resistor. Then check the data sheet to see if that much current will damage the pin.
You might also find Kirchhoff's circuit laws useful. Both sets of laws are fundamental to all electrical circuits.
@CrossRoads & @srnet, 1mA is plenty for the application, that's great information to have, thank you! Where have you found this specification?
@PerryBebbington, Yes, I had meant to elude to the "current" with consideration to the voltage potential as mentioned in the "pass through" statement, thanks for pointing that out. The datasheet does not provide any specifications for the protection diodes - they are only mentioned once in the datasheet as being present and clamped to VCC and ground.
It's not listed on the datasheet for the classic AVRs (it is for the tinyAVR 1-series and 0-series, megaAVR 0-series, and new DA-series parts; those quote 1mA when running at >4.9V, and 15mA (!!!) at lower operating voltage).
It is often referred to as "current injection".
As a bit of a detail here - you are not applying a voltage i excess of the specifications to an I/O pin if you are limiting the current as you describe! The protection diode and resistor will result in the voltage applied not being outside of the specs!
The datasheet does not provide any specifications for the protection diodes - they are only mentioned once in the datasheet as being present and clamped to VCC and ground.
That's frustrating! I kind of assumed it must be in there somewhere. Anyway, hopefully by now you have calculated the possible current into the pin and realised it's way below anything that's going to bother any protection diodes.
Excellent reference! Below I cite excerpts from the application note for the benefit of future readers of this thread. Thank you all for your responses and feed back.
AVR182: Zero Cross Detector APPLICATION NOTE
To protect the device from voltages above VCC and below GND, the AVR has internal clamping diodes on the I/O pins. The diodes are connected from the pins to VCC and GND and keep all input signals within the AVR’s operating voltage. Any voltage higher than VCC + 0.5V will be forced down to VCC + 0.5V (0.5V is the voltage drop over the diode) and any voltage below GND -0.5V will be forced up to GND - 0.5V. By adding a large resistor in series, these diodes can be used to convert a high voltage sinus signal downto a low voltage square wave signal, with amplitude within the AVR’s operating voltage ±0.5V. The diodes will thus clamp the high voltage signal down to the AVR’s operating voltage.
It is not recommended that the clamping diodes are conducting more than maximum 1mA, and 1MΩ will then allow a maximum voltage of approximately 1,000V.
Most resistors have an upper maximum voltage limit. Make sure that the resistors used in the application can handle the highest possible AC mains voltage, including high voltage spikes.