I've got a little bit of a (potential) problem in a design, under certain conditions - I have a chip that is powered down, however - some of the inputs to the chip may or may not still be high, so I've now got inputs way in excess of the chips supply voltage (which not surprisingly being unpowered is 0V).
However, i've noticed some IC datasheets don't actually list inputs as VCC +0.3V like some do, and instead just list a current. So if I use series resistors of the correct value to limit to current into these pins, then I shouldn't run into issues with latching up a chip when it's unpowered yet still got control signals going to it?
The concern is mainly damaging the input protection diodes, and then having the device be partially powered from current thru those diodes into the internal Vcc bus.
Current limit resistors might get you there.
Yes a series resistor can generally be safely used. It's a matter of looking up the maximum safe current value of the clamping diodes in a datasheet and calculating the needed resistance value. I don't recall the recommended maximum current limit value but from memory I think it should be less then 1ma?
Page 4 and page 7 suggests that the clamping diode current be limited to 1mA.
I've not been able to find the limit in the '328P datasheet outside of this reference in section 14:
All I/O pins have protection diodes to both VCC and Ground as indicated in Figure 14-1.
Refer to ”Electrical Characteristics – (TA = -40°C to 85°C)” on page 303 for a complete list of parameters.
You can also connect a simple double diode to the input (the one which has one anode and cathode common) and connect the other two pins to the power supply of the IC. If the digital signal has enough strength to kill your IC it will have enough to power it through this diode, and not the on inside your chip. You can combine this with the series resistor.
NANDBlog:
You can also connect a simple double diode to the input (the one which has one anode and cathode common) and connect the other two pins to the power supply of the IC. If the digital signal has enough strength to kill your IC it will have enough to power it through this diode, and not the on inside your chip. You can combine this with the series resistor.
The diodes would need to be Schottky if you want them to conduct before the internal protection diodes do. In this situation I use just a series resistor, however since no maximum input current is given on the datasheet, I normally aim for the maximum current into the input protection diodes to be no more than 0.1mA
retrolefty:
Yes a series resistor can generally be safely used. It's a matter of looking up the maximum safe current value of the clamping diodes in a datasheet and calculating the needed resistance value. I don't recall the recommended maximum current limit value but from memory I think it should be less then 1ma?
The datasheet doesn't actually state a current value for the protection diodes but 1mA is reasonable/practical.
Thanks for the sanity check, as I thought... the chip isn't actually an Atmel MCU, but I have clear maximum current limit values set out in the datasheet for the control input pins.