Hi Angelo,
In fact, every S12 MCU GPIO pin is protected by ESD diodes against power supply rails.
So, any voltage higher than VDDX + voltage drop at ESD diode will generate injection current.
The same is valid for voltage smaller than VSSX - voltage drop at ESD diode.
Note: VDDA/VSSA for port AD.
The voltage drop at ESD diode for a fully opened diode is approximately 0.6 - 0.7V. However, we have to calculate rather with 0.3V where ESD diode starts open.
The maximum allowed continues injection current for one pin is 2.5mA.
However, the ESD diodes are not designed continuous conducting current and therefore we should avoid that situation if possible. It might have also side effects:
The ESD diode is heated by this injected current and influence neighborhood silicon.
The ADC results at neighborhood channels might be influenced.
The injection current might cause the fault (overvoltage) of the voltage regulator in MCU power save modes.
…
Please look at our application note AN4731 Understanding Injection Current on Freescale Automotive Microcontrollers for more details about injection current:
http://www.nxp.com/files/32bit/doc/app_note/AN4731.pdf
You might look also on slightly older application note AN2434 Input-Output Pin Drivers on HCS12 Family MCUs:
http://www.nxp.com/files/microcontrollers/doc/app_note/AN2434.pdf
and AN2433 5V to 3V Design Considerations:
http://www.nxp.com/files/microcontrollers/doc/app_note/AN2433.pdf
Note: AN2433 is from 2002. There is plenty of newer voltage level translators which might be used:
http://www.nxp.com/products/discretes-and-logic/logic/voltage-level-translators
http://www.nxp.com/products/interface-and-connectivity/interface-and-system-management/ic-bus-portfo...
I hope it helps you.
Have a great day,
Radek
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