Possible analog input protection improvements #8
Description
The analog input dividers (GPIO1-6) have some unspecified protection devices (V1-V6).
What are they? (Varistors?) If these devices start conducting at 16.2V, in case of over-voltage these devices will get destroyed depending on allowed input current.
A much better way of protecting the delicate analog inputs of the processor is to use 3.3V ESD protection devices (or better zener diodes) on the CPU pins and increase the impedance of the voltage divider as much as possible within the design constraints. The current that needs to be clamped at the 3.3V devices (and in part possibly through the clamping diodes of the CMOS inputs) is minimal and won't destroy anything. Try to make the top resistors (R32...) of each divider 22k or higher if possible. Also consider some filter capacitance on the GPIO_ANA side of the divider. High frequency disturbance can be treacherous ;-)