Pin84 is Vpp only for F167, then should I stick pin84 to 12V, or switch 12V on only when erasing or programming flash?
For F168, pin84 is Vpp/RPD, then how to apply 12V to pin84 and maintain RPD function(this is a simple RC circuit without Vpp)? Should I switch 12V on to pin84 only when erasing or programming flash?
For the F167, you can connect the pin to 12V directly, but I recomend you connect it only when programing. It will ensure an application more robust and will avoid problem at power-up/down where the 12V need to be place after the 5V and removed first.
For the ST10F168 the pin has the 2 functions, therefore you cannot let the 12V during the reset phase. Have a look at