Hello Yu Ming,
I am not exactly sure about your requirement from the description given. I will try to answer from my understanding of your question. First of all, the absolute maximum voltage PSoC 4 can deal with is 5.5V(when VDD is 5V). So you will not be able to use it alone for 12 V switching. You have to add additional circuitry for the switching. Could you please let us know your exact application?
I just want to let PSoC 4 EVB be like a voltage switch.
But it's the same as I think,the maximum of the input voltage is around 5V.
it seems to need a additional circuit to realize it.