Tomas, firstly, piezos do not have a cathode and anode. They are
basically mechanical capacitors. Some may be polarised but they
are not semiconductors

If you want to protect the PIC, a Schottky diode k to PIC pin
a to ground to trap voltages below Vss, a 5V1 Zener diode also
k to PIC pin, a to ground, or a Schottky diode a to PIC pin, k
to Vcc to shunt piezo V > Vcc to the Vcc power rail

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