Send the PowerDown command to put the PN531 (including the
        contactless analog front end) into power down mode in order to
        save power consumption. The *wakeup_enable* argument must be a
        list of wake up sources with values from the
        :data:`power_down_wakeup_sources`.

def power_down(self, wakeup_enable):
        """Send the PowerDown command to put the PN531 (including the
        contactless analog front end) into power down mode in order to
        save power consumption. The *wakeup_enable* argument must be a
        list of wake up sources with values from the
        :data:`power_down_wakeup_sources`.

        """
        wakeup_set = 0
        for i, src in enumerate(self.power_down_wakeup_sources):
            if src in wakeup_enable:
                wakeup_set |= 1 << i
        data = self.command(0x16, chr(wakeup_set), timeout=0.1)
        if data[0] != 0:
            self.chipset_error(data)