Read MAC from OTP ROM

def read_mac(self):
        """ Read MAC from OTP ROM """
        mac0 = self.read_reg(self.ESP_OTP_MAC0)
        mac1 = self.read_reg(self.ESP_OTP_MAC1)
        mac3 = self.read_reg(self.ESP_OTP_MAC3)
        if (mac3 != 0):
            oui = ((mac3 >> 16) & 0xff, (mac3 >> 8) & 0xff, mac3 & 0xff)
        elif ((mac1 >> 16) & 0xff) == 0:
            oui = (0x18, 0xfe, 0x34)
        elif ((mac1 >> 16) & 0xff) == 1:
            oui = (0xac, 0xd0, 0x74)
        else:
            raise FatalError("Unknown OUI")
        return oui + ((mac1 >> 8) & 0xff, mac1 & 0xff, (mac0 >> 24) & 0xff)