Enables JTAG output on the controller. JTAG operations executed
        before this function is called will return useless data or fail.

        Usage:
            >>> from proteusisc import getAttachedControllers, bitarray
            >>> c = getAttachedControllers()[0]
            >>> c.jtag_enable()
            >>> c.write_tms_bits(bitarray("001011111"), return_tdo=True)
            >>> c.jtag_disable()

def jtag_enable(self):
        """
        Enables JTAG output on the controller. JTAG operations executed
        before this function is called will return useless data or fail.

        Usage:
            >>> from proteusisc import getAttachedControllers, bitarray
            >>> c = getAttachedControllers()[0]
            >>> c.jtag_enable()
            >>> c.write_tms_bits(bitarray("001011111"), return_tdo=True)
            >>> c.jtag_disable()
        """
        status, _ = self.bulkCommand(_BMSG_ENABLE_JTAG)
        if status == 0:
            self._jtagon = True
        elif status == 3:
            self._jtagon = True
            raise JTAGAlreadyEnabledError()
        else:
            raise JTAGEnableFailedError("Error enabling JTAG. Error code: %s." %status)