public byte [] recvInterrupt ()
    throws IOException
    {
	if ("interrupt" != getType ())
	    throw new IllegalArgumentException ();
	if (spi == null)
	    spi = iface.getDevice ().getSPI ();
	// FIXME getInterval() ms timeout (caller guarantees periodicity)
	return spi.readIntr (getU8 (2), getMaxPacketSize ());
    }