TCDS_IMER
bus_space_write_4(sc->sc_bst, sc->sc_bsh, TCDS_IMER, 0);
imer = bus_space_read_4(sc->sc_bst, sc->sc_bsh, TCDS_IMER);
bus_space_write_4(sc->sc_bst, sc->sc_bsh, TCDS_IMER, imer);