A time-to-digital converter with fine resolution of less than one inverter delay is described. The recessed geometry serves to reduce or eliminate charge trapping in regions from which the charge may be difficult to erase. On the other hand, if copy-prohibiting information exists, it processes it by either one of the methods of blank output or no output according to the user’s choice. The ground terminal is mounted at a lower level of an intermediate wall of the housing by insert molding, so that the tabs are in the electric-part side fitting recesses. The measurements are used to direct the path of the borehole.