An integrated circuit device includes a stack of integrated circuit memory dies having a plurality of through-substrate vias (TSVs) extending therethrough, and a buffer die electrically coupled to the plurality of TSVs. The buffer die includes a test interface circuit, which is configured to: (i) generate a plurality of internal test signals, which are synchronized with a second clock signal having a second frequency, from at least one control code, and from a plurality of external test signals, which are synchronized with a first clock signal having a first frequency less than the second frequency, and (ii) provide the plurality of internal test signals to at least one of the memory dies in said stack during a first test mode. The second frequency may be greater than three (3) times the first frequency.