This invention is an integrated circuit having at least one data pin connecting to external circuits. The invention includes a plurality of operational units each having at least one data input/output for data transfer and an enable input. The operational unit have a normal mode and a stall mode controlled by an enable input. The operational units can exchange data via the data input/output in normal mode and are not capable of exchanging data in the stall mode. A selection logic selectively enables an operation unit and connects the data input/output of the enabled operation unit to the at least one data pin. The selection logic is responsive to external signals on at least one data pin to selectively enable operation units.