In an implantable human tissue stimulator with a volatile memory an arrangement is provided to protect against the stimulating circuitry producing pulses as a function of unknown parameters in the memory, as a result of inadequate power to the memory from a rechargeable power source, e.g. a rechargeable battery. The arrangement includes voltage sensors, so that when the voltage from the battery drops below a selected level the stimulating circuitry is disconnected from the battery and only the memory is powered. If the voltage from the battery first drops, so that insufficient power is supplied to the memory and thereafter rises, as a result of recharging, to a level sufficient to power the memory, the memory is first reset with known parameter values. Only thereafter when the voltage level reaches the selected level, is the rest of the circuitry, including the stimulating circuitry, reconnected to the battery.