a even if the mask signal goes high briefly during the unstable period while power is being shut off this transient high does not lead to any flow of current into buffer 15 and does not prevent the logic core 10 a from powering down completely it is also impossible for the latch circuit 33 a to be left in the high output state after the logic core 10 a is powered off once the vddcore power supply voltage has fallen to the ground level resistor 33 a ensures that the output of the latch circuit 33 a will also go low the input signal i rtc will therefore still be masked the next time the logic core 10 a is powered up another effect of the first embodiment is that since the mask signal msk is generated inside the logic core 10 a the signal generator 40 can be simplified and an external terminal of the lsi chip is freed for other use the first embodiment can be modified in various ways within the scope of the invention for example a the switch controller 50 a need not be operated by pushbutton switches 51 53 as shown for simplicity in fig2 the switch controller 50 a may be adapted to detect various states and control its own switches or notify the cpu 11 accordingly b the cpu 11 may shut power off on its own.