This invention is a System On a Chip (SOC) requiring two tamper resistant externally generated power supplies. A first, higher power supply powers I/O and analog circuits. A second, lower power supply powers digital circuits and memory. A first voltage monitor circuit powered by said first power supply generates a first output signal when the first power supply is below an operational limit high level. A second voltage monitor circuit powered by said first power supply indicates when the second power supply is above an operational high limit level. A power switch is controlled by the first voltage monitor circuit. This power switch connects the second power supply and second load when closed and isolates them when open. Thus the memory cannot be accessed when the I/O and analog power supply is out of specification.