The single chip microcontroller unit includes a processing unit having normal power mode of operation and a low power mode of operation. Analog circuitry and digital circuitry are connected to the processing unit. A power management unit controls power operations of the processing unit, the analog circuitry and the digital circuitry. Power monitoring circuitry provides power control signals to the power management unit. The power monitoring circuitry further includes a system voltage monitoring circuit for generating a system voltage control signal responsive to a system voltage level with respect to a predetermined level. The power monitoring circuitry also includes a supply monitoring circuit for determining if a chip supply voltage level exceeds a threshold level.