Patent ID: 8638154

Claim:
A system clock monitoring apparatus loaded in an electric system driven by a system control clock based on a system clock, the system clock monitoring apparatus being configured to monitor a frequency abnormality of the system clock using a monitoring clock supplied from a source different to the system clock, comprising: a mode determination circuit configured to determine whether there is a status change of the electric system associated with a frequency variation of the system control clock, the status change is performed by a CPU/Peripheral circuit; and a clock change circuit configured to change the system control clock from the system clock to the monitoring clock based on a determination result obtained by the mode determination circuit, wherein the mode determination circuit is configured to make the clock change circuit change the system control clock from the system clock to the monitoring clock when the status change is detected and then suspend a system clock monitoring operation by the system clock monitoring apparatus during at least a period from a start timing of the status change to an end timing of the status change, and the mode determination circuit is configured to make the clock change circuit change the system control clock from the monitoring clock to a frequency-changed system clock when the end timing arrives and then restarts the system clock monitoring operation.