The OFD is a Toshiba original frequency detector which detects abnormal oscillation at the hardware level. It is compliant with IEC 60730 standard. A frequency of an external oscillator is always measured with a reference clock and a reset is generated when the frequency goes out of a predetermined range. The OFD requires no software program to operate it, which is quite different from using a watch-dog timer. The latter has to be assigned with the highest priority interrupt and needs a subroutine to detect the frequency of the oscillator. The OFD has more features. No external reference clocks are necessary, detected frequencies are selected with register values, and the detection time is as small as 100us min. to reset the microcontroller when the oscillator runs abnormally.