Microprocessor operating at high and low clok frequencies

Abstract

A microprocessor incorporating a PLL circuit using a clock pulse having a relatively low frequency as an input clock signal of a reference frequency to form an oscillating pulse of a relatively high frequency by multiplying the input clock signal. In the microprocessor, the operation of the PLL circuit is stopped in the low-speed mode to supply the clock pulse of the relatively low frequency to the microprocessor as a system clock signal, and, in the high-speed mode, the PLL circuit is activated upon reception of an event requiring high-speed processing. Until the operation of the PLL circuit is stabilized and the request for high-speed processing comes, the above-mentioned clock pulse having the relatively low frequency is kept supplied continuously to the microprocessor as the system clock signal. This novel setup permits the high-speed switching of the microprocessor from the operating mode to the high-speed operating mode. Accordingly, the microprocessor may be kept operating until the output frequency of the PLL circuit is stabilized, thereby allowing the microprocessor to cope with an unpredictable situation such as the occurrence of a priority event or a failure.