Variable frequency microprocessor clock generator

摘要

A microprocessor based system (10) includes a central processing unit (CPU) (12) that controls the operation of a display (20) through a controller (22). System storage is provided with a read only memory (16) and random access memory (14). A reference clock signal is generated by a clock generator (26) which is input to a clock control circuit (24). The control circuit (24) generates a CLK signal that is connected to the clock input of the CPU (12). The control circuit (24) is operable to reduce the rate of the clock input to the CPU (12) when accessing the controller (22) which has a slower speed of operation than the random access memory (14). The control circuit (24) includes a programmable counter (38) for generating a gating signal after counting a predetermined number of cycles of the reference clock signal and initiating a count cycle only after generation of the gating signal. Generation of the gating signal by the counter (38) causes a latch circuit (68) to become transparent during selected transitions of the CLK signal. The control circuit (24) also provides for overriding the programmable counter (38) via an event counter circuit (56) which is effective to generate the gating signal independently of the programmable counter (38) after counting a predetermined number of count cycles of the programmable counter (38). The control circuit (24) is thereby effective to reduce the rate of the CLK signal as input to the CPU (12) and to retain the reduced rate of the CLK signal for a time period sufficient for the CPU (12) to access peripheral devices of low operating speed. Thus, the CPU (12) is able to control peripheral devices that have different maximum rates of operation.