Clock disabling for power management has been implemented in some micro-controllers, but the wake-up time of Xtal/PLL-based systems is incompatible with fast interrupt response. On the other hand, hardwired on-chip clocking has been used for dedicated circuits. We illustrate the design issues of a general-purpose micro-controller core with a programmable on-chip fully-digital clock generator. The CPU is compatible with the PIC16C57 instruction set and supports software-controlled clocking modes - ranging from 44 MHz up to 124 MHz; on-line self-tuning of the maximum full-speed frequency in case of peak-performance requirements; ultra-fast wake-up even with totally disabled clock generator - namely 8.6 ns.