5.3 Wide-frequency-range resonant clock with on-the-fly mode changing for the POWER8TM microprocessor

A resonant-clock design for the IBM POWER8 processor core was implemented with 2 resonant modes (and a non-resonant mode), saving clock power over a wide frequency range from 2.5GHz to more than 5GHz. The POWER8 microprocessor is composed of 12 chiplets, each containing a single resonant clock grid for one core and its L2 cache, and a half-frequency, non-resonant clock grid for the L3 cache. The clock grids drive the local clock buffers (LCBs) that in turn drive the latches. The LCBs are gated off to measure the global clock power from the PLL to the LCBs. The resonant core communicates synchronously with the L3, requiring low skew between the domains. The chip was designed in a 22nm SOI process, including two ultra-thick-metal (UTM) layers (3 microns thick) for power distribution, I/O, all long global clock wires, and the resonant clock inductors. The UTM technology reduces wire resistance and simplifies inductor design, but requires accurate transmission line modeling and special routing.