AutoRex: An automated post-silicon clock tuning tool

Post-silicon clock-tuning is a technique used as part of speed-debug efforts to increase the allowable clock frequency of a chip. These days, it is not uncommon for high-end microprocessors to have cores containing a few thousand clock-tuning elements (i.e., variable-delay buffers). Each such buffer can be assigned to one of several possible discrete delay values, as part of the post-silicon speed debugging process. With the proper mix of assignments, many chips that initially could not meet targeted speed requirements, can now run within specification. With thousands of tunable buffers available on chip, the possible combination of assignments to the delay values is quite large. In addition, process variation causes the same design, once fabricated into silicon, to have different critical paths across different chips. Thus a specific buffer-delay assignment that most improves clock frequency for some chips may not be optimal for all chips. In this paper, we propose a tool we call AutoRex, that produces clock-tuning assignments automatically. AutoRex operates by taking data from a volume experiment across multiple process corners and analyzes this data using Satisfiability Modulo Theory (SMT) solvers to create a single “recipe” for delay buffer assignments such that the clock frequency of the chip is improved as much as possible over the entire sample of chips. Our results show up to a 9% improvement in frequency using AutoRex.