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Timing uncertainty in microprocessors is comprised of several sources including PLL jitter, clock distribution skew and jitter, across chip device variations, and power supply noise. The on-chip measurement macro called SKITTER (SKew+jITTER) was designed to measure timing uncertainty from all combined sources by measuring the number of logic stages that complete in a cycle. This measure of completed delay stages has proven to be a very sensitive monitor of power supply noise, which has emerged as a dominant component of timing uncertainty. This paper describes the Skitter measurement experiences of several IBM microprocessors including PPC970MP, XBOX360 TM , CELL Broadband Engine TM , and POWER6 TM microprocessors running different workloads.

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