Investigation of on-chip PLL irregularities under stress conditions - case study

In modern high performance VLSI design, on-chip phase locked loop (PLL) performance degradation due to intensive core switching activities is becoming an influential factor. Under certain borderline conditions, the PLL may become unstable. The analysis herein describes PLL irregularities under marginal mode, frequency and voltage conditions combined with intensive core operations. After lengthy analysis that included step-by-step elimination of all noise sources, the cause of instability was explained by coupling between a voltage spike on core power supply line and the internal control signal of the voltage controlled oscillator of the PLL through the chip substrate. The solution to the problem was suggested by changing the PLL dynamic characteristics. Through this investigation we studied the noise crosstalk issue in mixed mode (analog and digital) systems and also the PLL dynamics under stress conditions, which demonstrates the complexity of PLL analysis in a system-on-chip environment.