Mixing Drivers in Clock-Tree for Power Supply Noise Reduction

In today's process technologies, power supply noise may cause serious clock jitter and circuit malfunction. Noise occurs by the fast and simultaneous voltage switching. A primary contributor to the noise is the clock-tree and the underlying sequential circuits that switch simultaneously, thus causing high current peaks. This work proposes to spread the switching of the clock-tree drivers, while maintaining low skew at the sinks of the tree, where the clocked circuits are connected. Driver switching characterization has been used for fast computation of peak currents, delays and slopes, integrated in a two-phase algorithm. It first constructs the clock-tree in a top-down traversal, employing a mix of high threshold voltage (HVT) and weak low threshold voltage (LVT) clock-drivers. A bottom-up delay correction phase then takes place, aiming at clock skew minimization. The algorithm was implemented in 40 nanometers TSMC process technology, achieving 35% to 70% clock-tree peak current reduction, translated to similar reduction in power supply noise. The proposed method can easily be combined with other existing methods to further reduce the noise.