Slew-aware clock tree design for reliable subthreshold circuits

摘要

In the paper, we analyze the effect of clock slew in subthreshold circuits. Specifically, we address the issue that variations in clock slew at the register control can cause serious timing violations. We show that clock slew variations can cause latch timing metrics such as setup, hold and clock-to-q times to deviate by 90% from the design goals. Based on these observations, we recognize the importance of clock slew control in subthreshold circuits. We propose a systematic approach to design the clock tree for subthreshold circuits to reduce the clock slew variations while minimizing the power dissipation in the tree. We show that a tighter nodal capacitance control is necessary to control the slew in a subthreshold clock tree, which can increase the power dissipation. Recognizing that the wire resistances have a negligible effect in subthreshold circuits, we show proper wire sizing is necessary to reduce the clock power. Finally, we propose a dynamic nodal capacitance control technique that allows larger slew at the earlier nets of the tree while controlling it more aggressively near the sink nodes. The combined approach, including the wire sizing and dynamic nodal capacitance control, can achieve better slew control (and better timing control) at lower power in subthreshold circuits.