Timing characterization of clock buffers for clock tree synthesis

Abstract

It is formidable to embed iterative simulations into the clock tree synthesis process to verify the skew and slew constraints. Instead, accurate and simple timing models for clock buffers are traditionally used so as to perform clock tree synthesis with sufficient accuracy. Two-pole RC and/or piecewise linear models accurately models the gate delay without a waveform dependency for a wide range of waveform properties. However, they unnecessarily complicate the problem for the time modeling of clock buffers where, unlike logic gates, the input and output waveform properties are similar. Look-up table-based approaches are traditionally used in order to obtain the clock buffer timing with inputs being the input slew and the output capacitance. However, the effective capacitance estimation of the highly resistive wires of sub-45nm technologies is a challenge, making it hard to identify the output capacitance. Also, the multiple or dynamically-scaled voltage levels of the current designs necessitate a costly LUT-based pre-characterization process. In this work, a timing estimation scheme for clock buffers is proposed which models both the delay and the slew as linear equations, bypassing the costly LUT characterization process. The experimental results performed with SAED 32nm buffer library show that the proposed timing model can achieve a maximum absolute value error of ≈5ps to ≈10ps for the buffer timing compared to SPICE simulations. Furthermore, the proposed timing model provides an error from 0.2% to 4.6% at different timing constraints and operating voltage levels, when used for insertion delay computation.