Selective Glitch Reduction Technique for Minimizing Peak Dynamic IR Drop

Abstract This paper proposes a glitch co mpensation technique which involves reducing glitch power in selected combinational cells to reduce peak current which contributes to dynamic voltage or IR drop. The proposed methodology can be seamlessly integrated to existing physical design flo ws. A glitch is an undesired transition that occurs before intended value in dig ital circuits. A glitch occurs in CMOS circu its when d ifferential delay at the inputs of a gate is greater than inertial delay, which results into increased gate switching and hence notable amount of power consumption. When such large nu mber of logic gates switch close to the same t ime they will contribute to power integrity challenge called pe ak dynamic IR drop. The glitch power is becoming more pro minent in lower technology nodes. Introduction of buffers at the input of the Logic gate may reduce glitches, but it results into large area overhead and dynamic power. In the proposed methodology we are using transmission gate as a compensation circuit to reduce extra leakage and dynamic power. A flo w is proposed for charactering the pass transistor logic to cater different delay values. The proposed methodology has been validated on a plac e and routed Multiply Accumulate (MA C) layout imp lemented using Synopsys SAED 9 0n m Generic library. Experimental results show 12% to 50% reduction in top 10 peak transient IR drop numbers with just 12% g litch power reduction in selected combinational cell instances. When compared to traditional on-chip decoupling capacitor (Decap) cells insertion method the proposed technique could reduce the peak IR drop numbers by the same amount with just 5% increase in total core capacitance

Gate-level timing analysis and waveform evaluation

Static timing analysis (STA) is an integral part of modern VLSI chip design. Table lookup based methods are widely used in current industry due to its fast runtime and mature algorithms. Conventional STA algorithms based on table-lookup methods are developed under many assumptions in timing analysis; however, most of those assumptions, such as that input signals and output signals can be accurately modeled as ramp waveforms, are no longer satisfactory to meet the increasing demand of accuracy for new technologies. In this dissertation, we discuss several crucial issues that conventional STA has not taken into consideration, and propose new methods to handle these issues and show that new methods produce accurate results. In logic circuits, gates may have multiple inputs and signals can arrive at these inputs at different times and with different waveforms. Different arrival times and waveforms of signals can cause very different responses. However, multiple-input transition effects are totally overlooked by current STA tools. Using a conventional single-input transition model when multiple-input transition happens can cause significant estimation errors in timing analysis. Previous works on this issue focus on developing a complicated gate model to simulate the behavior of logic gates. These methods have high computational cost and have to make significant changes to the prevailing STA tools, and are thus not feasible in practice. This dissertation proposes a simplified gate model, uses transistor connection structures to capture the behavior of multiple-input transitions and requires no change to the current STA tools. Another issue with table lookup based methods is that the load of each gate in technology libraries is modeled as a single lumped capacitor. But in the real circuit, the Abstract 2 gate connects to its subsequent gates via metal wires. As the feature size of integrated circuit scales down, the interconnection cannot be seen as a simple capacitor since the resistive shielding effect will largely affect the equivalent capacitance seen from the gate. As the interconnection has numerous structures, tabulating the timing data for various interconnection structures is not feasible. In this dissertation, by using the concept of equivalent admittance, we reduce an arbitrary interconnection structure into an equivalent π-model RC circuit. Many previous works have mapped the π-model to an effective capacitor, which makes the table lookup based methods useful again. However, a capacitor cannot be equivalent to a π-model circuit, and will thus result in significant inaccuracy in waveform evaluation. In order to obtain an accurate waveform at gate output, a piecewise waveform evaluation method is proposed in this dissertation. Each part of the piecewise waveform is evaluated according to the gate characteristic and load structures. Another contribution of this dissertation research is a proposed equivalent waveform search method. The signal waveforms can be very complicated in the real circuits because of noises, race hazards, etc. The conventional STA only uses one attribute (i.e., transition time) to describe the waveform shape which can cause significant estimation errors. Our approach is to develop heuristic search functions to find equivalent ramps to approximate input waveforms. Here the transition time of a final ramp can be completely different from that of the original waveform, but we can get higher accuracy on output arrival time and transition time. All of the methods mentioned in this dissertation require no changes to the prevailing STA tools, and have been verified across different process technologies