An efficient transistor optimizer for custom circuits

This paper describes a combination transistor sizing/layout compaction tool used to synthesize high performance CMOS circuits. This optimization tool is part of a large integrated layout system, called Tailor. Given any CMOS circuit layout, Tailor's transistor size optimizer will simultaneously adjust transistor sizes and compact the layout so that the minimum required area (cell pitch) for a specified upper bound on circuit delay is achieved. All delay paths are considered by modeling circuit delay with a logic independent delay graph. Tailor's optimizer globally optimizes circuit area (in one dimension) and delay by use of compaction and nonlinear programming algorithms. The optimizer does not yet optimize in two dimensions simultaneously or optimize hierarchical circuits. Results for a few optimized CMOS circuits are presented.

We describe a procedure based on an existing static timing analysis tool for selecting path delay faults to target during test generation. The use of an existing static timing analysis tool ensures that a state-of-the-art process can be used for estimating path delays. However, static timing analysis, by itself, can be inaccurate as it does not take into consideration conditions that are necessary for detecting path delay faults. In the proposed method, these conditions are captured as what are called input necessary assignments, which static timing analysis tools are able to use. By providing the static timing analysis process with the input necessary assignments for a selected path, the static timing analysis process can estimate the delay of the path more accurately. It can also identify additional paths whose delays are at least as high as those of the selected paths. Thus, feeding back the input necessary assignments to the static timing analysis process enhances the correlation between static timing analysis and actual timing of tests on silicon. The result is a set of potentially detectable path delay faults associated with critical paths based on more accurate estimates of the path delays that can be exhibited by a test set, compared with the set that would be obtained by static timing analysis alone.

In this article, we present a critical path selection method that efficiently finds true (sensitizable) critical paths of a circuit in the presence of process variations. The method, which is based on the viability analysis, tries to select the least number of true critical paths that cover all of circuit critical gates. Critical gates are those that make a path critical with a probability higher than a predefined threshold value. Selecting fewer critical paths leads to less computation time for the algorithm and shorter test time of fabricated chips. For this purpose, an efficient Statistical Static Timing Analysis– (SSTA) based technique is suggested. This technique tries to find circuit-critical gates whose process parameter variations cover a major part of the process space. Improving the process space coverage using fewer paths is achieved by considering both spatial (proximity of gates) and structural (having common gates) correlations in the analysis of choosing the critical paths. In the selection process, paths with low similarities in their characteristics are preferred. In addition, only true paths whose delays affect the maximum delay of the circuit are included. The selected paths can be used in the test process of the fabricated chips to determine if the chip meets its timing requirements. Also, a modified viability analysis that incorporates statistical computations is used in the SSTA. The efficacy of the proposed method is evaluated by comparing its results for combinational and sequential ISCAS benchmarks with those obtained by exhaustive search. Results indicate although, on average, only 4.38% of all the critical paths found by the exhaustive search are selected by the proposed method, the maximum probability of criticality for the paths that are not considered in our method is, on average, less than 4%.

One of the critical issues in MTCMOS design is how to estimate a circuit delay quickly. In this paper, we propose a delay modeling and static timing analysis (STA) methodology targeting at MTCMOS circuits. In the proposed method, we prepare a delay look-up table (LUT) consisting of the input slew, the output load capacitance, the virtual ground length, and a power-switch size. Using this LUT, we compute a circuit delay for each logic cell by applying the linear interpolation. Experimental results show that the proposed methodology enables to estimate the critical path delay in a good accuracy.

This work addresses the problem of improving an nMOS PLA's speed and power consumption through modifications to the transistor sizes in the PLA. A simplified model of gate delay (lumped RC model) is used that allows rapid estimation of delays thus allowing interactive computation of optimal transistor sizes. Algorithms are presented for: a) minimization of delay through the PLA b) minimization of power consumption subject to a bound on maximum delay. Both these minima are computed subject to bounds on the transistor sizes. A prototype electrical optimization tool was compared to the PLA generation tools in the Berkeley CAD tools package (eqntott and tpla). The maximum delay through a PLA can often be reduced by a factor of 2, and power consumption along the critical paths can be reduced by 10 - 30% without increasing maximum delay.

A new field programmable gate array (FPGA) design algorithm, Maple-opt, is proposed for technology mapping, placement, and global routing subject to a given upper bound of critical signal path delay. The basic procedure of Maple-opt is viewed as top-down hierarchical bipartition of a layout region. In each bipartitioning step, technology mapping onto logic blocks of FPGAs, their placement, and global routing are determined simultaneously, which leads to a more congestion-balanced layout for routing. In addition, Maple-opt is capable of estimating a lower bound of the delay for a constrained path and of extracting critical paths based on the difference between the lower bounds and given constraint values in each bipartitioning step. Two delay-reduction procedures for the critical paths are applied; routing delay reduction and logic-block delay reduction. The routing delay reduction is done by assigning each constrained path to a single subregion when bipartitioning a region. The logic-block delay reduction is done by mapping each constrained path onto a smaller number of logic blocks. Experimental results for benchmark circuits demonstrate that Maple-opt reduces the maximum number of tracks per channel by a maximum of 38% compared with existing algorithms while satisfying almost all the path delay constraints.

With the aggressive scaling of the CMOS technology parametric variation of the transistor threshold voltage causes significant spread in the circuit delay as well as leakage spectrum. Consequently, speed binning of the high performance VLSI chips is essential and it costs significant amount of test application time. Further, the knowledge of the actual delay in the critical path of the circuit enables efficient use of typical low power methodologies e.g., voltage scaling, adaptive body biasing etc. In this paper, the authors have proposed a novel on-chip, low overhead and process tolerant delay measurement circuit which can estimate the critical path delay in a single clock period. This has the advantage of efficient on-chip speed binning.

Speed of operation depends on the longest critical paths in the multi-bit adders and also the MOSFET transistor gain which in turn depends on transistor size. The power consumption in MOSFET is depends on the switching frequency, sub threshold leakage and switching time. In this paper, author proposed the speed and area efficient transistor base adder using static CMOS pass transistor logic, and shortened the longest critical path to decrease the total critical path delay. The design simulation on microwind layout tool calculates the worst-case delay in nanosecond and total power consumption in microwatt range.

The effect of variations (process, voltage, temperature, and crosstalk) on circuit delay is increasing with technology scaling. As a result, the timing uncertainty of clock signal is increasing. In this paper, an efficient methodology for clock timing uncertainty- and skew-aware clock tree synthesis and analysis is proposed. We first present a statistical and less pessimistic methodology (as compared to the traditional static timing analysis (STA) methodology) for computing the clock timing uncertainty under the impact of parameter variations (process, voltage, temperature, and crosstalk). We also devise a technique to synthesize a clock tree that has zero skew and on which uncertainty can be computed efficiently. Finally, using the proposed uncertainty analysis algorithm, a post-processing scheme to reanalyze the critical paths reported by traditional STA is presented. We applied our zero-skew tree synthesis algorithm on a real industrial design. With our analysis methodology, the worst-case timing uncertainty on this tree was reduced from 388ps (used by traditional STA tools) to 63ps

Given the sensitivity of circuit delay to supply and ground voltage values, static timing analysis (STA) must take into account supply voltage variations. Existing STA techniques allow one to verify the timing at different process corners which effectively only considers cases where all the supplies are low or all are high. Cases of mismatch between the supplies of driver and load are not considered. In practice, supply voltages are neither totally independent nor totally dependent. In this work, we consider the supply and ground nodes of a logic gate to be either totally independent variables, or to be directly tied or connected to those of some other gate(s) in the circuit. We also assume that the exact supply voltage values are not known exactly, but that only upper/lower bounds on them are known. In this framework, we propose new timing models for logic gates and identify the worst-case voltage configurations for individual gates and for simple paths. We then give an STA technique that provides the worst-case circuit delay taking supply variations into account.

The authors describe a performance-driven module generator (perflex) for efficient generation of fast static combinational CMOS circuit modules. A new flexible CMOS layout style provides the foundation for implementing fast circuits. Timing optimization is performed via transistor sizing, transistor reordering, and the reduction of wiring capacitance on critical paths, all of which are performed in close interaction with the simulated-annealing layout process. The main contributions are the new layout style, which is well suited for the synthesis of high-speed circuits, and the way in which well-known performance optimization techniques are embedded into the layout generation process to achieve fine-grain optimization. >

In this paper, a novel low-power pulse-triggered flip-flop (FF) design is presented. First, the pulse generation control logic, an and function, is removed from the critical path to facilitate a faster discharge operation. A simple two-transistor and gate design is used to reduce the circuit complexity. Second, a conditional pulse-enhancement technique is devised to speed up the discharge along the critical path only when needed. As a result, transistor sizes in delay inverter and pulse-generation circuit can be reduced for power saving. Various postlayout simulation results based on UMC CMOS 90-nm technology reveal that the proposed design features the best power-delay-product performance in seven FF designs under comparison. Its maximum power saving against rival designs is up to 38.4%. Compared with the conventional transmission gate-based FF design, the average leakage power consumption is also reduced by a factor of 3.52.

As semiconductor technologies are aggressively advanced, the problem of parameter variations is emerging. Process variations in transistors affect circuit delay, resulting in serious yield loss. Considering the situations, variationaware designs for yield enhancement interest researchers. This paper investigates to exploit the statistical features in circuit delay and to cascade dependent instructions for reducing variations. From statistical static timing analysis in circuit level and performance evaluation in processor level, this paper tries to unveil how efficiently instruction cascading improves performance yield of processors. Cascading instructions increases logic depth and decreases the standard deviation of the circuit delay. That might improve performance yield of microprocessors. Unfortunately, however, it is found that variability reduction in the circuit level does not always mean yield enhancement in the microarchitecture level.

FPGA packages have maximum size constraints much larger than the number of IO pins. The resulting IO bottleneck during circuit partitioning means more required packages and more ordinary signal wires crossing between the packages. This cuts more critical timing paths between packages and drastically decreases the circuit operational frequency. In this paper, a bottom-up circuit partitioning method with cone structures is presented. The solution can minimize the delay of critical paths that slow down the circuit without changing the number of partitioned packages.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ensuring correct functionality is the basic objective of any digital design and this can be done only if the circuit obeys all the timing constraints. Static Timing Analysis is the procedure performed to calculate the setup and hold time of any digital circuit and to check for any setup and hold time violations. This paper presents the methodology to perform setup and hold time analysis for different designs of Master Slave D Flip-Flop using 18 nm FinFET technology. FinFET technology was chosen since it is the prevalent technology in the industry. Implementation of D Flip-Flop design, suitable test circuit and timing analysis is performed on the Cadence virtuoso analog design environment tool. Using appropriate excitations, parametric analysis is carried out to obtain the setup and hold time values for various designs of D Flip-Flop including the proposed design. From the results, it was observed that the setup time and hold time varied with different clock edge rates. For the edge rates of 0.01, 0.05, and 0.1 ns, it was observed that the proposed design gave setup times of 54.9, 61.2, and 69.8 ps and hold times of −30.7, −43.6 and, −48.8 ps, respectively. On comparing all the results, it was observed that the GDI MUX and pass transistor-based designs gave a good trade-off between transistor count, setup time and hold time. For the proposed design, although the setup times were slightly on the higher side, the hold time values were more negative which is always preferred.