Chun Po Huang

Effective and Efficient Approach for Power Reduction by Using Multi-Bit Flip-Flops

Power has become a burning issue in modern VLSI design. In modern integrated circuits, the power consumed by clocking gradually takes a dominant part. Given a design, we can reduce its power consumption by replacing some flip-flops with fewer multi-bit flip-flops. However, this procedure may affect the performance of the original circuit. Hence, the flip-flop replacement without timing and placement capacity constraints violation becomes a quite complex problem. To deal with the difficulty efficiently, we have proposed several techniques. First, we perform a co-ordinate transformation to identify those flip-flops that can be merged and their legal regions. Besides, we show how to build a combination table to enumerate possible combinations of flip-flops provided by a library. Finally, we use a hierarchical way to merge flip-flops. Besides power reduction, the objective of minimizing the total wirelength is also considered. The time complexity of our algorithm is Θ(n1.12) less than the empirical complexity of Θ(n2). According to the experimental results, our algorithm significantly reduces clock power by 20-30% and the running time is very short. In the largest test case, which contains 1 700 000 flip-flops, our algorithm only takes about 5 min to replace flip-flops and the power reduction can achieve 21%.

Common-centroid capacitor placement considering systematic and random mismatches in analog integrated circuits

One of the most important issues during the analog layout phase is to achieve accurate capacitance ratios. However, systematic and random mismatches will affect the accuracy of the capacitance ratios. A common-centroid placement is helpful to reduce the systematic mismatch, but it still needs the property of high dispersion to reduce the random mismatch [10]. To deal with this problem, we propose a simulated annealing [15] based approach to construct a common-centroid placement which exhibits the highest possible degree of dispersion. To facilitate this framework, we first propose the pair-sequence representation to represent a common-centroid placement. Then, we present three operations to perturb the representation, which can increase the degree of dispersion without breaking the common-centroid constraint in the resulting placement. Finally, to enhance the efficiency of our simulated annealing based approach, we propose three techniques to speed up our program. The experimental results show that our placements can simultaneously achieve smaller oxide-gradient-induced mismatch and larger overall correlation coefficients (i.e., higher degree of dispersion) than [10] in all test cases. Besides, our program can run much faster than [10] in larger benchmarks.

Mismatch-Aware Common-Centroid Placement for Arbitrary-Ratio Capacitor Arrays Considering Dummy Capacitors

Switched capacitors are commonly used in analog circuits to increase the accuracy of analog signal processing and lower power consumption. To take full advantage of switched capacitors, it is very important to achieve accurate capacitance ratios in the layout of the capacitor arrays, which are affected by systematic and random mismatches. A good capacitor placement should have a common-centroid structure with the highest possible degree of dispersion to mitigate mismatches. Several dummy units should be inserted to make the placement shape more square and compact. This paper proposes a simulated-annealing-based approach for mismatch-aware common-centroid placement under the above constraints. A pair-sequence representation is used to record a placement, and a couple of associated operations are developed to find better solutions. The experimental results show that the proposed placements achieve smaller oxide-gradient-induced mismatch and larger overall correlation coefficients (i.e., higher degree of dispersion) than those of previous works.

Performance-driven analog placement considering boundary constraint

To reduce parasitic mismatches in analog design, we usually care about the property of symmetric placement for symmetry groups, which would form several symmetry islands in a chip. However, routing is greatly affected by placement results. If modules with input or output ports are placed arbitrarily in a symmetry island, the routing wires, which connect these modules with other modules outside the island, may induce unwanted parasitics coupling to signals, and thus circuit performance is deteriorated. This phenomenon can not be identified by a cost function, which only considers placement area and total wire length. Therefore, we would like to introduce the necessity of considering boundary constraint for the modules with input or output ports in symmetry islands. Based on ASF-B∗ tree [3], we explore the feasible conditions for 1D and 2D symmetry islands to meet this constraint. Further, a procedure is presented to maintain the feasibility for each ASF-B∗ tree after perturbation. Experimental results show that our approach guarantees the boundary property for the modules with input or output ports in symmetry islands.

A 10-bit 50-MS/s SAR ADC with techniques for relaxing the requirement on driving capability of reference voltage buffers

A high speed successive approximation (SAR) ADC requires reference voltage buffers with high driving capability. Moreover, the power consumption of the reference buffers is usually several times larger than that of the SAR ADC itself. Three techniques are adopted to mitigate the requirement on driving capability of reference voltage buffers for SAR ADCs. A 10b 50MS/s ADC based on the proposed techniques is presented. The prototype ADC was fabricated in 40nm LP 1P7M CMOS technology. It consumes 0.47 mW at 50 MS/s from 1.1V supply voltage and achieves ENOB of 9.18-bit and figure of merit (FoM) of 16 fJ/conversion-step. The active area is 0.0114 mm2.

A 10b 200MS/s 0.82mW SAR ADC in 40nm CMOS

This paper reports a successive-approximation analog-to-digital converter (ADC) which combines the bypass window and direct switching technique to tolerate the incomplete settling error and reduce the control logic delay. A small unit capacitor cell reduces the power consumption and settling time. The 10-bit prototype is fabricated in a 40nm CMOS process. At 200 MS/s and 0.9-V supply, this ADC consumes 0.82 mW and achieves an SNDR of 57.16 dB, resulting in an FOM of 13.9 fJ/Conversion-step.

Routing-aware placement algorithms for modern analog integrated circuits

Due to continuous scaling in modern process technologies, more and more analog and mixed-signal circuits are integrated with digital units to realize system-on-a-chip. Since analog designs generally need comprehensive analysis to ensure circuit performances, it usually requires more development time to implement analog blocks than digital ones. Because of the difficulties in analog designs and the lack of support by design automation tools, analog circuits become bottleneck in the chip design flow. Although some studies have been proposed to consider placement of analog circuits recently, they usually ignore routing problems. In this paper, some issues about placement and routing for analog circuits are discussed, which include prevention of noisy signals in symmetry islands, congestion elimination in practical placement, and routing area reduction in capacitor arrays. By considering placement and routing at the same time, the routing-induced problems which may cause unwanted effects to deteriorate analog layout quality can be prevented.

A Systematic Design Methodology of Asynchronous SAR ADCs

Successive approximation register (SAR) analog-to-digital converters (ADCs) are widely used in biomedical and portable/wearable electronic systems due to their excellent power efficiency. However, both the design and the optimization of high-performance SAR ADCs are time consuming, even for well-experienced circuit designers. For system designers, it is also hard to quickly evaluate the feasibility of a given specification in a process node. This paper presents a systematic sizing procedure for asynchronous SAR ADCs based on design considerations. A sizing tool based on the proposed design procedure is also implemented, the sizing results of which are highly competitive in comparison with other state-of-the-art manual works. Moreover, the sizing time is relatively short due to the efficient and effective search algorithms employed. In addition to the simulation results, two silicon proofs with different specifications and process nodes are provided to demonstrate the feasibility of this design methodology.

A pipeline ADC with latched-based ring amplifiers

In comparison with conventional operational amplifier, ring amplifier can achieve better power efficiency for switched capacitor circuits. However, the cascade-inverter architecture of ring amplifier may suffer from undesirable oscillation which has a great impact on transient stability. This paper presents a latched-based ring amplifier which is capable of decreasing the probability of oscillation. Besides, two auto-zero schemes are employed in different pipelined stages to reduce the common-mode voltage offset and to increase the stability. The prototype ADC was fabricated in a 90-nm CMOS technology. The measured SNDR and SFDR are 52.06 dB and 63. 15 dB, respectively, for a Nyquist frequency input sampled at 35 MS/s, and the ADC consumes 3.65 mW.

Analysis of Nonideal Behaviors Based on INL/DNL Plots for SAR ADCs

This paper presents a comprehensive investigation of several important error sources for the successive-approximation register (SAR) analog-to-digital converters (ADCs). The error sources that we discuss in this paper include the dynamic comparator offset, the dynamic gain error of digital-to-analog converter (DAC), the capacitor mismatch of capacitive DAC, the incomplete settling of DAC, the undershoot of reference voltage, and the input signal coupling. The integral/differential nonlinearities (INL/DNL) of SAR ADCs that are resulted from these error sources are analyzed and addressed. A diagnostic procedure is presented to identify the possible error sources based on the INL/DNL plots. In addition, design suggestions for overcoming these problems are also offered and recommended in this paper.