저전력 고성능 디지털 시스템을 위한 고신뢰도의 클럭 네트워크 설계 방법론

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2015. 8. 김태환.오늘날의 회로 설계에서 공정변이가 회로 클럭의 타이밍의 변이에 미치는 영향은 매우 커짐에 따라, 전통적으로 사용되던 클럭 트리 구조를 기반으로 한 클럭 네트워크를 사용하는 것은 한계에 부딪히게 되었고, 이를 극복하기 위한 여러가지 기술들이 제안되었다. 본 논문에서는 변이에 강한 클럭 네트워크를 설계하기 위해, 연구 및 사용되고 있는 세 가지 기술에 대해 소개하고, 이들을 개선한 연구들을 제안한다. 첫째로, 이 논문에서는 클럭의 타이밍 문제를 회로 제작 이후 단계에서 조정할 수 있는 포스트 실리콘 조정 클럭 버퍼를 배치하는 문제에 대해 서술한다. 포스트 실리콘 조정 버퍼는 클럭의 지연시간을 회로가 제작된 이후의 단계에서 조정하 여 클럭의 타이밍 문제를 해결할 수 있지만, 버퍼 자체의 크기 때문에 최소한의 개수만 가장 효율적인 위치에 배치해야 하는 문제가 있다. 본 논문에서는 이전의 연구가 회로의 수율을 계산할 때 시간이 많이 걸리는 몬테-카를로 시뮬레이션을 사용하기 때문에 탐색 가능한 포스트 실리콘 조정 버퍼의 배치가 제한되는 문제가 있음을 지적한 후, 기존에 제안되었던 그래프 기반 회로 수율 계산 기법을 사용하여 효율적인 포스트 실리콘 조정 버퍼 배치를 찾을 수 있는 점진적이고 체계적인 방법을 제시한다. 다음은 클럭 시차 스케쥴링 방법에 대한 연구를 서술한다. 최근의 연구에서 제안되었던, 플립-플롭의 클럭에서 출력까지의 딜레이가 클럭의 준비시간과 유지시간에 의존한다는 유연한 플립-플롭 타이밍 모델 연구는 기존의 플립-플롭의 타이밍 특성들이 고정된 값이라는 가정에 기반한 정적 타이밍 분석의 정확성 문제를 해결할 수 있는 중요한 연구이다. 본 논문에서는 새로운 모델을 고려하여, 이전에 고전적인 플립-플롭 타이밍 특성 모델을 기반으로 진행되었던 클럭 시차 스케쥴링의 최적화 문제를 유연한 플립-플롭 타이밍 모델을 고려하여 해결하였다. 본 연구에서는 주어진 회로의 준비시간과 유지시간의 여유시간을 반복적이고 체계적으로 최대화하여 문제를 해결하였다. 마지막으로 클럭 스파인 네트워크의 합성을 자동화하는 문제에 대해 서술한다. 전통적인 클럭 트리 구조가 공정변이 문제를 해결하지 못했기 때문에 클럭 메쉬를 포함하는 다양한 대안적 구조가 제안되었다. 클럭 메쉬의 경우 공정변이에 의한 클럭 시차를 줄일 수 있었지만 이를 위해 와이어나 버퍼 등의 자원을 많이 소모하는 문제를 가지고 있다. 두 구조의 중간적 구조에는 클럭 트리의 노드를 연결하는 크로스 링크를 삽입하는 구조와 클럭 스파인 구조가 있다. 클럭 트리에 점진적인 수정을 가하여 만드는 크로스 링크와 달리, 클럭 스파인 구조는 트리나 이후에 제안된 메쉬와는 완전히 별개의 구조로, 이를 합성하는 방법도 매우 다르다. 그렇기 때문에 클럭 스파인을 합성하는 알고리즘은 필수적이라고 할 수 있으나, 합성 방법론이나 이를 자동화하는 방법에 관한 연구는 아직 없다. 본 논문에서는 우선, 클럭-게이팅을 지원하는 클럭 스파인을 주어진 클럭 시차 및 클럭 슬루 조건을 만족하면서 자원 및 전력 소모량을 최소화하는 문제에 대해 서술한다. 그리고, 회로에서 주어진 플립-플롭들을 클럭-게이팅 조건에서의 연관성을 고려하고 조직화하여 클럭 스파인을 삽입한 후, 클럭 시차 및 슬루 조건을 고려하여 버퍼를 삽입하는 알고리즘을 제안한다. 요약하면, 본 논문에서는 클럭의 타이밍 문제를 해결하기 위해 포스트-실리콘 조정 클럭 버퍼를 사용하는 테크닉과 클럭 시차 스케쥴링을 유연한 플립-플롭 타이밍 모델에서 적용하는 테크닉을 제시하고, 클럭의 타이밍 문제와 전력 소모 문제를 한번에 해결하기 위한 새로운 클럭 스파인 네트워크를 합성하는 자동화 알고리즘을 제시한다.As the process variation is dominating to cause the clock timing variation among chips to be much large, conventional clock tree based clock network is not able to guarantee the timing constraint of a digital system. To overcome the limitations of traditional clock design techniques, various techniques have been studied. This dissertation addresses three techniques that have been widely used for designing robust clock network and proposes developed methods. First, it is widely accepted that post-silicon tunable (PST) clock buffers can effectively resolve the clock timing violation. Since PST buffers, which can reset the clock delay to flip-flops after the chip is manufactured, impose a non-trivial implementation area and control circuitry, it is very important to minimally allocate PST buffers while satisfying the chip yield constraint. In this dissertation, we (1) develop a graph-based chip yield computation technique which can update yields very efficiently and accurately for incremental PST buffer allocation, based on which we (2) propose a systematic (bottom-up and top-down with refinement) PST buffer allocation algorithm that is able to fully explore the design space of PST buffer allocation. Second, clock skew scheduling is one of the essential steps that must be carefully performed during the design process. This dissertation addresses the clock skew optimization problem integrated with the consideration of the interdependent relation between the setup and hold skews, and clk-to-Q delay of flip-flops, so that the time margin is more accurately and reliably set aside over that of the previous methods, which have never taken the integrated problem into account. Precisely, based on an accurate flexible model of setup skew, hold skew, and clk-to-Q delay, we propose a stepwise clock skew scheduling technique in which at each iteration, the worst slack of setup and hold skews is systematically and incrementally relaxed to maximally extend the time margin. Lastly, clock tree with cross links and clock spine have an intermediate characteristics for skew tolerance and power consumption, compared to clock tree and clock mesh which are two extreme structures of clock network. Unlike the clock tree with links between clock nodes, which is a sort of an incremental modification of the structure of clock tree, clock spine network is a completely separated structure from the structures of tree and mesh. Consequently, it is necessary and essential to develop a synthesis algorithm for clock spines, which will be compatible to the existing synthesis algorithms of clock trees and clock meshes. To this end, this dissertation first addresses the problem of automating the synthesis of clock-gated clock spines with the objective of minimizing total clock power while meeting the clock skew and slew constraints. The key idea of our proposed synthesis algorithm is to identify and group the flip-flops with tight correlation of clock-gating operations together to form a spine while accurately predicting and maintaining clock skew and slew variations through the buffer insertion and stub allocation. In summary, this dissertation presents clock tuning techniques with consideration of post-silicon tuning, flexible flip-flop timing model, and clock-gated clock spine synthesis algorithm.Abstract i Chapter 1 INTRODUCTION 1 1.1 Clock Distribution Network . . . . . . . . . . . . . . . . . . . . . 1 1.2 Process Variation . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.3 Flexible Flip-flop Timing Model . . . . . . . . . . . . . . . . . . . 3 1.4 Clock Spine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.5 Contributions of This Dissertation . . . . . . . . . . . . . . . . . 6 Chapter 2 POST-SILICON TUNABLE CLOCK BUFFER ALLOCATION BASED ON FAST CHIP YIELD COMPUTATION 8 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2 Systematic Exploration of PST Buffer Allocation . . . . . . . . . 10 2.2.1 Observations . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.2.2 Problem Definition . . . . . . . . . . . . . . . . . . . . . . 15 2.2.3 Allocation Algorithm . . . . . . . . . . . . . . . . . . . . . 16 2.3 Fast Timing Yield Computation . . . . . . . . . . . . . . . . . . 17 2.3.1 Preliminaries . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.3.2 Incremental Yield Computation . . . . . . . . . . . . . . . 22 2.4 Experimental Result . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.5 PST Buffer Configuration Techniques . . . . . . . . . . . . . . . 31 2.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Chapter 3 POST-SILICON TUNING BASED ON FLEXIBLE FLIP-FLOP TIMING 34 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 3.2 Preliminary and Definitions . . . . . . . . . . . . . . . . . . . . . 40 3.2.1 Flexible Flip-Flop Timing Model . . . . . . . . . . . . . . 40 3.2.2 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 40 3.3 Motivational Examples . . . . . . . . . . . . . . . . . . . . . . . . 42 3.4 Clock Skew Scheduling for Slack Relaxation Based on Flexible Flip-Flop Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.4.1 Overall Flow . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.4.2 Finding Local Clock Skew Schedule . . . . . . . . . . . . 48 3.5 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . 51 3.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Chapter 4 SYNTHESIS FOR POWER-AWARE CLOCK SPINES 61 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.2 Preliminaries and Motivation . . . . . . . . . . . . . . . . . . . . 64 4.2.1 Clock Spine . . . . . . . . . . . . . . . . . . . . . . . . . . 64 4.2.2 Activity Patterns . . . . . . . . . . . . . . . . . . . . . . . 67 4.2.3 Power Computation . . . . . . . . . . . . . . . . . . . . . 67 4.3 Algorithm for Clock Spine Synthesis . . . . . . . . . . . . . . . . 68 4.3.1 Problem Definition . . . . . . . . . . . . . . . . . . . . . . 68 4.3.2 Power-Aware Sink Clustering . . . . . . . . . . . . . . . . 70 4.3.3 Spine Relaxation . . . . . . . . . . . . . . . . . . . . . . . 77 4.3.4 Spine Buffer Allocation . . . . . . . . . . . . . . . . . . . 80 4.3.5 Top-Level Tree Construction . . . . . . . . . . . . . . . . 86 4.4 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . 86 4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Chapter 5 CONCLUSION 95 5.1 Chapter 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 5.2 Chapter 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 5.3 Chapter 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Bibliography 97 초록 106Docto

Paleobiological Perspectives on Early Eukaryotic Evolution

Eukaryotic organisms radiated in Proterozoic oceans with oxygenated surface waters, but, commonly, anoxia at depth. Exceptionally preserved fossils of red algae favor crown group emergence more than 1200 million years ago, but older (up to 1600–1800 million years) microfossils could record stem group eukaryotes. Major eukaryotic diversification ∼800 million years ago is documented by the increase in the taxonomic richness of complex, organic-walled microfossils, including simple coenocytic and multicellular forms, as well as widespread tests comparable to those of extant testate amoebae and simple foraminiferans and diverse scales comparable to organic and siliceous scales formed today by protists in several clades. Mid-Neoproterozoic establishment or expansion of eukaryophagy provides a possible mechanism for accelerating eukaryotic diversification long after the origin of the domain. Protists continued to diversify along with animals in the more pervasively oxygenated oceans of the Phanerozoic Eon.Earth and Planetary SciencesOrganismic and Evolutionary Biolog

3D Computer Modeling Offers New Insights Into Diatom Ecology

Algae supply over half of the Earth’s global primary production and form the base of almost all aquatic food networks. Thus, changes in algal productivity or composition will induce profound shifts in many ecosystems. This research is guided by two questions. Herein I ask if 3D models of algae can be created accurately enough to use for research applications? If they can be accurately created, then how can these models be used to advance our understanding of functional trait evolution and paleoecology? Herein, I develop 3D computer models for estimating the volume of individual algae and their parts. I also examine pressures that influence algae biomass, resource requirements, and trait evolution. Further, I apply these to an annually resolved sediment records to reveal paleontological applications that can be used to reconstruct past ecosystems and evolutionary events. This dissertation provides the means improve historical ecological reconstructions, advance predictions of ecological changes that will occur under global climate change and allow for evolutionary cost benefit analysis of traits of microscopic organisms. For this study I exploit the sedimentary record from Herd Lake Idaho USA (44.089428, -114.173921). because it contains large (~0.5 cm) annual layers. These layers are extraordinarily abundant in the remains of the diatom Stephanodiscus niagarae. These qualities present an ideal setting to produce accurate 3D computer models of a diatom that influences a large portion of the nutrient cycles in an aquatic system. The abundance of individuals provides ample data and diversity for strong statistical interpretations of the diatom populations while the annual resolution of the sediment provides a means to compare the novel methods to weather and climate data that inform us of the ecological significance of these new methods. The bounds of this study are within the years 1927 – 2011 and pertain to aquatic environments on Earth. Although these works are constrained to the recent past and local ecosystems, future applications are bound only by the geological time frame in which algae have existed and are limited in space to wherever algae are found in the universe

Circadian Modulation of Neurons and Astrocytes Controls Synaptic Plasticity in Hippocampal Area CA1

Most animal species operate according to a 24-h period set by the suprachiasmatic nucleus (SCN) of the hypothalamus. The rhythmic activity of the SCN modulates hippocampal-dependent memory, but the molecular and cellular mechanisms that account for this effect remain largely unknown. Here, we identify cell-type-specific structural and functional changes that occur with circadian rhythmicity in neurons and astrocytes in hippocampal area CA1. Pyramidal neurons change the surface expression of NMDA receptors. Astrocytes change their proximity to synapses. Together, these phenomena alter glutamate clearance, receptor activation, and integration of temporally clustered excitatory synaptic inputs, ultimately shaping hippocampal-dependent learning in vivo. We identify corticosterone as a key contributor to changes in synaptic strength. These findings highlight important mechanisms through which neurons and astrocytes modify the molecular composition and structure of the synaptic environment, contribute to the local storage of information in the hippocampus, and alter the temporal dynamics of cognitive processing

Seven properties of self-organization in the human brain

The principle of self-organization has acquired a fundamental significance in the newly emerging field of computational philosophy. Self-organizing systems have been described in various domains in science and philosophy including physics, neuroscience, biology and medicine, ecology, and sociology. While system architecture and their general purpose may depend on domain-specific concepts and definitions, there are (at least) seven key properties of self-organization clearly identified in brain systems: 1) modular connectivity, 2) unsupervised learning, 3) adaptive ability, 4) functional resiliency, 5) functional plasticity, 6) from-local-to-global functional organization, and 7) dynamic system growth. These are defined here in the light of insight from neurobiology, cognitive neuroscience and Adaptive Resonance Theory (ART), and physics to show that self-organization achieves stability and functional plasticity while minimizing structural system complexity. A specific example informed by empirical research is discussed to illustrate how modularity, adaptive learning, and dynamic network growth enable stable yet plastic somatosensory representation for human grip force control. Implications for the design of “strong” artificial intelligence in robotics are brought forward

Clock Polarity Assignment Methodologies for Designing High-Performance and Robust Clock Trees

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2016. 8. 김태환.In modern synchronous circuits, the system relies on one single signal, namely, the clock signal. All data sampling of flip-flops rely on the timing of the clock signal. This makes clock trees, which deliver the clock signal to every clock sink in the whole system, one of the most active components on a chip, as it must switch without halting. Naturally, this makes clock trees a primary target of optimization for low power/high performance designs. First, bounded skew clock polarity assignment is explored. Buffers in the clock tree switch simultaneously as the clock signal switch, which causes power/ground supply voltage fluctuation. This phenomenon is referred to as clock noise and brings adverse effects on circuit robustness. Clock polarity assignment technique replaces some of the buffers in the clock trees with inverters. Since buffers draw larger current at the rising edge of the clock while inverters draw larger current at the falling edge, this technique can mitigate peak noise problem at the power/ground supply rails. Second, useful skew clock polarity assignment method is developed. Useful clock skew methodology allows consideration of individual clock skew restraints between each clock sinks, allowing further noise reduction by exploiting more time slack. Through experiments with ISPD 2010 clock network synthesis contest benchmark circuits, the results show that the proposed clock polarity algorithm is able to reduce the peak noise caused by clock buffers by 10.9% further over that of the global skew bound constrained polarity assignment while satisfying all setup and hold time constraints. Lastly, as multi-corner multi-mode (MCMM) design methodologies, process variations and clock gating techniques are becoming common place in advanced technology nodes, clock polarity assignment methods that mitigate these problems are devised. Experimental results indicate that the proposed methods successfully satisfy required design constraints imposed by such variations. In summary, this dissertation presents clock polarity assignments that considers useful clock skew, delay variations, MCMM design methodologies and clock gating techniques.Chapter 1 Introduction 1 1.1 Clock Trees 1 1.2 Simultaneous Switching Noise 3 1.3 Clock Polarity Assignment Technique 4 1.4 Contributions of this Dissertation 5 Chapter 2 Clock Polarity Assignment Under Bounded Skew 7 2.1 Introduction 7 2.2 Motivational Example 9 2.3 Problem Formulation 13 2.4 Proposed Algorithm 17 2.4.1 Independence Assumption 17 2.4.2 Characterization of Noise 18 2.4.3 Overview of the Proposed Algorithm 19 2.4.4 Mapping WaveMin Problem to MOSP problem 22 2.4.5 A Fast Algorithm 26 2.4.6 Zone Sizing/Partitioning Method 27 2.5 Experimental Results 28 2.5.1 Experimental Setup 28 2.5.2 Noise Reduction 28 2.5.3 Simulation on Full Circuit 29 2.6 Effects of Clock Polarity Assignment on Simultaneous Switching Noise 34 2.6.1 Model of Power Delivery Network 34 2.6.2 Peak-to-Peak Voltage Swing 35 2.7 Effects of Decoupling Capacitors 36 2.8 Effects of Clock Polarity Assignment on Clock Jitter 40 2.8.1 Noise in Frequency Domain 40 2.9 Summary 43 Chapter 3 Clock Polarity Assignment Under Useful Skew 44 3.1 Introduction 44 3.2 Motivational Example 45 3.3 Problem Formulation 47 3.4 Proposed Algorithm 49 3.4.1 Integer Linear Programming Formulation and Linear Programming Relaxation 49 3.4.2 Formulating into Maximum Clique Problem 49 3.4.3 Scalable Algorithm for Clique Exploration 51 3.5 Experimental Results 54 3.5.1 Experimental Setup 54 3.5.2 Assessing the Performance of UsefulMin over Wavemin 56 3.6 Summary 57 Chapter 4 Extensions of Clock Polarity Assignment Methods 60 4.1 Coping With Thermal Variations 60 4.1.1 Introduction 60 4.1.2 Proposed Method 61 4.1.3 Experimental Results 66 4.2 Coping with Delay Variations 70 4.2.1 Introduction 70 4.2.2 The Impact of Process Variations on Polarity Assignment 71 4.2.3 Proposed Method for Variation Resiliency 72 4.2.4 Experimental Results 73 4.3 Coping With Multi-Mode Designs 75 4.3.1 Introduction 75 4.3.2 Proposed Method 76 4.3.3 Experimental Results 84 4.4 Orthogonality with Other Design Techniques ? Clock Gating 87 4.4.1 Introduction 87 4.4.2 Proposed Partitioning Method 87 4.4.3 Experimental Results 88 4.5 Summary 90 Chapter 5 Conclusion 92 5.1 Clock Polarity Assignment Under Bounded Skew 92 5.2 Clock Polarity Assignment Under Useful Skew 93 5.3 Extensions of Clock Polarity Assignment 93 Appendices 94 Chapter A Power Spectral Densities of ISCAS89 Circuits 95 Chapter B The Effect of Decoupling Capacitors 99 초록 109Docto

Ancient origins of a diverse stygofaunal group: The Australian Bathynellidae (Crustacea)

Research on subterranean fauna on all continents has highlighted often high levels of endemism, for both troglobionts (terrestrial subterranean organisms) and stygobionts (organisms living in groundwater). In Australia, particularly in the arid zones of Western Australia, a rich hypogean assemblage has been discovered in the past few decades, largely due to surveys required as part of environmental impact assessments for, and imposed conditions on, mining operations. Bathynellidae (Crustacea) are part of the stygofaunal community, but to-date they are poorly understood, mainly because their conservative morphology, small size and delicate exoskeleton make their dissection, observation and study very difficult. Additionally, the incomplete description of the type genus and species of the family (Bathynella natans) have led to misidentifications and taxonomic/systematic uncertainties. Prior to this contribution, only one species of Bathynellidae had been described from Australia, and knowledge on their morphology, species/genera and distributional boundaries, inter- intraspecific variability, and origins were very limited. The overall aim of this research is to investigate diversity, patterns of distribution, and biogeographical history of Bathynellidae in Australia, focussing on the arid zones of Western Australia (particularly on the Pilbara region).The abundance of material already collected by different companies allowed a thorough morphological and molecular examination of many specimens. The study of multiple taxa occurring in different aquifers of the same and different catchments is carried out to explore intra-interspecific variability, species and distributional boundaries, and evolutionary histories. A molecular phylogeny including representative taxa from different continents is also built to put the Australian fauna into a worldwide context. Four new species and genera (Pilbaranella ethelensis, Fortescuenella serenitatis, Anguillanella callawaensis, and Muccanella cundalinensis) have been described, clarifying morphological and molecular intra- and interspecific boundaries. These represent the first four species described for Western Australia. One new subfamily, nine new genera, and 23 new species are recognised through molecular species delineation and preliminary morphological analysis. All species and most genera studied appear restricted to single aquifer systems in particular areas of a catchment. The molecular phylogeny of the Australian Bathynellidae indicates a complex history. The molecular clock analysis suggests that aridification processes only partly influenced the diversification, while most of their diversity predate the establishment of the drought climate. The results indicate very old origins dated back to the end of Devonian-early Carboniferous, and a substantial increase in the rate of diversification during the Triassic-Jurassic period, which could represent multiple freshwater invasions, facilitated perhaps by sea level fluctuations. A consequent dispersion thanks to a much wetter climate may explain the presence of Bathynellidae in areas of the country that have not been influenced by marine transgressions since the Proterozoic. The molecular phylogeny including genera and lineages from other countries indicates that the Australian Bathynellidae form a monophyletic clade comprising different subfamilies distinct from the ones described from other continents. This better understanding will help to predict the potential diversity of this group in unstudied areas, and enable improved advice to government regulators where protection of subterranean environments and their aquifers is a priorit

Population genetics and systematics of a species-rich clade of Neotropical reef fishes, the tubeblenny genus Acanthemblemaria

Neotropical coral reef fish communities are species-poor compared to those of the Indo-West Pacific. An exception to that pattern is the blenny clade Chaenopsidae, one of only three coral reef fish families endemic to the Neotropics. Within the chaenopsids, the genus Acanthemblemaria is the most species-rich. To understand the origin and maintenance of genetic and species diversity in these fishes, I characterized the population genetics for two Acanthemblemaria species, reconstructed the phylogeny of the group, and identified suites of correlated morphological characters responsible for the distinctive skull morphology of these fishes. By combining nuclear and mitochondrial sequence data I was able to recover the complex demographic history of two closely related Acanthemblemaria species, A. aspera and A. spinosa. Old population expansions in both species were obscured by a rapid mitochondrial substitution rate, but the mitochondrial DNA allowed the recovery of a recent expansion in A. aspera corresponding to a period of increased habitat availability. However, the older expansions that took place in both species were only recovered using the nuclear markers. Across the genus I found that mitochondrial COI is evolving nearly 100X faster than the nuclear markers and at an absolute rate of nearly 25% pairwise sequence divergence per million years. Replicate Bayesian phylogenetic analyses failed to converge on the same posterior distributions because proposals to update the rate multiplier parameter were rarely accepted, but when the tuning parameter was adjusted, all datasets converged quickly on to the same posterior distribution. When COI was included, posterior probabilities of the species tree were lower and topological estimates were worse than those from the nuclear-only dataset. The species tree that was constructed for the genus conflicted with the morphological phylogeny for the group, primarily due to the convergence of skull bones with spines. By performing phylogenetic analyses on these characters, I resolved some of the conflicts between the morphological and molecular phylogenies. Divergence time estimates recovered a mid-Miocene origin for the genus, with speciation both before and after the closure of the Isthmus of Panama. Some sister taxa were broadly sympatric, but many occur in allopatry