On-Chip Measurement and Compensation of Timing Imbalances in High-Speed Serial NoC Links

On-Chip Measurement and Compensation of Timing Imbalances in High-Speed Serial NoC Links

Sebastian Höppner (Technische Universität Dresden, Dresden, Germany), Dennis Walter (Technische Universität Dresden, Dresden, Germany), Georg Ellguth (Technische Universität Dresden, Dresden, Germany) and René Schüffny (Technische Universität Dresden, Dresden, Germany)
DOI: 10.4018/jertcs.2012100103

Abstract

This paper presents techniques for measurement and compensation of timing variations in clock and data channels of source-synchronous high-speed serial network-on-chip (NoC) links. Timing mismatch measurements are performed by means of asynchronous sub-sampling. This allows the use of low quality sampling clocks to reduce test hardware overhead for integration into complex MPSoCs (Multiprocessor System-on-Chip) with multiple NoC links. The effect of clock jitter on the measurement results is evaluated. Delay mismatch is compensated by tunable delay cells. The proposed technique enables compensation of delay variations to realize high-speed NoC links with sufficient yield. It is demonstrated at NoC links as part of an MPSoC in 65 nm Complementary Metal Oxide Semiconductor technology, where the calibration significantly reduces bit-error-rates of a 72 GBit/s (8 GBit/s per lane) link over 4 mm on-chip interconnect.
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Introduction

High-speed serial point to point connections are proven to be energy and area efficient solutions for on-chip data transmissions over long distances in the range of some mm in networks-on-chip (NoCs) (Mensink et al., 2010; Walter et al., 2012). Although conventional 2D-mesh NoC topologies require only shorter point to-point links between adjacent cores, the proposed long distance interconnects can enable efficient implementation of advanced NoC topologies like for example hierarchical 2D-mesh NoCs (Winter et al., 2010), where higher layer point-to-point links bridge distances over multiple cores. As link data rates increase, the influence of process variations gets more severe especially in sub-100nm CMOS technologies. To achieve high data rates and maximize yield, calibration techniques have to be employed to compensate static mismatch variations. In Höppner et al. (2010) a calibration strategy with optimal sizing of compensation delay elements has been proposed. However, measurement access to on-chip signal characteristics is required. Asynchronous sub-sampling, where a high-speed signal is periodically sampled by a low-speed clock, is widely used for on-chip measurement purposes. In Schaub et al. (2008) on-chip oscilloscopes using asynchronous sampling clocks are presented which allow measurement of high-speed signals but require low jitter sampling clocks. This is a major drawback for complex MPSoCs where measurement signals have to be distributed over longer distances with negligible circuit and area overhead. In Amrutur et al. (2010) measurement of static skews of periodic signals is proposed using statistical averaging. This approach eases integration due to relaxed requirements for sample clock quality. Using this method periodic on-chip signals whose period is in the range of sample clock jitter can be characterized.

This work presents an asynchronous sub-sampling technique (Höppner et al., 2011) to provide measurement access to delay characteristics of multiple high-speed NoC links in a 65 nm MPSoC. Therefore, a low frequency asynchronous sampling clock with relaxed jitter requirements can be used which simplifies integration and scalability.

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