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UBC Theses and Dissertations

Implementation considerations for "soft" embedded programmable logic cores Wu, James Cheng-Huan

Abstract

As integrated circuits become increasingly more complex and expensive, the ability to make post-fabrication changes will become much more attractive. This ability can be realized using programmable logic cores. Currently, such cores are available from vendors in the form of "hard" macro layouts. An alternative approach for fine-grain programmability is possible: vendors supply an RTL version of their programmable logic fabric that can be synthesized using standard cells. Although this technique may suffer in terms of speed, density, and power overhead, the task of integrating such cores is far easier than the task of integrating "hard" cores into an ASIC or SoC. When the required amount of programmable logic is small, this ease of use may be more important than the increased overhead. In this thesis, we identify potential implementation issues associated with such cores, and investigate in depth the area, speed and power overhead of using this approach. Based on this investigation, we attempt to improve the performance of programmable cores created in this manner. Using a test-chip implementation, we identify three main issues: core size selection, I/O connections, and clock-tree synthesis. Compared to a non-programmable design, the soft core approach exhibited an average area overhead of 200X, speed overhead of 10X, and power overhead of 15OX. These numbers are high but expected, given that the approach is subject to limitations of the standard cell library elements of the ASIC flow, which are not optimized for use with programmable logic.

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