COFFE: Fully-Automated Transistor Sizing for FPGAs

In this paper, we present COFFE (Circuit Optimization For FPGA Exploration), a new fully-automated transistor sizing tool for FPGAs. Automated transistor-level CAD tools are an important part of the architecture exploration flow because they provide accurate area and delay estimates of low-level FPGA circuitry, which must be obtained for each architecture. We show that modeling transistors as linear resistances and capacitances as has been done in previous FPGA transistor sizing tools is highly inaccurate for fine-grained transistor-level design in advanced process nodes. Therefore, COFFE's transistor sizing algorithm maintains circuit non-linearities by relying exclusively on HSPICE simulations to measure delay. Area is estimated with a transistor size-based model that incorporates a number of improvements to enhance its accuracy in advanced process technologies versus prior methods. In addition to more accurate area and delay estimation, COFFE considers more layout effects than prior published work by automatically accounting for transistor and wire loads, which are computed based on architectural parameters and layout area. This new FPGA transistor sizing tool requires only several hours to produce high-quality transistor sizing results for an entire FPGA tile; a task that would normally take months of manual effort. We demonstrate COFFE's utility in FPGA architecture studies by investigating an important new architectural question at the logic-to-routing interface.