Modeling, Optimizing and Exploring Multi-Die FPGA Routing Architectures

2026-06-04

AI Summary

This work addresses the lack of accurate and flexible interconnect modeling tools for multi-die FPGAs, which hinders the design and optimization of cross-die routing architectures. The authors extend the open-source FPGA CAD framework VTR to develop a high-fidelity interconnect modeling infrastructure supporting 2.5D/3D integration, enabling—for the first time—precise characterization of electrical properties across diverse cross-die technologies. They also enhance the VPR placement and routing engine to facilitate multidimensional design space exploration. Validation using HSPICE models based on a 7nm FPGA process and a 45nm silicon interposer demonstrates that 3D FPGAs reduce wirelength by up to 14% and improve critical-path performance by 6% compared to 2D counterparts, while 2.5D FPGAs with 32% cross-die connections incur only a 2% wirelength and 4% delay overhead, maintaining excellent routability.

Abstract

Die stacking has enabled 2.5D FPGAs by integrating multiple active dice on a passive silicon interposer for improved yield and capacity, and paved the way for 3D architectures that stack active dice directly atop one another. In these multi-die devices, the unique electrical and physical characteristics of the underlying die-stacking technology impose limitations on inter-die connection density and latency, necessitating a bespoke inter-die routing architecture. However, the absence of accurate and versatile modeling tools has left most questions about how to best design the inter-die routing architecture unanswered. To address this gap, we enhance the open-source FPGA CAD tool VTR to flexibly model a wide range of multi-die routing architectures, and augment VPR's placement and routing engines to improve optimization for both 2.5D and 3D FPGAs. We perform HSPICE-based circuit modeling of inter-die connections for active dice using a 7 nm process node and a 45 nm silicon interposer across several die-crossing technologies. Using this enhanced framework, we conduct a detailed design space exploration of inter-die routing architecture in 2.5D and 3D FPGAs, characterizing the impact of die-crossing technology, inter-die connection count, fan-in/fan-out, and interposer wire length on critical path delay (CPD), wirelength, area, and routability. Our results show that with suitable inter-die routing architectures, 2.5D and 3D FPGAs can increase capacity without significant routability or delay penalties. Specifically, 3D FPGAs achieve up to 14% wirelength reduction and 6% CPD improvement over 2D devices, and remain routable even with existing $10\,μ$m pitch technologies, while 2.5D FPGAs incur only a 2% wirelength and 4% CPD overhead at 32% inter-die connectivity. All extensions are open source and integrated with the VTR master branch.

Problem

• multi-die FPGA
• inter-die routing
• 2.5D/3D integration
• routing architecture
• die stacking

Innovation

• multi-die FPGA
• 2.5D/3D integration
• inter-die routing architecture
• VTR/VPR enhancement
• design space exploration