Department of Computer Science and Technology

Technical reports

Exploring networks-on-chip for FPGAs

Rosemary M. Francis

January 2013, 121 pages

This technical report is based on a dissertation submitted July 2009 by the author for the degree of Doctor of Philosophy to the University of Cambridge, Darwin College.

DOI: 10.48456/tr-828


Developments in fabrication processes have shifted the cost ratio between wires and transistors to allow new trade-offs between computation and communication. Rising clock speeds have lead to multi-cycle cross-chip communication and pipelined buses. It is then a small step from pipelining to switching and the development of multi-core networked systems-on-chip. Modern FPGAs are also now home to complex systems-on-chip. A change in the way we structure the computation demands a change in the way we structure the communication on-chip.

This thesis looks at Network-on-Chip design for FPGAs beyond the trade-offs between hard (silicon) and soft (configurable) designs. FPGAs are capable of extremely flexible, statically routed bit-based wiring, but this flexibility comes at a high area, latency and power cost. Soft NoCs are able to maintain this flexibility, but do not necessarily make good use of the computation-communication trade-off. Hard NoCs are more efficient when used, but are forced to operate below capacity by the soft IP cores. It is also difficult to design hard NoCs with the flexibility needed without wasting silicon when the network is not used.

In the first part of this thesis I explore the capability of Time-Division Multiplexed (TDM) wiring to bridge the gap between the fine-grain static FPGA wiring and the bus-based dynamic routing of a NoC. By replacing some of the static FPGA wiring with TDM wiring I am able to time division multiplex hard routers and make better use of the non-configurable area. The cost of a hard network is reduced by moving some of the area cost from the routers into reusable TDM wiring components. The TDM wiring improves the interface between the hard routers and soft IP blocks which leads to higher logic density overall. I show that TDM wiring makes hard routers a flexible and efficient alternative to soft interconnect.

The second part of this thesis looks at the feasibility of replacing all static wiring on the FPGA with TDM wiring. The aim was to increase the routing capacity of the FPGA whilst decreasing the area used to implement it. An ECAD flow was developed to explore the extent to which the amount of wiring can be reduced. The results were then used to design the TDM circuitry.

My results show that an 80% reduction in the amount of wiring is possible though time-division multiplexing. This reduction is sufficient to increase the routing capacity of the FPGA whilst maintaining similar or better logic density. This TDM wiring can be used to implement area and power-efficient hard networks-on-chip with good flexibility, as well as improving the performance of other hard IP blocks.

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BibTeX record

  author =	 {Francis, Rosemary M.},
  title = 	 {{Exploring networks-on-chip for FPGAs}},
  year = 	 2013,
  month = 	 jan,
  url = 	 {},
  institution =  {University of Cambridge, Computer Laboratory},
  doi = 	 {10.48456/tr-828},
  number = 	 {UCAM-CL-TR-828}