Computer Laboratory

Technical reports

Pulse-based, on-chip interconnect

Simon J. Hollis

September 2007, 186 pages

This technical report is based on a dissertation submitted June 2007 by the author for the degree of Doctor of Philosophy to the University of Cambridge, Queens’ College.

Abstract

This thesis describes the development of an on-chip point-to-point link, with particular emphasis on the reduction of its global metal area footprint.

To reduce its metal footprint, the interconnect uses a serial transmission approach. 8-bit data is sent using just two wires, through a pulse-based technique, inspired by the GasP interconnect from Sun Microsystems. Data and control signals are transmitted bi-directionally on a wire using this double-edged, pulse-based signalling protocol, and formatted using a variant of dual-rail encoding. These choices enable a reduction in the number of wires needed, an improvement in the acknowledgement overhead of the asynchronous protocol, and the ability to cross clock domains without synchronisation hazards.

New, stateful, repeaters are demonstrated, and results from spice simulations of the system show that data can be transferred at over 1Gbit/s, over 1mm of minimum-sized, minimally-spaced metal 5 wiring, on a 180nm (0.18um) technology. This reduces to only 926Mbit/s, when 10mm of wiring is considered, and represents a channel utilisation of a very attractive 45% of theoretical capacity at this length. Analysis of latencies, energy consumption, and area use are also provided.

The point-to-point link is then expanded with the invention and demonstration of a router and an arbitrated merge element, to produce a Network-on-Chip (NoC) design, called RasP. The full system is then evaluated, and peak throughput is shown to be 763Mbit/s for 1mm of wiring, reducing to 599Mbit/s for 10mm of the narrow metal 5 interconnect.

Finally, RasP is compared in performance with the Chain interconnect from the University of Manchester. Results for the metrics of throughput, latency, energy consumption and area footprint show that the two systems perform very similarly — the maximum absolute deviation is under 25% for throughput, latency and area; and the energy-efficiency of RasP is approximately twice that of Chain. Between the two systems, RasP has the smaller latency, energy and area requirements and is shown to be a viable alternative NoC design.

Full text

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BibTeX record

@TechReport{UCAM-CL-TR-698,
  author =	 {Hollis, Simon J.},
  title = 	 {{Pulse-based, on-chip interconnect}},
  year = 	 2007,
  month = 	 sep,
  url = 	 {http://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-698.pdf},
  institution =  {University of Cambridge, Computer Laboratory},
  number = 	 {UCAM-CL-TR-698}
}