Optical interconnect is coming to FPGAs

EDN Europe, 29 Mar 2011

Calling the disclosure a technology announcement – in other words, no specific product introductions or release dates have been revealed – Altera has described work-in-progress to add optical interconnect to the periphery of its fastest FPGA chips. In many bandwidth-hungry applications, Altera says, some of the most difficult traffic bottlenecks are at the printed-circuit board, rack and cabinet level, in chip-to-chip, inter- and intra-PCB connections. Altera has, in its most recent releases of top-end FPGAs (e.g. the Stratix and Arria families), implemented serial transceivers to route data on- and off-chip that are capable of handling up to 28 Gbits/sec. The company acknowledges what is scarcely a secret; that routing PCB tracks to sustain such data rates, or even a fraction of them, over more than a few cm. is “challenging”. The problems are well-known; signal integrity issues are difficult to handle, you must use exotic and expensive PCB materials, and the distance you can successfully transmit data at multi-Gbit rates is limited.

Altera therefore proposes to manufacture what will, in effect, be a multi-chip module based around its fastest FPGA devices; a special package will house the programmable logic die and optical transceiver dice; not disclosed is whether the technology will use a common substrate, or if the transceivers will be mounted directly to Altera’s silicon. The FPGA chips will retain their transceivers – the optical interfaces will not connect directly to the logic array – but they will be closely coupled to the optoelectronic driver chips.
Packages will accommodate micro-miniature connectors at their periphery, to which you will attach suitably-terminated multi-mode fibres: no details on package dimensions, or numbers of optical ports, are yet available. Altera anticipates sending optical signals over distances of up to 100m – enough to connect over rack-rack or cabinet-cabinet spans, as well as chip-chip or board-board.
Not envisaged at present, as far as Altera has disclosed, is any plan to connect to fibre incorporated into a PCB structure, or to any sort of PCB optical waveguide; optical connectors on the package edges will connect to discrete fibres, while conventional surface-mount termination on the underside of the package will handle electrical connections in the normal way. And, although Altera’s announcement talks of usage across backplanes, it is not yet clear if two PCBs would connect via an optical path through a backplane, or if direct board-board fibre connections would bypass the backplane itself.
Altera quotes 200 mW power dissipation for one of its 28-Gbit/sec transceivers operating at full speed, a figure which will likely reduce significantly when the transceiver no longer has to drive signals over long electrical lines; the company cites reducing power as one of the motivations for this development, saying, “For compute and storage-intensive applications such as data centers, the integration of optical interfaces into device packages could replace pluggable optics and reduce power by 70 to 80% while increasing port density and bandwidth by orders of magnitude. In backplane applications in the military, communications infrastructure and broadcast areas, these connections will replace expensive board material and connectors, increase bandwidth significantly and eliminate the signal integrity issues.”