Xilinx's 400 Gigabit Ethernet FPGA

Xilinx has detailed its latest 28nm CMOS Virtex-7 FPGA family that will support 400 Gigabit Ethernet on a single device. The Virtex-7HT completes the Virtex-7, joining the Virtex-7T and Virtex-7XT product families announced in June.

A single FPGA will support 400 Gigabit Ethernet duplex traffic. The FPGA can also support 4x100Gig MACs and 4x150Gbps Interlaken interfaces. Source: Xilinx

Why is it important?

Xilinx says its switch and router customers are more than doubling the traffic capacity of their platforms every three years. “They are looking for silicon that will support a doubling of capacity within the same form-factor and the same power budget,” says Giles Peckham, EMEA marketing director at Xilinx.  

An FPGA has an advantage when compared to an application-specific standard product (ASSP) chip or an ASIC: being programmable and a volume-manufactured device, it is easier for an FPGA design to contend with changes in standards and the escalating cost of implementing chip designs in ever-finer CMOS geometries.

The Virtex-7HT will support 28 Gigabit-per-second (Gbps) transceivers (serial/ deserialiser or serdes). Used in a four-channel configuration, a 100Gbps interface can be implemented. Indeed the largest member of the Virtex-7HT family - the XC7VH870T - will have 16 x 28.05Gbps transceivers, enabling 4x100Gbps or even a 400 Gigabit Ethernet interface.

The 28Gbps transceivers will be used to interface to optical modules such as the emerging CFP2 pluggable form-factor. The CFP2 multi-source agreement  is expected to be ratified in the second half of 2011 and start shipping in the second half of 2012, says Xilinx.

“Network processors and ASICs are typically a [CMOS] process node or two behind us"

Giles Peckham, Xilinx

And with the additional 72, 13.1Gbps transceivers on-chip, the XC7VH870T will have sufficient input-output (I/O) to support bi-directional 400 Gigabit Ethernet traffic. The FPGA's lower-speed 13.1Gbps serdes are included to interface to network processors (NPUs) or ASICs that only support the lower-speed transceivers. “Network processors and ASICs are typically a [CMOS] process node or two behind us – partly because of cost - such that they end up at a technology disadvantage, as in transceiver speed,” says Peckham.

The additional 13.1Gbps transceivers - only 40 of the 72 transceivers are needed for the 400 Gigabit Ethernet port – will enable the FPGA to interface to other chips.

Xilinx says it will be at least a year and possibly 18 months before samples of the Virtex-7HT FPGA family become available. But it is making the Virtex-7HT announcement now because it has tested successfully the 28Gbps transceiver design.

Front panel evolution from 48 SFP+ to 4 CFPs to 8 CFP2s. Source: Xilinx

What has been done

There are three devices in the Virtex-7HT family which have 4, 8 and 16, 28Gbps transceivers. Xilinx claims this is four times the transceiver count of any competing 28nm FPGA detailed to date. But Peckham admits that additional announcements from competitors are inevitable before the Virtex-7HT devices become available in 2012.

In September Altera announced that it had successfully demonstrated a 25Gbps transceiver test chip. And in November, Intel and Achronix Semiconductor formed a strategic relationship that will allow the FPGA start-up to use Intel's leading-edge 22nm CMOS manufacturing process.

The three Virtex-7HT FPGAs also come with different amounts of programmable logic cells, memory blocks and Xilinx’s XtremeDSP building blocks tailored for digital signal processing.

Xilinx says meeting the CEI-28G electrical interface jitter specification has proved challenging.  At 10 Gigabit the signal period is 100 picoseconds (ps) and the jitter allowance is 35ps, while the signal period at 28Gbps is 35ps. “When you realise the jitter spec on the 10 Gigabit interface is the same as the full period in the 28 Gigabit spec – 35 picoseconds – there is quite a lot of work to be done in reducing the jitter when migrating to 28 Gigabit,” says Peckham.

Xilinx uses pre-emphasis techniques on the signals before they are transmitted across the printed circuit board to reduce loss. In addition, the FPGA maker has enhanced the noise isolation between the FPGA's digital and analogue CMOS circuitry. “The short spiky current loads in the digital circuitry can impact the noise in the analogue circuitry and increase the jitter,” says Peckham.

What next?

Xilinx has created a test vehicle 28Gbps transceiver. This allows Xilinx to validate and fine-tune the design. The rest of the FPGA design needs to be completed while another design iteration of the 28Gbps test vehicle is likely. “We have a lot of things to do yet,” he says.

Meanwhile system vendors can start to design their systems based on the FPGA family in advance of samples that are expected in the first half of 2012.

• For a video demonstration of the 28Gbps test vehicle, click here.