i/oZONE Products for the week of January 12, 2004
National Semiconductor Says…
Beyond XAUI! National Semiconductor Extends High-end
Router Backplane Life With Quad 5 Gbit/s Full-Duplex SerDes Chip
National Semiconductor Corporation has introduced one of the
industry's fastest quad serializer/deserializer (SerDes) transceivers, which
extends the life of today's high-end router backplanes by quadrupling the
data throughput capacity. This new single-chip device, the SCAN50C400, allows
communication system designers to easily upgrade existing switch/fabric
cards and line cards to 5 Gigabits per second (Gbps) serial data rate performance
without a costly replacement of existing backplanes. With the announcement
of the SCAN50C400, National Semiconductor is enhancing its leadership in
the SerDes market and now offers a complete portfolio of backplane solutions,
ranging from Gigabit LVDS SerDes for broadband infrastructure to multi-Gigabit
solutions for core and metro routers.
"Service providers and OEMs tell us they want to increase capacity and optimize CAPEX expenditures by upgrading existing equipment rather than investing in new platforms," said Stephen Kempainen, marketing director of the Communication Infrastructure Products group at National Semiconductor. "National Semiconductor's new product makes this possible. The SCAN50C400's combination of 0.1 UI (unit interval) transmitter jitter performance and 0.65 UI receiver jitter tolerance enables our customers to upgrade their systems and achieve 5 Gbps serial data rate by just upgrading fabric and line cards."
About The SCAN50C400
Targeting the high-speed backplane interconnect between line cards and fabric
cards in applications like networking, storage and computing, National Semiconductor's
four-channel 5 Gbps SCAN50C400 transceiver works effectively on existing
FR-4 backplanes originally designed for lower speeds of 1.25 and 2.5 Gbps.
The device's CML (current mode logic) serial I/O supports multiple data
rates at 1.25, 2.5 or 5 Gbps over a printed circuit board backplane. In
total, the device provides a data link of up to 40 Gbps total throughput.
National's SCAN50C400 features a high-speed serial I/O that delivers unmatched signal integrity and jitter performance, as well as built-in self-test (BIST) circuitry and loopback modes to support at-speed testing. The device also has an IEEE P802.3ae D4.0 standard-compliant MDIO management bus to control the programmable features such as BIST, loopback modes and de-emphasis. In addition, the IEEE1149.1 JTAG-compliant SCAN features give the chip manufacturing testability. The SCAN50C400 is offered in a thermally enhanced 440-pin BGA package.
National's Total Backplane Solution
National leads the networking industry in providing complete backplane solutions.
In addition to the SCAN50C400, National is developing products such as fixed
and adaptive multi-Gigabit equalizer solutions and cross-talk cancellers.
The first in the family of equalizers is a 3.125 Gbps to 6.25 Gbps fixed
equalizer designed to compensate transmission medium losses and reduce the
medium-induced deterministic jitter. The equalizer will be available for
sampling in the first quarter of 2004.
analogZONE Says . . .
National was one of the original players in the LVDS SerDes market years ago, when the technology had first developed to provide high-speed, low-power, low-footprint interconnects. Their expertise earned them significant market share in applications like medium-speed backplanes and connecting LCD video displays to the main processor board in laptops and consumer electronics at 155-622 Mbit/s and eventually offered parts as fast as 1.25 Gbit/s. They were a bit slow out of the gate in updating their well-targeted XAUI product line to meet the emerging multi-Gigabit backplane market, but they have managed to leapfrog back into the thick of things with their latest 5-Gbit/s offering.
Although I'm very anxious to get on to the review of this very exciting introduction from National, I feel the need to clear the air about the misleading wording National used in the headline of its original press release (I edited the headline for accuracy before posting it at the top of the review). While the headline on the release says "National Semiconductor Extends High-end Router Backplane Life with Introduction of Industry's First Quad SerDes with 40 Gbps Throughput", I'd argue that this part is neither a "40 Gbit/s transceiver" nor an "industry first." I will say however that the SCAN50C400 is an exceptionally competent Quad-channel full-duplex 5-Gbit/s SerDes transceiver that is among the first on the market to deliver this level of performance. All this being said, the National's got an interesting blend of performance, flexibility and affordability that should help them win quite a few sockets in the upcoming generation of high-performance networking products.
Much like its cohort chips from Agere, Accelerant, Broadcom, and others, the transceiver targets the high-end router market where performance matters most. As we've discussed here before, the capacity demands in the infrastructure are growing more steeply as economic recovery continues, putting system operators in a bind about how to increase their capacity without scrapping their base of installed equipment. The best current backplane traces typically have a 2.5 Gbit/s capacity, and are usually trunked by 4X to form a 10-Gbit/s link. National's part doubles the capacity of a single trace to 5 Gbit/s.
Looking a bit more closely at the SCAN50C400's details, we find that the low-speed side of the chip employs four 1.25 Gbit/s LVDS channels per 5 Gbit/s stream. The high-speed side is implemented in current mode logic (CML). The chip's CML driver output has extremely low output jitter -- at or below 0.1 UI (unit interval) across all output conditions. It also enjoys a wide jitter tolerance of 0.65 UI at 5 Gbit/s, which translates to a generous130 ps.
Like most transceivers of its type, it employs 8B/10B NRZ line coding for signal integrity. The twist here is that the coding is generated off-chip, usually by a host ASIC. While 8B/10B is most commonly used, having the coding done off-chip allows you the option of using more efficient, higher-order codes like the 35B/36B scheme found in some emerging 10-Gbit/s applications. Moving the coding off-chip either to your design effort (and some negligible cost for a few gates), you get the advantage of having the 8B/10B running across entire data path instead of only in the SerDes link. While National could be accused of making lemonade out of lemons here, I'll agree with them that this feature/bug gives you the advantage of added signal integrity across a much larger chunk of your internal data stream.
The thing that most distinguishes National's offering from its competitors is that they have taken a fresh look at channel impairments and noise issues in the backplane environment, and reached some strikingly different conclusions about how to deal with them. For this reason, the SCAN50C400 incorporates several unique features including a programmable variable output voltage. Each channel's output swing can be programmed between 0.7 V and 1.4 V to minimize crosstalk by providing only the amount of power required to push a signal through a particular trace. This is important because it can greatly reduce the noise floor in a backplane environment, much of which is caused by excessive signal swing and over-zealous use of pre-emphasis circuitry.
And while most manufacturers swear by pre-emphasis to get their signals across stretches of backplane, the SCAN50C400 relies instead on an external equalizer to selectively boost the weak signals of the handful of "problem channels" found in nearly every backplane. National's philosophy is that rather than break out the "heavy artillery" for every channel whether it needs it or not, they will provide a cost-effective standalone solution that works in 50% or more of the traces, even in a challenging "legacy" backplane. You then get to drop in equalization, or even cancellation circuits, only where they are needed -- typically in runs with long trace length (over 26 inches), or serious channel impairments due to older-style connectors and bad layout practices. National is currently offering a reference design which specs a third-party equalizer, but this is an interim solution, and they expect to have a homegrown component produced in conjunction with Quellan to replace it in the near future.
National is so pre-emphasis averse that they have actually added a "de-emphasis" circuit to their transceiver. Its primary purpose is to overcome the inter-symbol interference caused by long strings of ones or zeroes building up a capacitive charge in the trace. The de-emphasis circuit looks ahead in the incoming bit stream to spot long chains (2 or more) of the same symbols, and then lowers the drive current on the initial transition to give a consistent zero-crossing time which minimizes jitter. While this goes against the conventional wisdom of the industry that advocates the use of some form of pre-emphasis, you can't argue with the results as shown in the two screen shots I've got here which show a radical improvement in deterministic jitter and a markedly lower signal that will be a much better neighbor to its adjacent traces.
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Another important thing about the device is that it can fall back to slower line rates (1.25, 2.5 Gbit/s LVDS). This allows an upgraded switch fabric equipped with these multi-speed SerDes to interoperate with both existing line cards and the higher-capacity units that will replace them as capacity demands dictate. This gives manufacturers a way to upgrade a chassis on a per-blade, or even a per-connection basis. Such a "pay as you grow" model could help manufacturers retain loyal customers, and encourage the sale of very profitable incremental upgrades. Between this, and the excellent price/performance ration offered here, it's a good bet you'll see these chips (and their companion EQ/canceller circuits) showing up in a good fraction of the equipment upgrades happening over the coming year.
Because they run contrary to much of the current conventional wisdom about SerDes transceiver operations, it would be hard to reach any firm conclusions about National's claims for its product without actually seeing it in operation and comparing it to other parts. That's why I'm so pleased that they have chosen to participate in our Equalize This! Gigabit Backplane Challenge. As of this writing, our research team at Georgia Tech's Electronic Design Center has collected the first round of test data on the SCAN50C400. I'll be posting the data and some analysis in the next week or two - as soon as I dig out from the holidays.
Samples of the SCAN50C400 and evaluation board are available today and the product will be priced at less than $100 per unit in large volumes.
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