Electronic Design

CMOS Transceiver Chip Allows 50-Gbit/s Serial Data Transmissions

This low-power IC makes possible serial backplanes that boost the performance of Internet routers, switches, and optical networks.

Serial data communications rates keep rising as the pressure for greater bandwidth continues. Recent developments in optical-fiber communications as well as 1-Gbit/s and 10-Gbit/s Ethernet have pushed semiconductor companies to produce chips that not only keep up speed-wise, but also consolidate as much of the logic as possible. The new nPower BBT3800 transceiver from BitBlitz Communications Inc. nicely fulfills those requirements, and it achieves a new level of CMOS speed/power performance.

The BBT3800 is an eight-channel full-duplex CMOS transceiver (Fig. 1). Every channel converts 8- or 10-bit parallel data into a serial signal with a data rate of up to 3.125 Gbits/s, for an aggregate bandwidth of 50 Gbits/s. Simultaneously, serial data can be converted into parallel form on each channel.

The chip's low-power feature of just 200 mW per channel allows multiple BBT3800s to work together in parallel. For example, designers could use 20 BBT3800 chips in parallel to design a serial backplane operating at a bandwidth of 1000 Gbits/s (1 Tbit/s).

BitBlitz's patent-pending Large-Amplitude Differential Logic (LADL) CMOS enables the device to operate at the 3.125-Gbit/s level with a power consumption of approximately one-third that of conventional CMOS transceiver ICs. Total power consumption is only 1.6 W or less if the high-speed transceiver logic (HSTL) mode is employed.

Depending upon the I/O requirements, the chip operates from one or two supplies. The core runs on 1.8 V, as does the HSTL parallel I/O. But implementing a parallel stub series-terminated logic (SSTL) I/O interface requires an additional 2.5-V supply. The parallel inputs and outputs comply with the JEDEC standards for SSTL2 or HSTL.

Furthermore, the transmitter contains a 4-byte (actually 10 bits wide) first-in/first-out (FIFO) memory to correct any drift associated with the transmit clock. The FIFO compensates for temporary phase drift within the transmit data stream. Should the application require it, such as 10-Gbit Ethernet, there's an 8B/10B encoder. The serializer translates the parallel data into a serial bit stream. Also, the serial I/O is designed to match a 50-Ω stripline or microstrip transmission line.

Included in the serial receive portion of the circuitry are a patent-pending PLL-type clock-recovery circuit, a deserializer, an optional 10B/8B decoder, and a 16-byte FIFO that performs byte alignment and clock compensation. Additionally, all channels share some circuitry, such as a PLL clock synthesizer, a channel alignment unit (deskewer), and a two-pin serial management data interface I/O. The internal clock is generated at the 3.125-GHz level by using an external 156.25-MHz crystal with the internal X20 PLL multiplier. Users access the management data I/O and management data control registers by using the two-bit interface.

The serial control interface complies with the standard proposed by IEEE 802.3ae (the 10-Gbit Ethernet Standard Task Force). Like earlier versions of the Ethernet standard, the serial management interface permits access to a group of 16-bit registers that control and monitor the transceiver chip.

The registers are loaded and read serially via the management data I/O pin by a 2-MHz clock on the management data control pin. Different codes let users enable or disable the 8B/10B encoding, select the clock mode, and enable serial or parallel loopback modes to allow testing and other operations. A 5-bit address is used to select one of 32 transceiver chips to activate the management data I/O.

The BBT3800 also has built-in self-test (BIST) features. The chip supports the IEEE-1149.1 JTAG testing functions as well as a variety of boundary scan codes. Furthermore, the chip contains an internal pseudorandom binary sequence (PRBS) generator for testing.

If the self-test is activated, the parallel PRBS word is serialized and sent to the output where a bit-error-rate (BER) tester can be used. Loopback connections are easily implemented via the management data I/O to facilitate additional testing.

High-Speed Serial Backplanes
The BBT3800 was designed to fit many of the newer high-speed applications, like Gigabit Ethernet switches and serial backplanes in the next generation of Internet routers, WAN/MAN optical networks, and terabit switches. One BBT3800 can be used to implement two eXtended Attachment Unit Interfaces (XAUI) being defined in the forthcoming 10-Gbit Ethernet standard. Many technical details have been defined so far, although publication of the completed and fully blessed standard isn't expected until sometime in 2002. One possible XAUI implementation is shown in Figure 2.

The Ethernet media-access controller (MAC) chip supplies 36 bits of data and clock signals to the BBT3800. This represents four bytes of data with a "k" control bit for each.

To achieve the 10-Gbit/s serial rate, the parallel input bus to the BBT3800—the eXtended Gigabit Media Independent Interface (XGMII)—must run at 312.25 MHz. That means very short (less than 5 cm) pc-board runs.

The serial outputs run at 3.125 Gbits/s. These are aggregated into the 10-Gbit/s serial signal by the optical interface. The serial outputs from the BBT3800 can drive a 50-Ω transmission line up to approximately a 50-cm length on an FR4 pc board.

The BBT3800 is also targeted at any application suggesting a serial backplane. A growing number of digital designers are turning to serial backplanes in place of today's more common parallel backplanes. Who would have thought that serial transmission would be faster than parallel transmission? Parallel data transmission is inherently faster, but only for distances of a few inches.

To understand the limitations of a parallel bus approach, consider this example. In a 16-port Gigabit Ethernet switch, each I/O port pair resides on one line card, supporting either one true full-duplex Gigabit Ethernet, or 10/100BaseT full-duplex links. A high-bandwidth bus needs to deliver the data across the backplane to and from the switch card.

At a full-duplex data rate of 1 Gbit/s, a parallel bus operating at 50 MHz requires a 40-bit width, while a 16-port system requires a backplane that is 640 bits wide. Accounting for extra bandwidth for clock, error-monitoring, and control signals, designers find that a parallel backplane can easily stretch to 800 or even 1000 bits wide.

Designers who need to support more ports or faster data rates could accommodate the increased bandwidth requirement by widening the bus or increasing the bus frequency. Beyond 50 MHz, upping the bus frequency usually isn't an option. This is due to the transmission-line effects caused by impedance mismatches, crosstalk, and signal skew. On the other hand, widening the bus results in higher connector pin counts, thereby increasing cost and insertion force (the force required to insert a board into the backplane connector).

Greater speeds can be achieved over longer distances with a minimum amount of noise by converting the parallel data and transmitting it over one or more serial paths. In a conventional three-state bidirectional data bus with four interface cards and connectors, bus speeds of 33 and 66 MHz are typical. Bus loads and distances must be reduced considerably for the higher speeds. In addition, bus-arbitration schemes have to be used, which further reduces throughput.

But the BBT3800 solves this problem. This chip accommodates 80 parallel inputs and 80 parallel outputs and converts them into eight serial inputs and eight serial outputs, all running at a speed of up to 3.125 Gbits/s.

Users can employ the device in a serial backplane whereby four devices or line cards talk to one another serially via a copper transmission line of up to 50 cm long, or over an even longer distance using a fiber-optic cable (Fig. 3). No contention is involved, and all interfaces are always on and capable of communicating in a full-duplex manner with any other device. Overall, serial backplanes like this one provide higher performance, are easier to scale, operate over longer distances, use fewer drivers, and enable a lower system cost.

Optical Applications
Other applications include optical fiber networks, like SONET and Fibre Channel. The BBT3800 can implement a SONET/SDH network at speeds of up to 9.953 Gbits/s (OC-192) by using four serial channels that each run at 2.488 Gbits/s (OC-48). When building line-interface cards, the BBT3800 allows the packaging of more channels per enclosure with lower power consumption.

Furthermore, the BBT3800 is made on a 0.18-µm CMOS process. It's contained in a 676-pin ball grid array (BGA) package.

Price & Availability
The nPower BBT3800 will be available in sample quantities in January. It's priced at $135 each in 1000-unit quantities. A four-channel version, the BBT3400, is packaged in a 289-pin BGA. Now sampling, it costs $70 each in quantities of 1000.

BitBlitz Communications Inc., 830 Hillview Ct., Suite 290, Milpitas, CA 950356. Contact Leo K. Wong, director of strategic marketing, by phone at (408) 586-9886, ext. 204; fax at (408) 586-9884; or via e-mail at [email protected] The company's Web address is www.bitblitzcom.com.

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