Programmable skew clock keeps pace with demands for lower jitter

Exar Corporation, a provider of high-performance, mixed-signal silicon solutions for the worldwide communications infrastructure today, expanded its market-proven communications portfolio by entering the market for clock distribution devices. The company unveiled a family of five new programmable skew clock components ” including an industry first: smallest package (32-pin) and the highest frequency range (200MHz) of any 3.3V or 2.5V devices.

Whether regulating data transmissions or executing system instructions, clocks perform the critical role of producing precisely timed and repeated impulses that synchronize the overall system or other embedded applications. Driving the need for better performing clocks are faster processors, faster memory and increasingly lower voltage requirements, especially in new designs. Keeping pace with these trends has put new demands on clocks for lower jitter, better skew specifications and unprecedented device reliability. Leveraging a rich history of analog and Phase Lock Loop (PLL) design expertise on a growing market opportunity, Exar has developed a series of innovative new clock and timing devices targeting wireless base stations, network switches and routers, among others.

“Technology innovation, an Exar core value, can be seen in this family of five new programmable skew devices,” said Dr. Roubik Gregorian, executive vice president and chief operating officer. “First in a series of new clocks to be offered by Exar, we have raised the industry bar for clock and timing devices by offering a robust feature set (seamless migration, simplified bill-of-materials, wide range of speed operations and more) that enables our customers to differentiate their products in the marketplace. This natural extension of Exar's Networking and Transmission offerings is built on our heritage of analog innovation and ingenuity seen in earlier generations of clock and timing devices that have performed extremely well worldwide.”

In addition to the other four new programmable skew clocks, Exar has developed an industry-first with the XRK4999. Three elements distinguish the component from competitive offerings: a 32-pin package for either 3.3V or 2.5V, a redundant input clock and the widest frequency range (3.75 to 200MHz) with a four bank skew and 250 pico second accuracy. Exar is the only company to offer these in a single solution, which is critical in designing highly integrated, small form factor components for leading edge Networking and Transmission applications.

Designers no longer have to contend with layout and/or clock tree redesigns to support industry-changing system voltage requirements. Customers do not have to carry two programmable skew clock inventories (single Bill-of-Materials [BOM]) for 3.3 or 2.5Volt applications. Exar's new devices eliminate these challenges. The XRK499x series (3.3V or 2.5V) clocks are offered in a small form factor (low-pin count), and standard pin-outs – this is in direct contrast to conventional devices that lack an easy (same form factor) migration path from 3.3Vto 2.5V. Designers are assured a seamless migration from 3.3V to 2.5V, and OEM customers can get their products to market faster than the competition.

“Exar's new devices are ideal for future 3.3V or 2.5V platforms where there is a need for the pin and functional-compatible programmable skew clocks,” said Sergis Mushell, clock product line strategic marketing manager.

The XRK499x portfolio of 3.3/2.5 Volt programmable skew clocks consists of five different products offering designers numerous options including different package sizes, faster speeds (output frequency of up to 200 MHz), plus better jitter and skew performance. Each product incorporates four pairs of programmable skew and zero propagation delay outputs that provide timing delays optimizing high-performance clock distribution circuits. The clock's skew control is guaranteed to be as low as 250 pico seconds, with selectable skew between outputs from 750 pico seconds to 18 nanoseconds. Also, all devices in the series have flexible output functions (divide-by-two, or four) which, when combined with the internal PLL, enable transmission of low-frequency clock impulses that can then be multiplied by a factor of two or four at the clock's destination. This flexibility means designers now have more options than before, and no longer have to contend with static, or fixed outputs.

The XRK4991 and XRK4991A are in 32-pin packages and have a frequency range of 3.75 to 100MHz with four bank skew, and are available with 250/500/750 pico second accuracy. In addition, the XRK4911 has a frequency range of 3.75 to 200MHz with 250 pico seconds accuracy. The XRK4993 is offered in a 28-pin package, and it has a frequency range of 3.75 to 100MHz with three bank skew and is available with 250/500/750 pico second accuracy.

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The Exar XRK499x programmable skew clocks are good matches for communications applications. The technology to build the clocks for these events is something Exar already possesses so it was no big stretch to make them for the telecom market. The family of clocks can be used in applications such as storage area networks and network switches and routers, wireless base stations, and LANs.

Many clocks perform synchronizing functions between the PC card and the overall system and they are found in a variety of electronic equipment. The typical PCB design has clocks situated all around the board to help distribute the clocking signal. There needs to be a compensation of the skew caused by the traces on the board, which is available on these Exar products. These skew clocks are unique because they have a programmable skew buffer that allows you to adjust the clock edges to compensate for delays caused by the board traces, and that's what I think will really help set Exar apart from the competition.

Exar decided to make it easy for you because many of the five devices introduced are pin compatible with industry standard devices. There are about 18 different products derived from the five products and they offer different accuracies, skews, frequency ranges, and the number of channels that you can provide skew (some have three or four).

The concept of the programmable delay is simply to enable you to change phase of the clock in relation to the master clock and to compensate for the effects of the traces in creating delay. You may want to move the clock edges around to reduce jitter and improve performance, and these devices allow you to program them. There are different granularities available and the tightest is a slick 250 pico seconds. So you can move +/-6 time intervals, which is 250 ps accuracy. However, the company expects that for now, the four bank skew product that runs at 100 MHz with 500 ps granularity probably will be the most popular version.

If you lay out a lot of boards and do a lot of time designing then you could probably know how to adjust for the timing issues, but it still takes a lot of time. These programmable clocks provide a lot of convenience for the designer so he doesn't have to spend a lot of time figuring it out. These devices can compensate for traces so you don't have to be concerned about managing all of them. Simply put, Exar helps you move the clock edges around to ensure that the timing is synchronized around the board. This is a real winner product for designers of telecom products.

Most of the designs are in 3.3 V but there is movement to 2.5V by some product manufacturers, so Exar offers both voltages. The 4991 through 93 packages and pin outs are drop-in compatible with competitive products from the likes of Cypress Semi, IDT, and Pericom in 32-pin (PLCC) or 28-pin (QSOP) packages. However, the 4999 is the only device with a redundant input that is in a 32-pin package with 3.3V/2.5V operation and offers 200 MHz frequency operation. It is not pin-compatible with competitive products; it is a proprietary device. It isn't pin compatible because the company thinks it can do business by providing additional capabilities that are not available now from other companies.

In 1,000 piece quantities, for commercial temperatures the XRK4991/91A/93CJ-2 (250 pico second accuracy) are $8.71, XRK4991/91A/93CJ-5 (500 pico second accuracy) is $6.71 and XRK4991/91A/93CJ-7 (750 pico second accuracy) is $5.15. The XRK49911 in 1,000 piece quantities (commercial temperatures) are $10.46 (250 pico second accuracy), $8.05 (500 pico second accuracy), and $6.18 (750 pico second accuracy). The XRK4999 is $11.94 in 1,000 piece quantities. The parts are also available in the industrial temperature, at 1,000 piece pricing the XRK4991/91A/93IJ-5 (500 pico second accuracy) are $7.38, and $5.67 for the RK4991A/93IJ-7 (750 pico second accuracy). Samples of all these products are available now.

XRK4999 data sheet (you will have to provide some contact info to get the data sheet, but I was able to get it after providing simple contact info.

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