Electronic Design

Field-Programmable Crystal Oscillator Beats The Clock On Delivery

Based on a programmable PLL, these units cut waiting times drastically whole supplying outputs with vey low jitter.

The old adage that "timing is everything" applies doubly for crystal oscillators. Used in countless applications, these workhorse components have more or less become commodity items. Also, they're often among the final pieces of the design to be specified. So with product-development cycles shrinkinxg, the timing of their delivery has become as important as the precise timing of their output signals.

Unfortunately, crystal or clock oscillators typically have very long lead times. According to James Magos, vice president of sales and marketing at oscillator-maker Cardinal Components, oscillator delivery ranges from four to 22 weeks for standard, fixed-frequency products and two to four weeks for some factory-programmable types. Now, Cardinal Components seeks to eliminate the wait entirely by introducing a field-programmable oscillator called the CP-2000.

Using tools supplied by the manufacturer, the oscillator can be programmed in a matter of seconds. The factory, Cardinal's distributors, or the customer can perform the programming.

By keeping a stock of unprogrammed units (or "blanks") on hand, designers will be able to immediately configure their oscillators as needed for prototyping. Alternatively, if customers desire preprogrammed samples, they can expect immediate delivery from the vendor or 24- to 48-hour delivery from Cardinal's distributors.

The company says that preprogrammed units in production volumes are delivered in three to seven days. Blanks are immediately available. Customers, then, may opt to program a large batch in-house, considering that 1000 units might be configured in just a few hours. And because the oscillator is twice-programmable, units in production can be reprogrammed once if the design requirements change.

The CP-2000 generates frequencies in the 1.0- to 133-MHz range while providing performance comparable to that of other clock oscillators (see the table). In addition to these specifications, the oscillator boasts very low jitter. Al-though the data sheet specifies output jitter at less than 100 ps p-p, the company claims that units built to date exhibit only 58 ps p-p maximum.

The oscillator is built around a second-generation Cypress Semiconductor chip, the CY2037, which is an EPROM-programmable PLL for crystal oscillators (see the figure). The Cypress PLL contains a 12-bit multiplier and a 10-bit divider that determine the value of the PLL's output frequency according to values programmed into the feedback (P) and reference (Q) counters.

The PLL output frequency is determined by the formula:

where FREF is the crystal frequency. The final output of the oscillator is FPLL/divider, where the divider can take on a value of 2N for N = 0 to 7.

Tuning the chip is essentially a two-step process. The software used to program the oscillator must solve the above equation for P, Q, and a divider that will roughly tune the oscillator to within ±150 ppm of the desired output frequency. That tolerance represents the crystal's pull range. Once the PLL puts the frequency within that range, an array of switched capacitors can be configured to fine-tune the oscillator.

After the tuning parameters are determined, they are programmed into the chip's EPROM along with duty-cycle and power-management selections. For duty-cycle levels, designers have a choice of TTL or CMOS. Power-management options permit operation in either a power-down mode or an output-enable mode. Either choice can be programmed to occur synchronously or asynchronously with respect to the oscillator's output.

The company offers a standalone programmer (PG-2000P), including a keypad and an LCD interface, priced under $1800. The unit is small enough to qualify as portable, and it operates off of a universal ac input. A second option is a lower-cost PC-based programmer (PG-2000) that requires an external frequency counter with an HPIB interface. Priced under $500, this tool features a Windows-compatible interface to prompt the user. With either setup, programming time is less than 5 seconds per unit.

Programming is fairly simple, but there is one caveat. If the desired output is related to the crystal frequency by a fractional multiple (that is, rational fractions like 1/1, 2/1, and 3/2), then the programmer may not be able to find values of P and Q that bring the PLL within the oscillator's fine-tuning range. So in some cases, it may not be possible to tune a specific unit to the desired frequency, depending on the tolerance of that unit. To lessen the likelihood of such an occurrence, the company chooses a nonstandard frequency for the reference crystal. Thus, the chance of finding a unit that cannot be tuned to the desired frequency is less than 1 in 1000.

The company also plans to develop programmable voltage-controlled and temperature-compensated crystal oscillators (VCXOs and TCXOs).

Price & Availability
Suggested pricing for the metal DIP versions of the CP-2000, which are now available, is $1.85 each in quantities of 1000. Surface-mount versions, including 5- by 7-mm outlines, will be introduced in the second and third quarters of this year, priced from $1.75 each. For volumes greater than 50,000, prices fall to about $1.10, which is said to be near that of standard oscillators. As production ramps up, the premium for the programmable oscillator should disappear.

Cardinal Components, Wayne Interchange Plaza II, 155 Route 46 West, Wayne, NJ 07470; James Magos, (973) 785-1333; [email protected]; www.cardinalxtal.com

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