Why Programmable Clocks Are Better than Crystal Oscillators (XO)

Yimu Guo
Timing Product Manager

It’s probably time to update your clocks...

Traditionally, crystal oscillators (XOs) have been the most commonly used timing components. However, with the advanced silicon technology available, it’s more and more common for designers to use a programmable clock generator to provide the clocks in their system. The diagram comparison here shows the space savings such a change can provide without sacrificing performance:

programmable-clock-vs-crystal-oscillator-xo.png

The advantages of using a programmable clock solution are quite clear when you examine:

Space Savings

  • As seen in the diagram above, there’s plenty of space saved here.

Flexibility

  • A programmable solution isn’t just limited to the common frequencies. It can generate any frequency within the frequency range. This saves you the time of going shopping for custom oscillators, which usually require a significantly longer lead-time.
  • One part can drive multiple logic types (LVDS, HCSL, LVPECL, CMOS, etc.) and different voltages.

Lower BOM Cost

  • When replacing multiple crystals or XO’s (typically more than two), a programmable silicon timing solution is usually better in terms of BOM cost; especially if replacing several custom frequencies and frequencies higher than 70 MHz.

Reduce Cost of Ownership and Logistics

  • With the programmable solution, you can qualify the device once and reuse everywhere else. The qualification effort is much lower and you’ll have far fewer components to manage on your approved vendor list.

There are a few concerns we hear from customers, mainly regarding EMI and layout efforts:

  • When the clock generator cannot be close to all target inputs and relatively long clock traces are needed, there are concerns with system EMI. Tests done in our Timing Design Center have shown that programmable IC solutions are often the cleaner solution when IC bypassing is done properly. See the app note Layout Considerations for VersaClock® Products for test plots and more details about layout considerations.
  • IDT’s VersaClock products also have spread spectrum clocking to reduce EMI. This would save additional effort in designing shielding mechanisms.

At first glance, an XO might seem like an easier solution, but really, the programmable clock offers a lot more flexibility and can ultimately lead to a more elegant solution for your design.

For more technical details on designing with programmable clock generators, please see our application note Programmable Clocks vs Crystal Oscillators.

Visit idt.com/versaclock for more details on the VersaClock family of programmable clock generators, including samples and evaluation kits. 


 

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