PLL Stability and Any Gotchas?


We'll be using the PLL for the first time in a product, out of necessity (the micro only allows a crystal up to 8MHz, but we need a 32MHz clock). The PLL will also be clocking a CAN module within the microcontroller.

I'd be grateful for insight into any of the following questions:

a)How do long do PLL's take to start?

b)Do PLL's ever fail to start?

c)If there is more than once choice for a crystal frequency where we can multiply it up to 32MHz (for example, 8Mhz x 4 = 32MHz, but 4MHz x 8 = 32MHz also), what are the best choices?

d)Is there anything to watch out for in the hardware or software design (any gotchas)?

e)Is there anything we can do to make the system as robust and stable as possible (i.e. decrease the probability of a PLL unlock)?

f)Can anyone recommend any books or technical papers they've found useful?

Thanks for the help, Dave Ashley.

Reply to
David T. Ashley
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"Phaselock Techniques" by Floyd M. Gardner is pretty much my bible on the subject.

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Reply to
Jim Stewart

That depends on the manufacturer. Motorola was pretty good about letting you know in their data sheet once you found the one page in 1000 that was pertinent. I would expect any manufacturer to be pretty up-front with this.

Egad! I would expect that if a chip with a PLL in it had that sort of problem you'd hear the screaming from wherever you were.

In general the higher the reference frequency the better the jitter, so your 8MHz choice is probably best in that regard. But -- do you need that sort of jitter performance?

Here again this is highly manufacturer-specific, and I would expect to find it in a data sheet.

Hardware: Pay attention to grounding and bypassing. Do what the manufacturer tells you.

Software: Don't change the PLL rate unless you _really_ know what you're doing. Set it at start up and leave that thing alone!

The PLLs in those things are, AFAIK, pretty simple and robust.

The Garner book mentioned is a good one, AFAIK. I prefer "Phase-Locked Loop Circuit Design" by Wolaver (Prentice-Hall, 1991), but only because I took the class from the guy who wrote it. Mostly what understanding PLL theory will do for you is help you understand why the manufacturer tells you what they do. The details of how to best make _their_ circuit work depends so heavily on the details of _how_ they made their circuit that you can't really influence much.

Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

Unusual Micro Spec - which one ?

Do you need a PLL ? A 32MHz xtal module could feed straight into the uC ?


Reply to
Jim Granville

If budget, PCB real state, etc. allow it, wouldn't it be simpler to use an external 32Mhz oscillator + divider to provide 8Mhz for the CPU?

Reply to
Roberto Waltman

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