Dear
I've 2 stupid questions related to crytstal
From my understanding, crystal is used to generate CLK for MCU. But what is the relationship between crystal and PLL? And what can be measured in the crystal pin when the MCU is in reset or run state?
Also I would like to know how to read crystal value marked on the crystal
Thanks
JC
A mastery of understatement. You need to learn to read datasheets.
Maybe.
Maybe nothing. Does your micro have a PLL? Does it have a DCO? Are you using either, or are you using the crystal as a raw clock source?
There is no way of answering that generically.
Crystals are generally marked with characters from the Roman alphabet and digits from the Arabic numeral set . In rarer cases you might find letters from the Cyrillic alphabet .
Those are the generalities; of course for specifics you would refer to the crystal manufacturer's datasheet, would you not?
The MCU uses the crystal as the frequency determining element for its clock generation circuitry. The crystal itself is a passive device, and does not generate anything directly.
Whatever the MCU manufacturer makes it. To find out, read the MCU data sheet -- it varies wildly by manufacturer and part, so there is little generic information to be had. At best, I can promise you that usually if the MCU has a PLL it's output frequency can usually be related to the crystal oscillator frequency by some integer or rational ratio. That's not very helpful, is it?
That depends on the MCU. Generally, not much, because unless you take pains to do it right, attempting to measure oscillator behavior at a crystal pin will stop the oscillator. Getting accurate measurements of the oscillator behavior without changing the oscillator behavior significantly usually involves board mods.
That varies with the manufacturer, too. It may be a part number, it may be the design frequency of the crystal -- but it usually doesn't include the full information you need to use the crystal in a circuit.
-- http://www.wescottdesign.com
A PLL requires a standard frequency with which to compare the generated frequency. That's the function of the crystal. For reading, see the manufacturers data sheet.
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[mail]: Chuck F (cbfalconer at maineline dot net)
[page]:
Try the download section.
The use of crystals is poorly understood by most digital engineers. It turns out that there are a number of parameters that need to be matched to the MCU, but most MCUs don't spec those parameters. Sometimes you can get the values for these parameters from the MCU makers, if you ask. Less often they are in the data sheets.
If you want to learn about crystals, go to the web sites of multiple crystal makers and read all the data sheets and app notes you can find. Some are good enough that you can actually learn a few things from them. That is how I learned, that and also asking dumb questions of the crystal makers. I learned nothing from the MCU makers, but I have taught a few a bit.
Another approach is to use an oscillator instead of a crystal. Then you only have to worry about the power supply voltage, frequency and a few parameters on stability if it is important to your app. You also need to read the MCU data sheet to see how to connect an oscillator and what range of frequency is acceptable.
Rick
Fundamental mode crystals are made for frequencies up to about 20 MHz. In a typical MCU, the crystal connection is just an internal inverter and the crystal is connected across the inverter input and output with some small (about 30 pF) load capacitors from both ends to ground.
In a proper external oscillator circuit, the crystal can be forced to oscillate at an odd overtone (usually 3rd, 5th or 7th) thus generating frequencies up to 60, 100 or 140 MHz, by including a frequency selective element (usually an LC circuit) tuned close to the desired harmonic. The generated frequency can then be connected to the MCU oscillator inverter input.
If the MCU requires over 140 MHz clock frequencies, these can not be generated directly by the crystal, but some form of frequency multiplication needs to be done. This can be done with a traditional frequency multiplier chain consisting of non-linear elements and LC filters, but is much easier to integrate a PLL on to the MCU chip itself. Assuming an external fundamental mode 20 MHz crystal and an internal PLL with a divide by 16 divider, a 320 MHz system clock frequency can be generated.
Paul
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