How does the load capacitance of a crystal effect its clock?

rwgast_logicdesign

Ok so I have a bunch of ds1302 RTC chips and they require a 32khz 6pF crystal the ds1307 apparently requires a 12pf crystal. So hoe exactly doe load capacitance effect a clock line, I mean on the prop you can usually throw an 18 or 20pf crystal on without any problems. I always though the PPM were the big deal.

I was looking at experimenting with both clocks and just wanted to buy a set of crystals in bulk but they have different the two clock have this different pF requirement

Leon

A crystal with the wrong load capacitance will result in an inaccurate clock.

Phil Pilgrim (PhiPi)

Every crystal has both serial- and parallel-resonant oscillation modes. Which mode is invoked depends upon the the oscillator circuit driving the crystal. The series-resonant and parallel-resonant frequencies for any crystal will be slightly different. Moreover the resonant frequency of a crystal oscillating in parallel-resonant mode will depend upon the load capacitance of the circuit. That's why the required load capacitance is included in the specs for crystals characterized for parallel resonance. IOW, the crystal will work with a different load capacitance, but it won't oscillate at exactly the frequency stamped on its case.

-Phil

rwgast_logicdesign

Ya thats what I thought. So why is it when you see schematics for an arduino the crystal always has 22pf caps tied to an apparently 22pf crystal? I never see this practice on a propeller? The arduino seems to be hooked up in parrallel like phil described not series.

Also the prop manual says 5-16mhz xtal 36pF but all the crystals for the prop are 18/20pF cant remmber which at the moment.

The one last thing im trying to understand is if you were to use another chip to generate a clock for the prop, avr, rtc whatever how does load capacatine play into that equation?

Im sorry this stuff has just been bugging me for a long time and i cant really find a whole lot of information about the details here.

Phil Pilgrim (PhiPi)

The Propeller has the load capacitance built in, obviating any need for external caps.

There was some discussion awhile back about the 36pF figure. I believe that's the total capacitance (both pins), whereas the crystal spec is per pin. I could be wrong about that, though.

-Phil

rwgast_logicdesign

So do you mean you can use any crystal with the prop? I have some series crystals and they dont work with the propeller at all, I ordered them on accident. Or are you saying the the prop needs an 18pF xtal then has 18pF internally and thats where the 36 figure comes from?

RobotWorkshop

Here are some more references about crystals and using them:

http://www.maximintegrated.com/app-notes/index.mvp/id/726

http://en.wikipedia.org/wiki/Crystal_oscillator

Tracy Allen

In a Pierce oscillator like the one here, the load capacitors C_G and C_D are effectively in series. Go around the loop with the xtal and the two capacitors. Ignore the connection to ground, which provides a DC reference point for the CMOS amp and R_A that feed energy into the circuit to keep the oscillation going.

The oscillator in the Prop is of this type, and C_G=C_D=36pF. Make each of those 40pF to account for the capacitance of the pin and outside wiring. Two 40pF capacitors in series is 20pF. When you buy a 5MHz crystal that states a C_L load of 20pF, you should find that it oscillates in fact very close to 5MHz. If you buy a crystal that is specified at 12pF instead of 20pF, that would oscillate a few or 10s of ppm higher than the specified 5MHz, but likely still within tolerance. The manufacturer of the crystal designs it and calibrates it to have the stated frequency when loaded with the stated capacitance. You can go to them and ask for what you want (for a price). For example, if a standard crystal is listed in the catalog as 5MHz 12pF, you can call them up and ask if they can do 5MHz 20pF in the same package, and they will provide a quote for as many as you want!

rwgast_logicdesign

@robotws, thank you the maxim link was a bit over my head but both full of good information.

@tracey ok that cleared up most of my confusion with how the prop interacts with crystals

so this is the basic jist of it if i understand right. i thought load capacatence had something to do with how much energy the crystal stored. load capacatence is acually just a property of the crystal that tunes it to the exact frequency when it is matched with the specifed amount of capacatence, this is only relevant in a circuit where the xtal is parallel with the oscillator though. chips like the propeller and ds1302 have the caps built in while chips lile the avr omit the caps from there silicpn.

so in a situation where your using a chip or oscillator to clock the propeller you wouldnt worry about load cap as it only tunes a crystal. you would then connect the out put of the clocking chip or oscillator to the xi pin only becuase there is no reason for these to get the feedback from the xo pin?

the one thing im confused about still as far as the prop is it says you can connect a crystal to the xi pin only. would this be a series resonant circuit? why would someone take this path and why dont you see alot of prop schematics that do this? i would guess becuase parallel resonence is more accurate?