I have a 32.768 kHz crystal connected to a driver chip, and I cannot get the correct oscillation frequency out of it. I have tried two crystals, and I am getting about 196 kHz at the driver output for both and I cannot figure out why. The crystal has a 7pF load capacitance, and the driver has a stray capacitance of 1.5 pF, so I used 2 parallel load capacitors of values 12 pF, calculated from CL = (C1*C2/C1+C2) + Cs. I really dont know what I am missing from this circuit to make it work, does anyone have any suggestions? These are the datasheets.

http://www.nxp.com/documents/data_sheet/74AUP1Z04.pdf
http://www.abracon.com/Resonators/ABS10.pdf

 

Parallel caps ADD. You need to put them in series to use your formula. Resistors in parallel and caps in series use similar formulae.

C=KA/T where C is capacitance, K is dielectric constant of insulator and T is the thickness of the dielectric. Effectively, two equal sized caps in parallel double the AREA where two equal sized caps in series double the thickness of the dielectric.

 

I believe you are exceeding the maximum crystal drive power of 1μW and overdriving the crystal, which can cause operation at a harmonic frequency. For 1μW maximum power, the voltage across the 70kΩ equivalent resistance of the crystal at resonance should be <264mVrms. Thus the series resistor (which you show as 30kΩ) should likely be no smaller than 115kΩ for a 2V supply.

 

Parallel caps ADD. You need to put them in series to use your formula. Resistors in parallel and caps in series use similar formulae.

C=KA/T where C is capacitance, K is dielectric constant of insulator and T is the thickness of the dielectric. Effectively, two equal sized caps in parallel double the AREA where two equal sized caps in series double the thickness of the dielectric.

But I think the capacitors can be viewed as being in series with the crystal and thus it is the series capacitance of the two external capacitors and not the parallel capacitance that should be calculated.

 

Hi, thank you. Yes I'm aware parallel caps add, however I'm confused because the datasheet (pg. 18/19) gives a circuit like the one I attached but then gives equation for series caps with the crystal and stray capacitance of the device. Am I missing something?

 

I agree that the caps to ground on each end of the crystal are in series when calculating Cload, which is the capacitance across the crystal.

 

Thanks for clearing that up for me.. A day without learning something is a wasted day!

 

The crystal is oscillating at the 3rd harmonic frequency.
Those two capacitors are too low in value. Try 33pF.

 

The crystal is oscillating at the 3rd harmonic frequency.
Those two capacitors are two low in value. Try 33pF.

196k/32,768k≈6

 

Sorry, I did the math in my head and missed the 1.

 

Thank you for the comments. I don't believe I was getting any oscillation at that resistance value of 115k and above, but I will check again. I am confused as to how my load caps are too small. Solving for CL using C1 and C2 as 12 pF as per the equation in my post, I get a value of 7.5 pF, almost identical to the 7 pF spec on the crystal.

 

Thank you for the comments. I don't believe I was getting any oscillation at that resistance value of 115k and above, but I will check again. ...............

Then try reducing the value below 115K in increments until it works.

 

I used a 1M resistor and reduced the supply voltage to the device and it worked at .9V, anything above and I lose the signal. Even if I increase the resistor value above 1M I always lose the signal at or around 1V. I would like to use this device at 2 or 2.5V, is there a reason why I can't get the signal? The datasheet says it works above 3V.

 

Any ideas why this won't swing above 0.9V?

 

Page 18 in the datasheet tells how to select the drive resistor:

13.1.1 Design
Figure 14 shows the recommended way to connect a crystal to the 74AUP1Z04. This
circuit is basically a Pierce oscillator circuit in which the crystal is operating at its
fundamental frequency and is tuned by the parallel load capacitance of C1 and C2. C1 and
C2 are in series with the crystal. They should be approximately equal. R1 is the
drive-limiting resistor and is set to approximately the same value as the reactance of C1 at
the crystal frequency (R1 = XC1). This will result in an input to the crystal of 50 % of the
rail-to-rail output of X2. This keeps the drive level into the crystal within drive
specifications (the designer should verify this). Overdriving the crystal can cause damage.

This results in a value of 405kΩ. Try 390k or 430k .

BTW, is this built on a breadboard? If so, what kind? Stray capacitance may be much higher than you allowed for.

 

Thanks for the info Ron. Those resistance values work fine, however I am not having a problem getting the right frequency output, I just can't get the output to go above about 0.9 V. Any VDD value above 1V I lose the signal, or at lower resistances the frequency increases. Bumping up the resistance value with a high voltage doesn't work either.