Anybody have experience with low power crystals?

[Space Varmint]

I got a couple of 32KHz small crystals. What I fear is that I may have blown them some how. Do they blow easy? I have not ever destroyed a crystal so don't know. What I did was hook it up in a 74HC4060 and it appeared to resonate but might have been an over tone. I walked away for a bit then came back and it was dead. Just in case, I built a low power oscillator and appeared to get the other one running. So I was trying to check the other crystal and did some de-soldering and left the power supply on. Well the other crystal did not work, then I put the 2nd one back in and it didn't work anymore. Does anyone have a known good circuit for them to check these crystals? These are the small one lie what they use in a watch.

Thanks

 

[Nigel Goodwin]

32KHz crystals are easily damaged, they operate at very low power levels, so feeding them as a normal crystal will kill soon them.

 

[Space Varmint]

Shoot! That's what I was afraid of. Thanks Nigel. I looked and looked and could not find that info anywhere.

 

[schmitt trigger]

Unless you use a low voltage supply V< 3, you need to add a resistor ~4.7k to 10k to the output of your 4060 to limit the drive into those crystals.
To learn this trick, I also damaged a couple of those.

I also have read -and used- unbuffered inverters 4069U for these crystals, and they appear to be less susceptible to damage, although the evidence is limited.

 

[audioguru]

The datasheets for the crystals show recommended oscillator circuit using ordinary Cmos (not HC high power Cmos) and the recommended current-limiting resistor value for feeding the crystal.

 

[Space Varmint]

Thanks fellas. I guess I had to learn the hard way this time. Thanks to Nigel I did not have to look anymore to see if that's what actually happened though it did appear to be that way. So Schmidt? I actually use Schmidt triggers for square-wave oscillators from time to time. But you think I can still use the '4060 oscillator then if I use the right series resistance? That would be great since I am after low chip count as well as low power.

 

[HiTech]

Sell them to a new-age crackpot who's into crystal healing power!

 

[Space Varmint]

LOL! What's up HiTech?

 

[RCinFLA]

Here is typical specs for 32 KHz xtal. Notice the 1 uW max drive level. You have to be very careful about stray capacitance of the PCB layout which can change the cap divider ratio and thus the drive level, not to mention the extra capacitance load causing error in freq (lower in freq then expected). Also because the input is very high impedance you have to be careful about stray noise pickup. Worst is routing a squared up 32 Khz drive output near oscillator input pin.

Nominal Frequency 32.768 kHz
Make Tolerance +/- 30 ppm
Temperature Stability -0.038 ppm /C^2 (inverse parabola freq vs. temp)
Series Resistance 80 kOhms
Maximum Drive Level 1 uW
Operating Drive Level 0.25 to 0.5 uW
Nominal Load Capacitance 9 pF
Pin-to-pin Capacitance 1.3 pF
Aging 3 ppm/year

 

[kchriste]

Here is an example from the 74HC4060 datasheet, though they don't specify the xtal frequency:
(Click on image to enlarge)



And from the original 4060 datasheet:



And finally, this datasheet has a detailed explanation on how to choose the value of the power limiting resistor.

 

[Space Varmint]

Why thank you so much kchristi (spelling). You know who I mean. Thanks allot! I did not get that data sheet. That was my next question was would I be able to drive the TTL level logic using that low power crystal. This is great because I have my new ones on the way.

 

[kchriste]

That was my next question was would I be able to drive the TTL level logic using that low power crystal.

The 74HC series is not TTL. It is a high performance CMOS version of the old TTL chips. You can use the 74HC4060 to drive 74LS TTL chips though.

 

[Space Varmint]

Well I know that. I just meant logic level. I understand they are lower power. What is the term now days? CMOS logic maybe?