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32khz crystal selection

Jeff Labute
Prodigy 130 points
Other Parts Discussed in Thread: MSP430G2231

Hi Everyone...

Being a noob with the msp430, we purchased two launchpad kits. I noticed the 32khz crystal (MS3V-T1R) is a micro can style. Since we are short for space on an existing PCB we'd like to use an SMD crystal if possible.

Any of you forsee an issue using these crystals? I see some specs are the same, such as similar load capacitance, 20ppm, Rs is 50/70K. I would tend to think it should be fine. I am compelled to ask since working with atmel a lot I've come across uC chips that won't work without a low ESR  (low swing) crystal despite everything else being fine.

http://www.microcrystal.com/Products/Quartz-Crystals/10---2100-kHz/SMD-Ceramic.aspx

I'd be curious to know if I could use the smallest crystal on this page... the CM8V-T1A. All the specs seem the same except series resistance which is 90K instead of 50K. Uses only 0.5uW too... less than the launchpad crystal..

http://focus.ti.com/lit/an/slaa322b/slaa322b.pdf

I noticed in this document, it says that typical ESR is 30K to 100K... should be ok to use CM8V-T1A?

Thanks

 

Jeff

  • 0
    Expert 2080 points

    Concerning the quartz selection you have to calculate the safety factor (SF) wich is the ratio between oscillator strength (oscillator allowance) and the mechanical loose (equivalent serie resistance) of your quartz (SF=OAL/ESR).

    For example the 12,5pf version of CM8V-T1A have a maximal ESR of 100k, the OAL of F2252 for 12.5pF is 200k, so you minimal SF is 2, wich is considered as "unsafe", on an other hand, the 6pF still have 100k, but the oscillator have an OAL of 500k, so the minimal SF is 5 which is "very safe", so the 6pF should be ok.

    The power consomption will depend on the load capacitance (CL, lower is better), and the "dynamic" stability too (higher is better), so you have to choose what you want.

    On some microcontrollers of serie 2, there is a problem with low ESR quartz (XOSC8) a chip revision should come.

    Concerning the selection, approximatively all manufacturer have recommended crystal for MSP430, for eample:

    Microcrystal : http://www.microcrystal.com/getdoc/e712e482-1336-4ee1-a40c-b8f58bb50e26/TI_MSP430_Crystal-Recommendations.aspx

    Epson toyocom : http://www.epsontoyocom.co.jp/english/C_support/C_design/MSP430series/msp430_E.html

    You have to know that more the crystal is small, more it is sentitive to mechanical shock, depending your application it is perhaps not good to use small crystal.

    Anyway, you have to be very carefull with the oscillator circuit, it is ultra low power so extremely sensitive of external disturbance, your routing have to be very good.

  • 0 in reply to Argail
    Prodigy 130 points

    Thanks Argail!

    That makes sense. I was un-aware of selection details like this. So, the crystal we are looking at has an ESR of 70K, the msp430g2231 has an OAlf of 200K at 12.5pf so the safety factor is 2.86 which is considered 'suitable'. Is suitable safe? haha... can't believe I have to ask this kind of question. Does suitable always work? Any disadvantages to a suitable crystal rather than a safe crystal?

     

    http://focus.ti.com/lit/an/slaa322b/slaa322b.pdf

     

    Thanks :-)

    Jeff

  • 0 in reply to Jeff Labute
    Guru 227245 points

    Jeff Labute said:
    Is suitable safe?

    No, else it would be save and not suitable :)

    In theory, even a safetyfactor of 1 will work. Provided that there are no device variations, no capacitance tolerance, no parasitic capacitance on the PCB, no EMI influence, no thermal stress, no shock, no...

    An unlimited safety factor would ensure that nothing can go wrong. Reality is somewhere in between. So the more critical your applicaiton is, the higher the safety factor should be.

    'Suitable' means that it won't normally fail but can do if things go more wrong than usually. 'Safe' means that it will work unless there's smething awfully wrong (which would usually cause more havoc than just disturbing the oscillation).

    Or in other words: the higher the safety factor, the easier is it to blame someone else if it stops working.

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