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MSP430F5436A: randomly LF crystal stalled at power up

MichRFT
Expert 2110 points
Part Number: MSP430F5436A


Hello,

We use MSP430F5436A and LF crystal CM7V-T1A-12.5pF for the 32.768kHz..

With some boards and sometimes we face stalled LF crystal at power up. A reset or a power down then up does not change anything. The same board can start correctly a few hours after then stay stalled the day after...

Reading the document slaa322d.pdf, at chapter 2.1 I can see: "Hence, if using two equal capacitors, the values of these capacitors must be twice the required load capacitance."

So with XCAP configured at 12pF inside the MSP430F5436A, can I consider that the CM7V-T1A-12.5pF is the correct choice or must I use CM7V-T1A-7.0pF ?

Best regards and many thanks for any suggestions

Mich

  • 0
    TI__Guru 56685 points

    Hello Mich,

    The section you found in SLAA322 explains this the best as you have found. You need to match the effective load capacitance of the crystal. When using external capacitors, that equation is given to get you to match the effective capacitance of the xtal.

    Now, the XCAP settings inside the MSP430F5436A are effective load capacitance equivalents. See datasheet snippet below. So the setting you would use would need to match the effective capacitance of the xtal being hooked up. The highest setting has a typical value of 12pF, so it could be a little high or a little low. Your board parasitics come into play as well, see notes in datasheet as what we utilize is an estimated 2pF (common). 

    Now you are trying to match a 12.5pF effective load capacitance while utilizing a 12pF setting. That can be enough to cause you issues, especially taking into account variation of the specification, and board parasitics. 

  • 0 in reply to Jace H
    Expert 2110 points

    Hello Jace,

    So should I consider that the term "effective load capacitance" means the result of the 2 // capacitors?

    Sorry I did not see your "datasheet snippet below" ...

    If 12pF is the appropriate choice for 12.5pF crystal, I do not think that 0.5pF could be the issue. What's more I check references given in the slaa322 document at table 3 and they are 12.5pF crystal but not 12pF and the msp430F54xxA does not offer the 12pF choice with XCAP so crystal and microcontroler designer missed something?

  • +1 in reply to MichRFT
    TI__Guru 56685 points

    Hello,

    MichRFT said:
    So should I consider that the term "effective load capacitance" means the result of the 2 // capacitors?

    Yes, but this is saying it a little backwards. It's more of you need to balance the capacitance the Xtal is seeing to match its effective load capacitance value. Its impedance matching basically. 

    Sorry about the missing snippet. Here it is below:

    A 0.5pF difference for effective load capacitance can be significant. Take the equation from the app note and plug in 12.5 instead of 12. You arrive at needing 24pF caps instead of 22pF. Depending on board layout or environmental factors, you can certainly have issues with xtal startup. The 12pF XCAL setting within the MSP430F5436A device, is the appropriate setting for a 12pF effective capacitance xtal. If utilizing a 12.5pF Xtal, you would need external caps to properly balance the circuit. There are many choices of Xtals with different load capacitance values. When this part was designed, the integrated caps where put on certain values to help address the bulk of the choices for xtals we saw at the time. You can of course use other xtals with different values with the addition of external capacitors. 

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