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Original question:

ADS131A04: crystal oscillator not oscillating

ADS131A04: sharing external crystal oscillator with the ADS converter and MCU

charles Duruaku
Prodigy 190 points
Part Number: ADS131A04

Good day I may have committed a bit of a blunder on my PCB for the ADS circuit. In an attempt o achieve max sync I routed the oscillator pins (24 and 25) in parallel with the processor oscillator pins . I just figured this affects the load capacitance and will certainly affect the crystal operation. SO please I have attached the  schematic and PCB below as i yet to couple it i wanna know how I can calculate/estimate the capacitance on the lines and possibly reduce the attached   external capacitors to mitigate the effects of the additional IC pins I also took a queue from the evaluation and made an option for external clock. You can notice its routed fro two layers I am already on 8.2pF which is far from the recommended 20pF. I am coupling now though just need pointers for problems ahead.

THanks

  

  • 0
    TI__Guru 63196 points

    Hello Charles,

    Thank you for your post and Happy New Year!

    The design may still work as you currently have it, but it is generally not recommended to share a crystal in this fashion. A crystal oscillator, on the other hand, should be able to drive multiple clock inputs.

    Do the outputs of the crystal, XTAL11 and XTAL21, go directly to the MCU? If you notice the crystal has difficulty starting up correctly, perhaps you can try uninstalling the 0-ohm jumpers J1 and J2 and provide an external clock input for the ADS131A04 on P1[1]. This will remove some of the parasitic capacitance which may be loading the crystal oscillator output.

    Regards,