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XTALO pin in designs with no crystal

Thread Summary

The user is using an external oscillator for the ADAU1787's CLK instead of a crystal. The final answer confirms that the XTALO pin can remain unconnected and the XTAL_EN register should be disabled to prevent RFI emissions. It also advises ensuring the external MCLK is supplied early and waiting for the PLL to lock before writing to registers, as detailed in the datasheet's System Clocking and Power Up section.
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Category: Hardware
Product Number: ADAU1787

Hi guys, 

Me and my team have been working on a custom design using the ADAU1787 with a Nordic processor and Zephyr RTOS. 

Our driver works fine and we can load and use the ADAU normaly. Nevertheless, in some of our PCBs we had some problems and therefore we are double checking everything in the design once again. 

For the ADAU, we decided to use an oscilator for CLK instead of a crystal for saving size. Our XTALI pin is connected to the osciliator's output pin and we have have nothing connected to the XTALO pin. In fact, the XTAL_EN register was enabled until one hour ago when I realized it. 

When not using it, should the XTALO pin be connected to anything? Would there be any problem if we had nothing connected to it and the XTAL_EN register was set? 

Thanks in advance.

Cheers,
João Colombari

Parents
  • Hello João.

    No, it is not a problem to have nothing connected to the pin and you should disable it to help prevent and RFI emissions from the part. The unterminated pin with a clock on it is a nice antenna!!

    The purpose is to make the crystal oscillate so since you are just using an external oscillator you do not need it. 

    In your testing make sure the external MCLK is supplied early enough and you wait for the PLL to lock before trying to write to all the registers and memory. This should be detailed in the datasheet. Actually it is in the System Clocking and Power Up section early in the datasheet. 

    Thanks,

    Dave T

     

Reply
  • Hello João.

    No, it is not a problem to have nothing connected to the pin and you should disable it to help prevent and RFI emissions from the part. The unterminated pin with a clock on it is a nice antenna!!

    The purpose is to make the crystal oscillate so since you are just using an external oscillator you do not need it. 

    In your testing make sure the external MCLK is supplied early enough and you wait for the PLL to lock before trying to write to all the registers and memory. This should be detailed in the datasheet. Actually it is in the System Clocking and Power Up section early in the datasheet. 

    Thanks,

    Dave T

     

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