ST25R3916B – XTI/XTO Pins, Crystal Requirement, and MCU_CLK Usage

[MERSI]

Hi everyone,

I'm currently working with the ST25R3916B NFC reader IC and I have a few questions regarding the XTI and XTO pins as well as the MCU_CLK output.

From the datasheet, I understand that:

• XTI is the input and XTO is the output of the internal crystal oscillator circuit.

• The oscillator is designed to operate with a 27.12 MHz external crystal.

Now, here’s what I’d like to confirm:

1. Is the external 27.12 MHz crystal absolutely required for normal operation, or is there an internal oscillator that can be used as a fallback?

2. Regarding the MCU_CLK output: this seems to be a clock signal derived from the oscillator.

   • Is this optional?

   • It is used to clock an external microcontroller (e.g. STM32)? Do I have to use it? Many STM32 MCUs already have decent internal oscillators.

3. Are there any recommended crystal components (specific part numbers or manufacturers) that are known to work well with this IC?

Any clarification or insights from those with experience using the ST25R3916B would be greatly appreciated!

Thanks in advance!

[Ulysses HERNIOSUS · Accepted solution]

Hi,

ad 1) Yes, it is obligatory. No fallback available if you want to perform NFC comm

ad 2) MCU_CLK is optional. MCU could run synchronous to the NFC field in case this is desired, e.g. when using transparent mode

ad 3) Please check the DS and the various demos we are providing.

Regards, Ulysses

[MERSI]

@Ulysses HERNIOSUS thank you for your reply. 

How would i connect the MCU_CLK f.ex. to the STM32F030CCT.. ? On a TIM Channel?

I can't find a specific recommendation for the external 27.12 MHz crystal. The link in the evaluation board for the BOM does not work anymore (leads to an error page). 

[Ulysses HERNIOSUS]

Hi

if you don't plan to use transparent mode, then just don't connect it. I don't have a good recommendation which pin to connect to. In the past we had connected it to a pin which allowed the MCU to drive an SPI master using the MCU_CLK. I don't have any details to do such on STM32 as it was never required. 

About the crystal: For me I could get to the BOM. Please report which of the below links may not work for you:

1. https://www.st.com/en/evaluation-tools/steval-25r3916b.html#cad-resources
2. https://www.st.com/resource/en/bill_of_materials/steval-25r3916b-bom.pdf
3. https://www.st.com/en/ecosystems/x-nucleo-nfc08a1.html#cad-resources
4. https://www.st.com/resource/en/bill_of_materials/x-nucleo-nfc08a1-bom.pdf

If you followed a different link, then please let us know.

BR, Ulysses

[MERSI]

Hi,

oh thanks. 

I'have tried to open the BOM link via datasheet of the STEVAL-25R3916B Board. That still doesn't work for me. (https://www.st.com/resource/en/bill_of_materials/STEVAL-ST25R3916B_bom.pdf) 

[Ulysses HERNIOSUS]

Hi,

that link does also not work for me. Seems to be an incorrect link in this document. Please try the links which I provided.

BR, Ulysses

[Ulysses HERNIOSUS]

Please check also

AN6134: Guidelines for crystal oscillator design on ST25 NFC readers

BR, Ulysses