How does an nfc tag where to save the NDEF-message?

[xnormanw]

Hey guys,

I'm wondering how data is received from the ntag. Using arduino, I can choose which eeprom page to write. Using the NXP App, or any NFC App with my smart phone, I always only write one message to the tag. Is there a way to change said writing by eg. adding a start byte from the register I want to write to?

And how exactly does the NXP Ntag (or any nfc tag) know, which eeprom register to write to? I can't find any evidence if said register is transceived while using the app.

Thank you in advance.

Regards

Norman Wiedemeyer

[driesmoors · Accepted solution]

Hi Norman,

I suppose you are referring to writing NDEF messages? When writing one, the tag reader will try to overwrite the first NDEF message it has found when the tag was tapped.

So: if your tag contains a capability container followed by a bunch of non-NDEF TLVs (such as lock and proprietary), and then on page X onwards its NDEF TLV, that page X will be the page to write. This is the behavior when using the high-level API on Android or iOS. I assume the NXP App Tag Info uses the same approach, whether it uses the high-level or the low-level API (I don't know which).

If you want to deviate from that behavior you'll need to use the low-level APIs offered by Android and iOS, i.e. you'll need to write each and every NFC page yourself, in the correct way.

Aside: there also exist mobile APPs which allow you to write any content in any NFC page. One I know but lack a lot of experience with is the iOS APP "Smart NFC".

Kind regards,
Dries.

[xnormanw]

Hi Dries,

Thanks for your reply, this perfectly answered my question. One small thing: Which low-level APIs do I have to use? Is there a certian one, which is coming to your mind?

Regards,

Norman

[driesmoors]

Hi,

• The "high level" api would be to use https://developer.apple.com/documentation/corenfc/nfcndeftag on iOS and
  https://developer.android.com/reference/android/nfc/tech/Ndef on Android,
• the "low level" API to get an instance of your tag and write/read pages, eg. for MIFARE UltraLight compatible tags
  • https://developer.android.com/reference/android/nfc/tech/MifareUltralight and readPages/writePage for reading resp. writing on Android and
  • https://developer.apple.com/documentation/corenfc/nfcmifaretag and sendMiFareCommand/sendMiFareISO7816Command for interacting with native commands (check the datasheet) resp. with APDUs (similar to what NFC USB readers expose to a PC application).

Best,

Dries.

[xnormanw]

Thanks Dries, for the great support! Everything was answered!

[nxf58474]

Hi Norman,

Hope you are doing well. Could you please specify which tag are you using? Are you using a NFC Reader with the Arduino?

Regards,

Ricardo

[xnormanw]

Hi Ricardo,

thanks for your reply. I'm doing fine and you?

I'm currently using the npx I2C NTag plus.

Regards

Norman