Is it possible to identify that a tag is being removed with the Capacitive Sensor of ST25R3916?

[TDumb.1]

Dear Sir/Madam,

I am using the capacitive sensor of the ST25R3916 to "wake-up" the system when a user inserts its tagRFID in the equipment. While the tag is in, the equipment distributes a liquid uninterruptedly and stops when the tag is removed when I want to debit the consumed credit of the tag according to the volume delivered. My doubt is if it possible to use the same sensor to identify that the tagRFID started to be removed? Is there a way to generate an Interrupt for wake-up and another to detect that a tag is being removed due to the ∆cm (capacitance variation)?

Many thanks for your support.

[TDumb.1]

Find attached an imagem of my application, where in yellow you can see the badge with the TagRFID

[Ulysses HERNIOSUS]

Hi TDumb.1,

yes capacitive sensor could be used for such. You just need to set a new reference value cm_ref of the measured capacitance with the card in position and define a proper delta.

The other alternative would be to perform NFC-based presence check to see when the communication breaks by the card having exited the slot.

Best Regards, Ulysses

[TDumb.1]

Hi Ulysses,

I am really impressed with your responsiveness. I really appreciate that!

Unfortunately, I cannot use the NFC-based presence check as I need to detect the moment as soon as the card started to be removed, and then perform a write cycle in the tag (with the updated consumption value) before it is out of the NFC range. I chose this procedure to avoid writing many times on the tag and extend the lifetime of the EEPROM of the tag.

So, one possible solution would be:

After the reader ST25R3916 wakes up and detects the card, I will set the new delta (cm_ref). So, I'll put it back to wake-up mode to detect the new variation(when the card started being removed) and generate a new IRQ to the MCU when I will able to send the write cycle and update it before the card is out of range of the reader.

Please, correct me if I am wrong.

Many thanks,

[Ulysses HERNIOSUS]

Hi,

yes sounds feasible. However not sure about the required timings. Capacitive wake-up is expected to be performed with field-off. So you would need to turn the field off. You could then activate the capacitive wake-up, as soon as you see a a movement you could go back to field-on, reactivate the card and issue your write request. However this will take some time during which the card will have been moved some more. Don't know how fast a card is possibly moving out of range....

A more reactive solution could be to keep the field enabled and just perform Measure Amplitude/Phase very frequently and do your own "wake-up" if the card moves then you will have a change in those values and you can immediately perform the writing. The advantage is that you can react almost immediately not relying on the wake-up timing (minimal every 10ms), avoid the 5 ms and the time to re-activate the card (which technology is the card?).

Best Regards, Ulysses

[TDumb.1]

Yes sure,

That's my main worry! Thanks for giving me these recommendations.

I'll use the tag st25tb04k (ISO 14443-2 Type B) and I2C fast Mode(400MHz) interface to keep it compatible with the previous project.

In the tests that I performed with the ST25R3916Disco, I felt that the inductive wake-up modes are not really precise for detecting a movement of a card within the RFID range. But I will do few more tests before order my prototype for validation.

Rind regards,

Theo