Reduce power consumption PN7150 while tag present

[mwernsen]

Hello,

For one of our products we are making use of the PN7150 in a low-power environment. In the product application a tag (ntag212, NFC-A) is presented and has to be read once, after which removal has to be detected. The tag will be present for most of the product awake time time, so low power usage with the tag present is important.

Without the tag present, energy consumption of the PN7150 is low while waiting for tag discovery in DISCOVER mode. However, after tag discovery and reading, energy consumption remains high in the POLLING mode while waiting for the PRESENCE_CHECK function to return, informing us that the tag has been removed.

What options are there to detect tag removal without remaining in POLLING mode? I've tried solutions using starting and stopping discovery modes, however this didn't provide a reliable solution.

Greetings,

Mathieu

[Kan_Li]

Hello @mwernsen ,

Just wondering if the notification of card removal should be real time or allow some delay. Indeed a loop-back transition on state RFST_POLL_ACTIVE, corresponding to the RF_PRES_CHECK_CMD which can be sent by the DH to know if the Card/PICC is still in the field. so the device stays in RFST_POLL_ACTIVE in that case.

Have a great day,
Kan

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[mwernsen]

Hello @Kan_Li,

Thanks for the response. Sadly only a minor delay (~1s) is acceptable for our application as the nfc reader is used for near-realtime presence detecton of different objects.

Ideally we are looking for a solution that does not involve the loop-back transittion on state RFST_POLL_ACTIVE using RF_PRES_CHECK due to the high RF power consumption in this state.

Our current solution makes use of the RF_DEACTIVATE_CMD (Discovery) right after the tag has been read. Tag presence detection is then done by waiting on a RF_DISCOVER_NTF notifcation with a timeout (1s). We have however noticed that in some environments this timeout of 1s is not enough to receive the notification eventhough the tag is present (false negative), i.e. tag activation takes too long. We did however see a major power improvement using this loop-back.

So:

1. Is there a way to reduce power consumption in RFST_POLL_ACTIVE mode, taking into account that we are not interested in writing or reading, just simply checking if the tag is still present?

2. Can our custom loop-back solution be improved by e.g. updating the polling times and frequency during discover mode? So to prevent the false negatives.

Hope everything is clear!

Greetings,

Mathieu

[Kan_Li · Accepted solution]

Hi @mwernsen ,

Thanks for the clarification! 

Indeed, the PRESENCE_CHECK mechanism implemented in the NXP-NCI example is keeping the RF field ON (which is significantly power consuming) and continuously send command until the tag is removed.

One possibility to implement a lower power consuming presence check procedure after the tag has been read is the following

1. stop the discovery loop: NCI CMD 21 06 01 00

2. wait for some time (trade off between power consumption optimization and latency for the tag removal detection)

3. restart the discovery loop only with the technology of the detected tag

  o if the tag is detected -> verify the detected Tag UID match the previously detected Tag (only possible if Tag is not using Random ID)

    - if UID match, restart from step 1.

    - if UID doesn't match, stop discovery loop and restart the complete scenario with the newly detected Tag

  o if no tag is detected in short time (depends on the TOTAL_DURATION value) -> Tag was removed, stop discovery loop and restart the complete scenario for next Tag detection

Hope that makes sense,

Have a great day,
Kan

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