Trouble with ST95HF tag detection mode

[CSmur]

I am attempting to design a low power application utilizing the tag detection mode of the ST95HF.

When operating in reader mode, performance is excellent, I am able to communicate with tags reliably at reasonable range (approx 50mm).

However when attempting to use tag detection mode, I am seeing almost no variation in DAC output when a card is nearby. When running calibration, a value of 0x48 is always returned. When a card is touching the antenna the DAC value is either 0x48 or 0x4C.

If I enter card detection mode setting DacDataL/DacDataL with a guard of +/- 2 then I am able to detect a card if it is physically touching the antenna. With a guard value of +/-4, a card is never detected. I am not able to detect a card at any distance.

My antenna tuning has not been tested, I am purely using calculations based on the ST RFID antenna design tool, however I have been assuming that the antenna is adequately tuned because performance is excellent in reader mode.

Any advice would be greatly appreciated. Thanks!

[Peter Boringer · Accepted solution]

Dear Charlie,

You will found in st.com an application note "Optimizing wakeup time and power consumption in CR95HF and STRFNFCA devices" that develop calibration procedure, wakeup and tag detection .

​https://www.st.com/resource/en/application_note/dm00034250.pdf

Just one tip: the calibration have to be done in free air with nothing around the antenna. By adding a tag close to the antenna the calibration doesn't make sense and the DAC values should be different from the free air measurement as you will modify the antenna loading.

Please have a look on the AN 3433.

Hope it will help

BR

PB

[CSmur]

I have now better tuned my antenna, and card detection is working correctly at very short range (approximately 10mm). I will continue to improve my antenna tuning as it seems that this is definitely the problem. Thanks.