ST25R3911 Capacitance measurement

[RFlod.2]

I have RFID reading with ST25R3911 working now with RFAL and  I would like to sense if there is a RFID tag nearby before starting RFID reading.

I did not find any example code in RFAL. I looked into data sheet and tried to read the registers:

st25r3911ReadRegister(0x3C, &c);
printf("CapA %d\n\r", c);
st25r3911ReadRegister(0x3D, &c);
printf("Cap %d\n\r", c);
st25r3911ReadRegister(0x2B, &c);
printf("Volt %d\n\r", c);

but I just get zeros:

CapA 0
Cap 0
Volt 176

How shall this be done?

[Ulysses HERNIOSUS]

Hi,

Capacitance measurement is a feature of the Wake-up system. Best if you get the ST25R3911B-DISCO whose GUI has a tab for experimenting with it. Also the related AN4985 will give some explanation. 

From RFAL perspective you have the following entries:

• st25r3911MeasureCapacitance()
• st25r3911CalibrateCapacitiveSensor()
• rfalWakeUpConfig.cap

Please beware that this feature will also react to water/human body and is more susceptible to emission from other radios / fluorescent lamps, etc. than the inductive wake-up system.

BR, Ulysses

[RFlod.2]

I tries the functions and they seem to work. But I get very small changes (on ST devkit) like :

No card: 140

One card:  137

Another card:142

When using 

st25r3911MeasureCapacitance(&c);
printf("Cap %d\n\r", c);

Is this expected?

[Ulysses HERNIOSUS]

Hi, 

which devkit are you using?

BR, Ulysses

[RFlod.2]

X-NUCLEO-NFC05A1

[Ulysses HERNIOSUS]

Hi,

I was not expecting such results. But in my quick experiments I was also not able to get it working as expected X-NUCLEO-NFC05A1. Likely this needs some more investigation.

But I tried again on ST25R3911B-DISCO where this feature was mostly evaluated and there it works within its boundaries. I recommend you get one yourself and use this as basis?

BR, Ulysses

[RFlod.2]

What values do you get with  ST25R3911B-DISCO using st25r3911MeasureCapacitance  when card is present/not present?

Would be nice to get it work on NUCLEO-NFC05A1 since I alredy have it. Maybe changing capacitators and/or resistors?

[Ulysses HERNIOSUS]

Hi,

I have done quick analysis and didn't spot any difference to the ST25R3911B-DISCO schematic and BOM. So, this will need some detailed analysis. Sorry, but I cannot promise a short term solution for the NFC05. The value I am getting there is 116 at the settings shown here:

BR, Ulysses

[RFlod.2]

Can you show difference from 116 when a RFID card is close to reader?

[Ulysses HERNIOSUS]

I first approached with my hand and then with a small ~22mm ST25TV02K.

BR, Ulysses