ST25DV and energy harvesting

[Luca G.]

Hi to all.

I should make an application with NFC where the entire circuit is powered by energy harvesting.

For this reason I thought of using the ST25DV and I have already purchased the ST25DV_Discovery_C1 and ST25DV_Discovery_C6

On these discovery I closed ST3 to connect 3.3V with EH, I connected the pins of GND and 3.3V of connector J1 to an oscilloscope to monitor the voltage supplied, and then I approached the cell phone to the discovery with the active NFC (and with the application of the open ST).

I have set (as from the manual) the EH_MODE register to 0 and therefore I have once again allonato and reapproached the mobile.

What I expected to find from the oscilloscope is a continuous voltage whereas what I find is a signal varying between approximately + 500mV and about -500mV, except for an initial transnitory in which the voltage rises up to about 30V but is extinguished in about 50ms.

Furthermore this behavior occurs both with EH_MODE at 0 and with EH_MODE at 1, except for the initial transient that occurs only when EH_MODE is at 0.

Is it right or am I doing something wrong? Is energy harvesting only the initial transient or even the variable signal between + 500mV and - 500mV?

I also noticed that the EH_CTRL_Dyn register has in any case the first three bits (EH_EN, EH_ON and FIELD_ON) always at 1.

I hope I was clear.

Thanks in advance.

Luca G.

[JL. Lebon]

​Dear Luca,

Your tests setup seems correct: the only condition to get voltage on EH output is to enable it by setting the EH_MODE register to 0 and to present the antenna to an RF field.

If you close the ST3 jumpers ad you did, you should get voltage output (~3V) on J1 pin 2 (you can check between pin 2 and pin 14 (GND) for example).

Please,not it is better not to connect the main discovery board when trying to measure this (the white board one with the LCD), since the STM32 on board would draw too much current when ST3 is closed and  distort the measurements.

I guess something may be wrong in your measurement setup (may be scope ground ?), since it is not possible to get a 30V output out of EH. Max will be around 4V in very strong RF field. Just to be sure, how did you connect the scope ?

Best regards.

[Luca G. · Accepted solution]

Dear JL. Lebon,

thanks for your reply and sorry for delay.

It was my mistake: I connected the oscilloscope to the 3.3V pin of the J1 connector but I didn't connect the ST3 jumper. Now it works well.

If I wanted to connect ST25DV64K with a microcontroller, do I have to supply VCC and VDCG (then connect them with EH)?

Thanks again.

Luca G.

[JL. Lebon]

​Dear Lucas,

VCC is needed if you want to access the ST25DV from I2C interface. Enabling energy interface doesn't automatically power Vcc, so you need to connect Vcc to the V_EH output.

VDCG is used to power the GPO output. If you plan to use the GPO output, then you also need to connect the VDCG pin to V_EH output.

Best regards,