How to use the X-NUCLEO-NFC08A1 with 3.3V power?

[doesitwork]

Hi, I'm currently examining the X-NUCLEO-NFC08A1 as a NFC chip for our project. And I've stumbled across a problem. Our own Hardware runs predominantly on 3.3V. In theory, this would not be a problem as the Chip st25r3916b supports the 3.3V operation, but I've found no resources on how to use it.
I've implemented a startup routine, that resets the chip (0xC1), then sets the Chip to 3.3V mode (0x01,0x80) and then tries to read the Chip ID (0x7F), but the Chip ID is not returned.

As far as my understanding of the Protocol goes, there is no way to only use the Chip resp. the Eval-Board with only 3.3V. As the chip would have to be powered up with 5V and then set to 3.3V mode to use it that way. Otherwise, the Chip won't be able to hear the MCU, as the SPI signal threshold is below the minimum 3.6V, that are required for the 5V use case.

Am I wrong in my understanding or is there some sort of jumper, that I need to set?

[Ulysses HERNIOSUS]

Hello doesitwork!

I think you are looking into two unrelated issues:

1. ST25R3916B supports 5V and 3.3V operation for its transmission path (VDD/VDD_TX). However the internal logic (VDD_D via regulator from VDD) and I/O (VDD_IO) are separated from it. The STEVAL-25R3916B has even a physical switch for changing VDD between 3.3 and 5V. So no problem running it at 3.3V from the start. Please see the Minimum system configuration diagram in Figure 1 of the DS.
2. In your system ST25R3916B is not answering to SPI commands: Did you check I2C_EN pin, how is it connected? Did you verify MISO is high-ohmic from MCU side? Are VDD_IO, VDD and VDD_D there?

BR, Ulysses

[doesitwork]

Hi Ulysses

I think it is only one issue, as the SPI works, when I amplify the Signal with a Schmidt-trigger to 5V.

In regard to the first point, I'm not quite sure, what that switch on the STEVAL-25R3916B really does and how It would Influence the SPI signal or tell the Chip it has to change the to be expected high-level. It also eludes me how I could use the same logic on the X-NUCLEO-NFC08A1 Board.

BR, doesitwork

[Ulysses HERNIOSUS · Accepted solution]

Hi,

please measure the voltage levels on VDD_IO. MOSI, MISO, SCLK and CS are set/interpreted according to its voltage. Please refer to symbols VIH and VIL in DC/AC characteristics for digital input and outputs in the DS which defines the levels to be relative to VDD_IO.

The switch S100 on ST25R3916 Discovery board allows to supply the ST25R3916 with either 3V3 or with 5V (via V_RF to pins VDD and VDD_TX). The rest of the systems including VDD_IO is at 3V3. It does not change any SPI signaling.

And I can assure you that this board works independent of the setting of the S100 switch. Similar exists on the STEVAL-25R3916B board but allows to provide external supply.

I also performed now a quick experiment on NFC08 to prove operation at 3V3 with existing X-CUBE-NFC6 firmware for it:

I was bending away the 5V pin (CN6.5) and used a jumper to shorten the board's 3V3 and 5V. Resulting in all supplies being 3V3. Voila the board initializes again and reads cards.

BR, Ulysses

[doesitwork]

Hi,
Thanks to your post, I found out what my problem was.
I never used a jumper to connect the 3V3 and the 5V pins.

It works now.

Thank you very much
BR, doesitwork, yes it does