dynamic nfc business card

After diving into the datasheet and Arduino Library (STM32duino ST25DV) for the ST25DV NFC tags, I was able to get output from the NFC IC's GPO pins as well as output transmitted power form the Energy Harvesting output pin. However, the energy harvested power level seems to be inconsistent and sometimes very weak. Unsure if i messed up a setting in the code or if it is something else. The output was able to light up an LED at the very least.

The trick ended up being a setting written over i2c to the Energy Harvesting Config register and the GPO setup registers. Refer to the GPO and EN register sections to see which bits to set. See the code below for bits used in my example. They output energy when a field is present. 

The code looks a bit like this:

#include "ST25DVSensor.h"
#include "ST25DV_IO/st25dv_io.h" // arduino library for st25dv
void setup()
{
    // attach interrupt to GPO pin
    pinMode(NFC_GPO_PIN, INPUT_PULLUP);
    attachInterrupt(NFC_GPO_PIN, nfc_tag_cb, RISING);

    // enable Energy Harvesting output
    st25io.ST25DV_i2c_ConfigureGPO(0b10001000); 
    st25io.ST25DV_i2c_WriteEHMode(ST25DV_EH_ON_DEMAND);
}

It just occurred to me that I should have routed the energy harvesting output to the charging circuit so that I could put this device on any NFC enabled wireless charging pad and add some juice to the battery. I went ahead and tried that, however I am not sure if the Energy harvesting output is going to generate enough power. The charge light sure doesn't turn on. I will try other things at a later date.

I don't plan to order a rev2 of this board (definitely not in time for the contest) because I am contemplating changing things up for this idea and using the PN7150 reader/writer IC instead to open the door for even more NFC fun, just as someone else in the contest has also done. I have a breakout board I designed a while back using the PN7150 but didn't use it due to having mediocre antenna performance. This path seems much more interesting to me. 

Stay tuned to see how this journey goes as it changes to a read/writer as well as tag emulator. 

Did anyone really think there would be zero mistakes?

Turns out I got the display pins rotated the wrong way, oops....

The device is shown running a test sketch that cycles through some preset NDEF messages to update the NFC data with. Left and right buttons change the URI message and the center button commits the data to the IC. By using the proper URI protocol, the receiving device can recognize phone numbers to call, emails to send a message to, and URLs to open in the default browser with.

I added some jumper wires to get the display spun the right way. Looks alright enough to keep going with rev1 of this project. Adds a lot to the thickness unfortunately. In the end, this will be soldered in place as flat as possible. It looks much nicer top down where you cannot tell how tall the display sits. 

Here is a side view of the device. the battery is much thicker than the end goal but for testing this is working out well. It fits in my pocket without any problems (aside for being stabbed by the programming pins still soldered in place). The battery can be charged via the programmer's input power pin. The end goal is to find a paper thin Li-Po for the bottom side to keep the entire thickness down (I found a company "PowerSteam" that offers some viable choices). Obviously, getting the display connection correct will also help reduce that thickness. 

Unsure exactly what the issue is. Unable to get enough power through the UART TTL programmer into the charge and regulator circuit. Bypassing the charge circuit for the time being to work on programming the device and ensure all parts are working as expected. I know the programmer isn't intended to deliver a lot of power but surprised this isn't working since I used an identical circuit in other functional projects. Could be component selection. Will explore this later. Also planning to use pogo-pins for programming (4 pin header for now...)

The on off switch I chose is easily stuffed up with flux after some rework. Boot button also seems to occasionally have this issue as well. Will need to be careful when doing rework, or buy extra parts.. 

I was successfully able to update the NFC chip via i2c from the ESP32-c3 to share URL data. Hoping to spend time learning more about the various messages able to share using NDEF.  It is covered by the display once everything is soldered. Initial tests will use a board to board connector just in case I need to rework the NFC IC, however it seems to work fine, so far...

PCB's arrived. Still waiting for ESP32-C3 and ST25dv16k IC's since JLCPCB didn't have the specific part number I planned to use. Those should arrive tomorrow and I will solder them on as soon as possible. Crossing my fingers this works right away....

PCBA ordered. ETA ~5 days.