Very much in the spirit of the 90-90 rule, this took much longer than I assumed.

So my goal is to put the galvinic skin response, hand temperature and SPO2 sensors onto a ring connected to a bluetooth ‘watch’, and have some phone app which can stream the data. To play around with a microcontroller which has bluetooth, I got the STEVAL-IDB008V2 dev board, which has a BlueNRG2 chip. Because I’ve only played with the Nucleos so far, I was overoptimistic about linux compatibility. On the plus side, my setup for development has gotten much more comfortable-I’m back to using emacs and makefiles, and I’ve learned how to use st-util to flash and debug and I’ve also gotten my first experience using logic analyzers with sigrok and pulseview.

The issue which held me up was actually getting the programs onto the chip. While the dev board has a JTAG header, the default application disables those pins so it cannot be programmed in that way. The alternative is to flash it with the UART bootloader, which requires the chip be booted into a specific mode and a different pair of pins.This board has a USB, but unlike the nucleos, it is not connected to a ST-Link. All of the software for flashing the chip was windows-only, and I had a lot of trouble getting that working.

Anyway, after much toil I managed to work out a way to reliably flash the chip using the windows software on a VM, but at this point I had already mostly written a bootloader using a nucleo as an intermediary. A warning on the very slim odds a reader will be stuck on the bootloader like I was. As far as I can tell, the bootloader will respond with a positive acknowledge even to invalid addresses, only to fail when receiving the data to flash. To work the last bugs out of the bootloader I was working on, I compared the output of the logic analyzer for each of them flashing the chip with the same data. This revealed that the address bytes I was using were off allowing me to fix the program.

~10usd logic analyzer that’s already worth it
Address bytes and acknowledge in Pulseview
Full writing in pulseview. First is the erase command, followed by blocks of 256 bytes.

Photoplethysmogram 3

I’ve gotten all of the sensors working at once now. One of the changes is that I’m now using a ’24’ bit differential external ADC. I’m using the NAU7802, which communicates with I2C and runs at a very slow 10-320 SPS. This is plenty fast and seems to be high enough resolution to measure GSR without any external amplifier. The ADC also has an internal PGA which hasn’t been nessesary so far.

More restricting than it could be

The spikes in graphs below for red and IR correspond to heart rate.

I’m currently using the MAX30102’s temperature sensor (which is meant for the LEDs) to measure finger temp. I will have to decide whether this is good enough.

I made several attempts dead-bug soldering the NAU7802. What worked best was to glue to face down on a peice of cardstock and then just solder to the prongs from there.