Before You Begin
There are a few choices you need to make up-front about which IDE you wish to use for development, plus which development framework. I have chosen Microsoft's Visual Studio Code since it is cross-platform, feature-rich without hogging resources, free to download, and based on an open-source product similar to how Google Chrome is derived from Chromium. It comes with extensions for language support and IntelliSense, which are must-haves when authoring code. You are free to use an IDE of your choice; it won't really hamper your ability to enjoy the rest of this article.
The other decision lies in your development framework. I investigated two choices -- the Arduino framework and ESP-IDF. When doing my research, mostly back in the summer, I found several posts where people indicated running into problems with support for various pieces of the Bluetooth stack. Plus, I decided to be closer to the bare metal and have less abstracted away for me, so I went with ESP-IDF. Now if you don't go along with this choice, you may not find much value from the rest of this article. But you're welcome to look at these two alternatives for what you want to write your code.
Anyway, if you choose the VS Code with ESP-IDF, then you can follow along with this Instructable that details how to set up the entire toolchain: https://www.instructables.com/id/Develop-ESP32-With-PlatformIO-IDE/ Despite one commenter that complained loudly about the poor quality of the Instructable, I didn't find it too troublesome to follow along with. You might just need to look up what any missing pieces are called nowadays, as some things have changed, but I can assure you I got through setup just this past weekend with mostly just that article.
Strange Things You May Encounter
After getting a good way through the setup steps, it was time to compile the code for the first time. Unfortunately, some odd errors appeared that indicated I hadn't defined some constants and some functions. I Googled for the missing constant name and eventually found it on GitHub, and saw that it was missing from my local copy of the header file (which leads me to believe the Instructable or the accompanying example code must be being updated). Fortunately, it is easy to update your development library in case you end up getting an old version and finding that such library code is missing.
From the VS Code Command Palette (Ctrl+Shift+P or Cmd+Shift+P), just look for:
PlatformIO: Update All (platforms, packages, libraries)
Or, from the command line/terminal, write:
Once I updated the library code, I was able to compile the code successfully. However, when using the toolbar at the bottom left of the VS Code window, it is not always apparent which terminal window the command you selected from the toolbar is actually running in. Whenever you click on the toolbar, it seems to generate a new Terminal instance that's selectable from a dropdown at the top right of that window pane. Usually, once the command is done running, you can close that terminal instance by pressing a key, but you are always left with one PowerShell instance from within VS Code where you can run your own commands.
After uploading the binary to the Sparkfun Thing for the first time, it displayed nothing but garbage on the serial terminal and didn't show up in the list on my Android's copy of the Nordic nRF Connect BLE scanner app. This compelled me to reinstall the PlatformIO Core and the platforms/packages/libraries again, especially since after the first time, it apparently failed to remove a file due to a "Permission denied" error. After seeing an error about something basic being missing once more when rebuilding, I did what you do with any Microsoft product to fix it -- you restart it. A quick restart of VS Code fixed the problem, and now I was able to rebuild the binary once again without problems.
There was another problem when building the binary: all of my directory paths involved with this project have spaces in them, and the tool did not end up putting quotation marks around some of these. As such, I would see errors such as:
esptool write_flash: error: argument <address> <filename>: [Errno 2] No such file or directory: 'd:\\Programming\\BriteBlox'
Fortunately, a bit more Googling allowed me to find out a command that would reveal the command that caused this error:
pio run -t upload -v
Here is the command:
"c:\users\user\.platformio\penv\scripts\python.exe" "C:\\Users\\user\\.platformio\\packages\\tool-esptoolpy\\esptool.py" --chip esp32 --port "COM6" --baud 921600 --before default_reset --after hard_reset write_flash -z --flash_mode dio --flash_freq 40m --flash_size detect 0x1000 D:\\Programming\\BriteBlox Wearable ESP-IDF\\.pioenvs\\esp32thing\\bootloader.bin 0x8000 D:\\Programming\\BriteBlox Wearable ESP-IDF\\.pioenvs\\esp32thing\\partitions.bin 0x10000 .pioenvs\\esp32thing\\firmware.bin
Now to make the modifications, change the full path to Python at the beginning to just python (it seems to freak out with a fully-qualified path), then add quotes as needed. Run the command in your own terminal, and the code will deploy as desired. Meanwhile, this Github issue seems to indicate a fix might be imminent.
Alas, rebuilding the binary did not solve the problem of garbage coming through my serial monitor. I Googled around for this some more, and found out that the Sparkfun Thing was running at the wrong frequency, or at least a different one than expected. It runs by default at 26MHz, but the development platform expects that the device is running at 40MHz. As such, by taking the device's baud rate of 115200, and multiplying by 26/40, I was able to find the true baud rate: 74880. By opening up RealTerm on COM6 and entering 74880 as the baud rate, I was able to see the expected serial output from the Sparkfun Thing, instead of garbage finally:
BLE Advertise, flag_send_avail: 1, cmd_sent: 4
Now, to solve the mismatch in expected vs. actual frequency, you could either change the crystal to the correct frequency or adjust the development framework to work with the crystal on the board as it is. In this case, I chose the latter approach. Many people write about running make menuconfig in the root directory of your project in order to adjust the board settings, but that only seems feasible in Linux. For Windows users, go into the sdkconfig.h file in your project, and alter the following lines:
#define CONFIG_ESP32_XTAL_FREQ 40
#define CONFIG_ESP32_XTAL_FREQ_40 1
#define CONFIG_ESP32_XTAL_FREQ 26
#define CONFIG_ESP32_XTAL_FREQ_40 0
(Nota Bene: I was compelled many times to write 24 in places rather than 26, which led to lots of confusion. The baud rate if you write 24 will be 124,800.)
By changing the two lines from what is in red (on top) to what is in green (on bottom), you will be able to read serial output at the expected 115200 rate, and see the device appear in the nRF Connect app as "ESP-BLE-HELLO" if you copied the Instructable tutorial code.
Happy coding for BLE!