Serial upload works.

Currently uploading via the Arduino IDE and PlatformIO works. But it's not perfect yet:
1. We're using a custom-compiled bootloader for the e73 board, which is nice;
2. We're still using `board = adafruit_feather_nrf52832` in the `platformio.ini` file. Using our custom board `boards/e73-tbb.json` doesn't work yet. Uploading results in a `*** [upload] IndexError : list index out of range` error. Most likely has to be fixed in https://github.com/platformio/platform-nordicnrf52/blob/develop/builder/main.py.

code/parasite/platformio.ini

@@ -11,8 +11,8 @@
 ; [env:adafruit_feather_nrf52832]
 [env:e73-tbb]
 platform = nordicnrf52
-; board = adafruit_feather_nrf52832
-board = e73-tbb
+board = adafruit_feather_nrf52832
+; board = e73-tbb
 framework = arduino
 build_flags = -DNRF52 -DS132 -DNRF51_S132
 ; upload_port = /dev/cu.usbserial-14330

code/parasite/src/main.cpp

@@ -4,13 +4,13 @@ constexpr int kLED1Pin = 17;
 constexpr int kLED2Pin = 18;
 
 void setup() {
+  Serial.begin(9600);
   pinMode(kLED1Pin, OUTPUT);
   pinMode(kLED2Pin, OUTPUT);
-  // Serial.begin(9600);
 }
 
 void loop() {
-  // Serial.print("Hello, world\n");
+  Serial.print("Hello, world\n");
   digitalWrite(kLED1Pin, LOW);
   digitalWrite(kLED2Pin, LOW);
   delay(1000);

code/playground/blinky/blinky/blinky.ino

@@ -0,0 +1,14 @@
+#include <Arduino.h>
+
+#define kLED1Pin 18
+
+void setup() {
+  pinMode(LED_RED, OUTPUT);
+  Serial.begin(9600);
+}
+
+void loop() {
+  Serial.println("Hello, world");
+  digitalToggle(LED_RED);
+  delay(1000);
+}

resources.md

@@ -8,6 +8,10 @@
 
 # Articles
 * Great article about using Rust and Apache Mynewt, but also covers J-Link, ST-Link, openocd, unlocking the nrf52. [Link on medium](https://medium.com/@ly.lee/coding-nrf52-with-rust-and-apache-mynewt-on-visual-studio-code-9521bcba6004)
+*
+# nrf command line utilities
+* [nrfjprog](https://www.nordicsemi.com/Software-and-tools/Development-Tools/nRF-Command-Line-Tools) Used for programming hex files using the J-Link programmer.
+* [nrfutil](https://github.com/NordicSemiconductor/pc-nrfutil) Used for [DFU](https://en.wikipedia.org/wiki/USB#Device_Firmware_Upgrade) - device firmware update - via the USB port. No J-Link. This is what Adafruit uses for uploading code via USB. This is "higher level" and requires a pre-existing bootloader in the nrf52 chip that understands DFU. The [Adafruit_nRF52_Bootloader](https://github.com/adafruit/Adafruit_nRF52_Bootloader) is such a bootloader, which we can burn using nrfjprog + J-Link once. Adafruit has its own version, [`adafruit-nrfutil`](https://github.com/adafruit/Adafruit_nRF52_nrfutil).
 
 # SWD vs. Bootloader vs. DFU
 * How `nrfutil` is used in the platformio nordicnrf52 package: [link](https://github.com/platformio/platform-nordicnrf52/blob/develop/builder/main.py#L319)
@@ -19,6 +23,18 @@
 1. `pip install intelhex`
 1. `PATH=$PATH:/Users/rbaron/.platformio/packages/framework-arduinoadafruitnrf52/tools/adafruit-nrfutil/macos make BOARD=feather_nrf52840_express all`
 
+## Burning our custom bootloader
+The error I was making was using a non-softdevice bootloader. The following works:
+```bash
+# Compile
+$ PATH=$PATH:/Users/rbaron/.platformio/packages/framework-arduinoadafruitnrf52/tools/adafruit-nrfutil/macos make BOARD=e73_tbb all
+
+# Flash (make sure you use the _s132_ hex file)
+$ JAVA_HOME=/Users/rbaron/homebrew/Cellar/openjdk/15.0.1 nrfjprog --program _build/build-e73_tbb/e73_tbb_bootloader-0.4.0-2-g4ba802d_s132_6.1.1.hex --sectoranduicrerase -f nrf52 --reset
+```
+
+Now we can drop the JLink and use the USB serial for uploading/monitoring.
+
 ## Uploading with nrfutil
 ```
 $ ~/.platformio/packages/framework-arduinoadafruitnrf52/tools/adafruit-nrfutil/macos/adafruit-nrfutil dfu genpkg --dev-type 0x0052 --sd-req 0x00B7 --application .pio/build/e73-tbb/firmware.hex dfu-pkg.zip
@@ -35,5 +51,3 @@ Upgrading target on /dev/cu.usbserial-14330 with DFU package /Users/rbaron/dev/p
 Activating new firmware
 Device programmed.
 ```
-
-It flashes okay, but the program doesn't seem to run - the leds don't flash.