My development board

E73-TBB(52832)
Manual PDF
512KB flash
64KB RAM
Using the Generic board variant from arduino-nRF5. This is kinda similar to the Adafruit feather nrf52832 board definition, but seems to use sandeepmistry:openocd instead of nrfutil and uses a different linker as well.

Articles

Great article about using Rust and Apache Mynewt, but also covers J-Link, ST-Link, openocd, unlocking the nrf52. Link on medium

nrf command line utilities

nrfjprog Used for programming hex files using the J-Link programmer.
nrfutil Used for DFU - 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 is such a bootloader, which we can burn using nrfjprog + J-Link once. Adafruit has its own version, adafruit-nrfutil.

SWD vs. Bootloader vs. DFU

How nrfutil is used in the platformio nordicnrf52 package: link
Seems like we can use DFU if we install a DFU-enabled bootloader like the Adafruit_nRF52_Bootloader
Using the adafruit nrf52 bootloader with the E73-TBB link

Burning the Adafruit bootloader

$ brew cask install nordic-nrf-command-line-tools -> Didn't work. I had to manually install the pkgs from nordic
pip install intelhex
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:

# 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
Zip created at dfu-pkg.zip

$ ~/.platformio/packages/framework-arduinoadafruitnrf52/tools/adafruit-nrfutil/macos/adafruit-nrfutil dfu serial --package dfu-pkg.zip -p /dev/cu.usbserial-14330 -b 115200
Upgrading target on /dev/cu.usbserial-14330 with DFU package /Users/rbaron/dev/parasite/code/parasite/dfu-pkg.zip. Flow control is disabled, Dual bank, Touch disabled
########################################
########################################
########################################
########################################
########################################
#######################
Activating new firmware
Device programmed.

Linking

In platformio-core/tools/pioplatform.py:

if "build.ldscript" in board_config:
    env.Replace(LDSCRIPT_PATH=board_config.get("build.ldscript"))

In platformio-core/platformio.py:

# append into the beginning a main LD script
if env.get("LDSCRIPT_PATH") and not any("-Wl,-T" in f for f in env["LINKFLAGS"]):
    env.Prepend(LINKFLAGS=["-T", env.subst("$LDSCRIPT_PATH")])

The -T flag expects a path to a linker script, as per ld docs.

Frameworks, such as the nordicnrf52, contain boards definitions that specify the ldscript.

For the adafruit_feather_nrf52832 board, we have:

"arduino":{
  "ldscript": "nrf52832_s132_v6.ld"
},

Which is defined here.

For the nrf52_dk, we have:

"arduino":{
  "ldscript": "nrf52_xxaa.ld"
},

Which is the most common linker script, and is part of the nordic official SDK, it seems, available here. For reference, check out the linker for the Adafruit nRF52 Bootloader. Here you can see the memory map of the nRF52 Feather.

Same platform, different framework

Both the Feather nRF52832 and the Generic boards above point to the same platform, the nordicnrf52. Inside this platorm, there's a switch for selecting which framework to use, based on the board name, here:

if self.board_config(board).get("build.bsp.name",
                                            "nrf5") == "adafruit":
                self.frameworks["arduino"][
                    "package"] = "framework-arduinoadafruitnrf52"

Memory layout

The SoftDevice S132 spec specifies the layout as the soft device begins at addr 0x0 and the application code comes after it, at the address APP_CODE_BASE.

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