This is perhaps the first in a series of personal investigations into the nRF52840-Dongle. The dongle first attracted my attention when I was looking into usb dongle devices with microcontollers for a new Adventure Labs kit. Shortly afterwards it was referenced on a Linux news podcast I listen to mentioning the Google OpenSK which can be flashed onto this device as a secure key. This has piqued my interest as I’ve considered getting a yubikey for some time but being proprietary it’s put me off.
So Why Micropython?
OpenSK installs onto TockOS and is written in rust. However, the guides on the OpenSK site states the need for a JTAG programmer and these can be quite expensive. An alternative programmer can be via SWD, and there are various ways of programming the nRF52840 via SWD. However, the comments on OpenSK’s github issues suggest that this hasn’t been successfully implemented yet.
There is a way of installing OpenSK onto a chip that already had TockOS and there are guides on installing TockOS via OpenOCD for other chips. Again, however, the guide to install TockOS onto the nRF52840 specifies using the JTAG programmer and a tiny connector.
Micropython has a port for the nRF52840 dongle and has guides for flashing the firmware onto the dongle using various methods via SWD.
Therefore maybe it’s possible to burn TockOS onto the nRF52840 if I can follow the same steps as developers of Micropython but use the binaries for TockOS.
Also I’d like to get familiar with the chip and the dev board, using Micropython will give me a development environment which I’m familiar with to explore some of the features.
Preparing the Dongle
Underneath there are 10 small pads, 5x2 rows, designed for a specific connector TC2050. This is a connector for a JTAG programmer and most of the guides I’ve come across recommend this method for burning new bootloaders. looking at the Segger site for a J-Link programmer and the necessay cables etc I estimate the required equipment to cost £100s…
But there is an alternative. Next to these pads are 10 rectangular pads that can be used with other Serial Wire Debuggers - two of these pads, the CLK and the DATA are broken out on the the edge of the board, near to the usb connector. It is here I have soldered pins to connect jumper wires. I’ve also soldered a pin onto one of the GNDs too.
Wiring up the ST-Link Programmer
I’m going to use an ST-Link SWD programmer and I’ve written a post about setting this up.. In that post I make sure that I have openocd installed through my os package manager and pyocd installed through pip into the python virtual environment I’m working in. From this point on note that I have these installed and I’m working in the virtual env.
I connect the nRF52840 to the ST-Link in the following way, without VDD or RESET.
| nRF5280 | ST-Link |
|---|---|
| SWCLK | CN2 p2 |
| GND | CN2 p3 |
| SWIO | CN2 p4 |
Burning Micropython onto the nRF52840
I cloned Micropython from github and built the project
git clone https://github.com/micropython/micropython.git Micropython
cd micropython
make -C mpy-cross
I then change directory to ./ports/nrf and plugged both the ST-Link
and the dongle into my PC. I then ran:
make submodules
make V=1 BOARD=10059 FLASHER=pyocd flash
The output should list lines like INFO:pyocd.coresight.... and then
show an upload progress bar. Once complete micropython should be on
the dongle. If there are any issues maybe check the wiring, check
there ST-Link can be seen (run pyocd list) and check the error
messages.
I didn’t have any issues to resolve.
Testing Micropython on the nRF52840 Dongle
I am using the repl on the chip and accessing it using picocom.
picocom -b 115200 /dev/ttyACM0
picocom v3.1
port is : /dev/ttyACM0
flowcontrol : none
baudrate is : 115200
parity is : none
databits are : 8
stopbits are : 1
escape is : C-a
local echo is : no
noinit is : no
noreset is : no
hangup is : no
nolock is : no
send_cmd is : sz -vv
receive_cmd is : rz -vv -E
imap is :
omap is :
emap is : crcrlf,delbs,
logfile is : none
initstring : none
exit_after is : not set
exit is : no
Type [C-a] [C-h] to see available commands
Terminal ready
MicroPython v1.12-167-gf020eac6a on 2020-02-24; PCA10059 with NRF52840
Type "help()" for more information.
>>> from machine import Pin
>>> grnled = Pin(6, Pin.OUT)
>>> grnled.off()
>>> # Green LED has turned on!
Conclusion
Once I had the ST-Link set up I found flashing micropython onto the nRF52840 very straight forward. I notice in the Makefile there are commands using the pyocd programme which take a .hex file as a parameter. The next step for me is to try this same process with TockOS.