Week9: Embedded programming

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The nrF52 is a familly of ARM based MCU fabricated by Nordic. It has quite interesting feature both for the Gamma-project and the Gamma-Cool-Box.

• 32 bit, 64Mhz Cortex-M4 core
• 12bit 200ksps ADC with "easyDMA" => easy buffering?
• 16Mhz PWM clock => 60kHz PWM @ 8 bit resolution
• fully configurable I/O mapping ("AHB multilayer") => one can do what one wants with watever pin.

And of cource, last but not least:

• It can nativelly do short range radio comunication (2.4GHz) ant talk BLE!

This last feature, will allow to completelly isolate the power electronic PCB (that handles thermal PID with fans and peltiers, very noisy...) from the main PCBs, that is very sensitive.
Plus, it will should be possible to do what Nordic calls "OTA-DFU": Over The Air Device Firmware Update. Meaning that I could wirelessly program and monitor my MCU directly from a laptop, a RaspberyPi or even a smartphone!

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As no MCU is perfect, the nRF52 still has some drawback:

• 3.6 Volt operation MAX. And it is not 5V tolerant => I'll have to be carefull
• No native USB driver

The worst drawback of the nRF52, that I initially ovelooked since I didn't know that the ARM development workflow was differen from the AVR/Arduino one:

• The dev tools advirtised and supported by Nordics are almost entirelly closed sourced, expensive, and windows based only :/

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As it was part of the courses, I decided to start with the BT832A module of Fanstel. This minimalist module only integrates MCU, quartz, RF antena+filter on a very tiny PCB.
Here under is a little memorandum i did about it:

NB: The BT832A uses the nRF52810 which is similar to the nRF52832 but has less memory (192kB Flash + 24kB RAM), and peripheral resources).

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The BT832A cost 3.3€/unit, theoretically. I bought 20 of them and ended up paying 135€ = 75$ of parts + 43$ of "handling & shipping" + 33€ of "DHL import theft".
So, all in all, for a french maker, the BT832A is 6.8€/unit. Not bad.
They were delivered in 1 week. I could't resist to crack one open ^^

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I designed a little breakout board for this module, using KiCAD. I created myself the shematics blocks and the footprint. I ignored the "bga style" outputs, and only considered the 16 lateral pins.
The Kicad project is available in this KiCAD project folder on gitlab. Fabrication files are in the fab/ sub-directory.
The essential fabrication files are:

• The shematics
• The plot of the pcb for color map laser cut + engraving, "classical" laser engraving, or milling

I laser engraved the PCB and soldered it:

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Up till the Fabacademy, I was used to work with MCU only with "big" dev-board intergrating the MCU and a lot of periferic tools as well as pinheah connectors. Exemples of such dev-boards based on the ATmega32u4 and the PIC32MX.
Those dev-boards cost 10 to 40€ (or more for the one embeding WiFi or other fancy modules), when the MCUs chips cost only 1 to 3€. But with this "high" price comes confort: one can easyly bootload and program the MCU's dev-bord through USB using the Arduino IDE.
With Fabacademy I did learn how to bootload and programme AVR MCU with the ISP protocol. I can directly talk to the AVR MCU whith the open-source-cross-platform Arduino IDE and the fablab-made ISP programmer PCB. I dont need the "expensive" dev-board no more :)

The ARM MCUs have a different architectures and works differently. Theis interface is not ISP (In-System Programming) but JTAG. JTAG can be used for advance debuging, with up to 20 pins. It also have a simple variant: SWD (Serial Wire Debug).
SWD uses only few connections: SWDCLK, SWDIO, GND, 3.3V and an optional Reset. It requires an ad-hoc SWD programmer module like the ST-Link whitch cost 20€ or the Atmel ICE that cost 100€!
Needless to say that I dont like that...

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My goal: set-up an nRF52 workflow with open-source-cross-platform softwares and minimal use of exotic programmer modules. For this quest I had three starting points:

• The Nordics online doc, on getting started with nRF5 MCUs
• This repo of the CBA, on programing the nRF52. NB: this project use the BC832 module of Fanstel, which is similar but not pin to pin compatible with my BT832 module.
• This repo of Sandeepmistry which explain how to pimp the Arduino IDE to program nRF52 using a SWD programmer.

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Lets review the 4 tools nativelly supported and documented by Nordics:

• Keil is an ARM SDK. Closed-sourced and for Windows only. Plus, the free version is limited to 32kB program size! The cheapest licence is 1200€/year. No thanks.
• IAR is an other closed-source, Windows only, ARM SDK. Free trial of 30 days limited to 32kB. Pricing secret, probably over 1000€. No thanks.
• SES, the Segger "Embeded Studio". It is cross-platform (Linux/iOS/Windows) C/C++ IDE. Through a ressent partnership with Nordics it is completelly free for non-profit use (otherwise 1000€ licence). It sounds great! There is even a youtube tuto made by Nordics. But there is a catch... SES is only compatible with the proprietary segger J-link programmer, a USB harwares thats cost several 100€! No thanks!
• GCC and especially its fork: the GNU Arm Embedded Toolchain. Completelly open-source and inherently multi-platform. The only really acceptable choice.

NB: all the tuto proposed by Nordics only covers big expensive nRF52 dev-board (like the this one). Even in this tuto on bootlading for OTA, it requires the big dev-board to be used as an USB->RF transponder.

The CBD repo strongly advises beginners to develope with a dev-board allowing direct USB flashing, like the nRF52 Feather of Adafruit (25€) which come pre-bootloaded. But I'm no beginner no more ^^. I want to work with MCUs soldered on my custom PCB, not dev-boards.

The repo explore two methods to talk directly to the nRF52 chips, using SWD:

• First with the expensive proprietary Segger-J-Link programmer :( and the Nordic's command line tools.
• Secondly with the RaspberyPi as SWD programmer and the openOCD compiler which, contrary to GCC, is not supported by Nordics. This methods also quotes:
  • This tuto of Adafruit, on using a RaspberryPI as SWD programmer.
  • this
  • this

I got stuck on the edition of the config files and the compilation procedure. I've never compile anthing at the command line before. Make files and all are new to me.

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Following this other tuto on Nordics forum. I struggled in my windows environment to program the nRF52 using the Arduino IDE and then the Eclipse IDE. I could not make the ST-Link talk to the chip. ST-link driver issue?..

No. It wasn't. It is just that ST-link is not J-link. And the GCC tools of Nordics forces needs a J-Link programmer... The was a Segger "Atmel Ice" J-link in the digiscope. It didn't work nor with the commandline GCC, nor with the Segger IDE.

This strategy of Nordics is really anoying... Even to use the GCC Nordics tool, you need to buy a dedicated 300€ programmer from SEGGER or a complete 60€ nrF52 dev-board. Sad.