TS MK II – Mesh Network Pt.2

As part of my Temperature Sensor upgrade, I’ve started looking into Mesh Networking support.

Documenting my progress

Rather than try and put a post together at the end, I’m going to try and document my progress as I go, with additions to this post each time I do something significant.

The beginning

I recommend you read the first post before picking up here.

31th July

I carried on through the developer study guy and implemented the basic Light. The messaging flow is now making sense to me. I knew the Microbit’s 16KB of RAM wasn’t going to be enough for any real project, so I decided to bite the bulled and pick up a Adafruit nRF52832 device.

Ordered from www.coolcomponents.co.uk, they arrived pretty quickly. I plugged it in, downloaded the Bluefruit app from the app store and was able to discover and connect to the device pretty quickly. So far, so good.

Unfortunately, I couldn’t flash it my Zephyr project. More digging around, using the Nordic Tools and I discovered that the West tool couldn’t detect the device. Strange, as I could flash it with simple Arduino sketches. I trawled the documentation. Tried changing drivers. Updated the bootloader. Nothing had any effect.

During the umpteenth reading of the Zephyr docs, I finally realised why:

Flashing Zephyr onto the nrf52_adafruit_feather board requires an external J-Link programmer. The programmer is attached to the X1 SWD header.

https://docs.zephyrproject.org/latest/boards/arm/nrf52_adafruit_feather/doc/index.html

I have emphasized external in the quote above. I totally missed that key piece of information. More googling and I learn that Segger J-Link is a special protocol and that a small unit is required to interface your computer with the board! Everything fell into the place. The different drivers and the talk of the SWD header.

Unfortunately, my Adafruit boards don’t have a SWD header. Fortunately, the board has space and solder pads for one to be added.

The SWD header goes here

Adafruit sell the headers, but nobody in the UK had them available. I searched around and found something on RS that looked right.

I wasn’t prepared for the tininess of the header

I found a J-LINK device on The PiHut which I ordered, along with a short cable. I’ll wait for that to arrive before I attempt to solder this header onto the board. I’m going to need some steady hands!!

3rd August

J-Link Device arrived and connected up. Managed to solder on the header without any issue.

Added the RS 10 pin SWD header to my Adafruit nRF52

I think hooked up my newly purchased J-LINK device. Side node here – I ordered a serial cable with this as PiHut said it was recommended. One came in the box, so the extra one I ordered was redundant! I’ll be writing to them about that.

Hooked up (I should point out that this shows the correct cable orientation – see below)

Running the nordic command shows it has found the J-LINK connector!

Sadly, nothing is easy…

Big cryptic error trying to flash the device

My attempt to flash the hello_world sample failed. Ah man – with some help from this https://embeddedcomputing.weebly.com/segger-j-link-with-adafruit-feather-nrf52.html I realised I had the cable connected the wrong way around. I could find no information on the orientation of the header or cable, but I reversed it based on that pic and boom…..

SUCCESS!

Zephyr hello_world sample successfully written to device

It appeared that I had successfully flashed the device with the sample code.

Terminal output shows hello_world is working

Bloomin’ heck. It did actually work!

With the flash process *finally* working, I create a new “light” project, using what I’d learned from the BBC Microbit. I was then able to reuse the BBC Microbit switch.

I’d consider that a success! I think I’ve enough of a basic grasp of BLE Mesh networking to park this part of the project and move on!

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TS Mk II – Mesh Network Pt.1

As part of my Temperature Sensor upgrade, I’ve started looking into Mesh Networking support.

I was aware that the Expressif ESP32 supports the new Bluetooth LE Mesh protocol, and as Bluetooth Low Energy would help stretch battery life, it seemed like the natural place to start my investigations.

I was initially going to try the WiFi mesh, but it didn’t really seem to suit devices like temperature sensor as they are actually asleep most of the time. If nodes in the mesh keep turning on and off, the other devices in the mesh would be constantly reorganizing themselves to plug the gaps. I do want to learn about ESP’s WiFi mesh, but for now, I’ll focus on Bluetooth

Documenting my progress

Rather than try and put a post together at the end, I’m going to try and document my progress as I go, with additions to this post each time I do something significant.

Getting started

I’ve already got the ESP-IDF environment setup and have two different dev boards. I’m going to try and approach this in stages

  • Copy one of the sample projects and get it running
  • Add another node
  • Send data from one node to another
  • Add MQTT and WiFi to get data out of the mesh

9th July 2019

I pulled down the ESP-IDF code from their mesh branch and spun up on the samples. After faffing around with the menuconfig settings, I got the Node example compiled and deployed onto the device. It also appeared appeared in the Nordic iOS Mesh App.

I found a good post on the provisioning process, https://www.novelbits.io/bluetooth-mesh-tutorial-part-3/ and this helped me make sense of what I was seeing.

Once I tapped on the node, I was able to tap “Identify”, which pulled back more information. I think this is what novelbits called “Invitation”. I then tapped “Provision” and after a few seconds, and a lot of logging from the device, I had the first Node in my mesh!

11th July

Now that I had my first mesh node, I quickly realised why the Expressif code was only at version 0.6 – none of the provisioning data was being saved. This meant that each time I flashed the device, I had to provision it again. This became annoying after the fifth time, so I started digging into the actual provisioning process, to see if I could put in something temporary.

I was digging around on google and I came across the Zephyr RTOS. In their examples, they had a simple 2 node mesh demo. The readme page states that no provisioning is required, and whilst it’s insecure, it’s sufficient for the demo. In their code, I could see a few lines of code where they manually provision the device with their own network and device keys.

static const u8_t net_key[16] = {
	0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef,
	0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef,
};
static const u8_t dev_key[16] = {
	0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef,
	0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef,
};
static const u8_t app_key[16] = {
	0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef,
	0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef,
};

err = bt_mesh_provision(net_key, net_idx, flags, iv_index, addr, dev_key);

This seemed ideal for my tinkering. I also found reference to the Zephyr code in Expressif’s news release about their BLE Mesh, where they state they are basing their implementation on top of the Zephyr . This meant I could probably find simple code in the esp-idf repo.

Running this code on the device resulted in it saying that provisioning was complete.

19th July

After more digging around the ESP-IDF examples, I came across some testing code, which appeared to provision using the esp code.

bt_mesh_device_auto_enter_network()

Try as I might, I couldn’t get that to work. Just kept reporting an error of -22.

UPDATE 13th August: During some experimenting with how to bridge the mesh network with MQTT, I managed to get this code working eventually, meaning:

static const u16_t net_idx;
static const u16_t app_idx;
static const u32_t iv_index;

struct bt_mesh_device_network_info info = {
        .net_key = { 
                    0x01,
                    0x23,
                    0x45,
                    0x67,
                    0x89,
                    0xab,
                    0xcd,
                    0xef,
                    0x01,
                    0x23,
                    0x45,
                    0x67,
                    0x89,
                    0xab,
                    0xcd,
                    0xef,
                    },
        .net_idx = net_idx,
        .flags = flags,
        .iv_index = iv_index,
        .unicast_addr = 0x0002,
        .dev_key = {
                    0x01,
                    0x23,
                    0x45,
                    0x67,
                    0x89,
                    0xab,
                    0xcd,
                    0xef,
                    0x01,
                    0x23,
                    0x45,
                    0x67,
                    0x89,
                    0xab,
                    0xcd,
                    0xef,
        },
        .app_key = {
		0x01,
		0x23,
		0x45,
		0x67,
		0x89,
		0xab,
		0xcd,
		0xef,
		0x01,
		0x23,
		0x45,
		0x67,
		0x89,
		0xab,
		0xcd,
		0xef,
},
        .app_idx = app_idx,
        .group_addr = 0xc000
    };

    err = bt_mesh_device_auto_enter_network(&info);

20th July

After more attempts to provision the device directly, I’ve decided to kinda give up. As I mentioned, the Zephyr code is easier for me to understand at this stage, so I’m going to try and get a build of the Zephyr samples running on my ESP32 boards.

23rd July

After hours of playing around trying to get the Zephyr SDK compiling against the ESP32 toolchain, I decided to stop. I found an excellent resource at over on the Bluetooth SIG website, called “An Introduction to Bluetooth Mesh Networking“. The samples in their course use the Zephyr SDK and the BBC Microbit. I had a glance though it and most of the main concepts are covered, so I think I’ll order some BBC Microbits and run through it.

I also came across an interest post at https://hutscape.com/, which is worth checking out. The author, Sayanee, uses an Adafruit board to build a UV sensor. Pretty cool. At £26 (and no Friend Node support in the Nordic SDK), I decided against buying any for the time being.

24th July

I got a BBC Microbit and tried the Zephyr mesh_demo code. After more mucking around, I managed to flash it with their code.

Building the mesh_demo firmware
The output from the microbit shows it working

As suggested by the Bluetooth study guide, I turned on the logging and then

Enable the logging and I use up too much RAM

The damn firmware, with logging, was too big for the BBC Microbit’s 16KB of ram. Bloody hell. Nothing is every easy. Some googling revealed that the small amount of SRAM makes it almost impossible to run the mesh. The Apollo Computer ran on 4KB of RAM 🙂

Hardware development is tough! That said, these microbits are perfect for me to dabble in the fundamentals of Bluetooth Mesh and to get to grips with the models. Hopefully there is enough space for me to try my own code. Perhaps it’s possible to compress the Zephyr RTOS runtime down a little??

25th July

Been reading about how to figure out the size of the runtime. You just run the command ninja rom_report

The mesh_demo code takes up just over 3% of the total size
The bluetooth code takes up 66%

I don’t know what those numbers actually mean. I had assumed bytes, but that would mean the bluetooth library, if measured in bytes, was around 50KB and I know that’s not right.

28th July

I experimented with the Zephyr mesh_demo sample, but couldn’t make it work. Kept getting error codes when trying to send messages. I tried to tweak it here and there, but I was just strumbling around in the dark. I returned, instead, to the Bluetooth developer guide and gave that go. The guide starts at the Switch module.

The sample code wasn’t up-to-date with the latest Zephyr code, so I had to tweak it somewhat, but I got it to compile and, amazingly, it worked.

One Microbit is the light, the other the switch
Turning the light on and off over BLE Mesh!

You can continue following my adventures here