BLE GATT characteristic parsing

Martin_Wood · February 2, 2020, 1:15pm

Hi There,

Sorry if this has already been answered elsewhere but I wanted to know what the easiest way of parsing BlueTooth characteristic types, in particular ones such as the example below which have several fields to pick from:

https://www.bluetooth.com/wp-content/uploads/Sitecore-Media-Library/Gatt/Xml/Characteristics/org.bluetooth.characteristic.rsc_measurement.xml

I have managed to extract the heart rate data but just by messing with string manipulation but for more complex ones I don’t see how it’s possible in App Inventor. If there is an extension which allows you to use an XML spec to expose each field that would be ideal.

Martin

ABG · February 2, 2020, 5:46pm

Look into the Web Component for XML decoding.

Also see

bfasula · February 3, 2020, 12:03pm

I have been using the BytesReceved for sensor data like Heart Rate and Speed Sensor. Then you can manipulate each byte as needed.

Martin_Wood · February 3, 2020, 8:48pm

Thanks - I’ll see how I get on

Martin_Wood · February 12, 2020, 3:21pm

Hi guys - I’m trying to filter what’s coming through the bytes received, but wondered if the data can easily get mixed up e.g power value and battery level - is this possible? I’ve been selecting the 3rd string along for power but think it might be a moving target:

. Heart rate works all the time and HRM battery level.

bfasula · February 12, 2020, 7:14pm

I think you just need to register for bytes once for each device. I am using an HRM, Speed Sensor and Cadence Sensor and Register for bytes once for each one. I use three Bluetooth LE components one for each device.

Martin_Wood · February 13, 2020, 1:51pm

I’ve figured out what’s happening, when the PWR gets above 255W it clips and I just get the left-over e.g if it’s 257W I get 2W, so looks like I was only getting literally one byte, but when I tried to get the one next to it no luck. Ideally I’d have a bit mask I can AND over the whole data - think that’s what I need to do. Think it’s the 2nd block of 16 bits I need to focus on for cycling power data.

I tidied up all characteristic literals so it’s all done with lookups in plain English. I was also doing > 1 read bytes operation per bytes received which didn’t help! So stored it in a local variable at the top

ewpatton · February 13, 2020, 4:14pm

Hi @Martin_Wood,

If the characteristic is intended to be larger than a single byte (and it’s fixed width), consider using RegisterForShorts instead as that will give you 2-byte wide values, which implies an unsigned range of 0-65535. Do you have a pointer to the exact characteristic you’re reading that’s going beyond a single byte?

Martin_Wood · February 14, 2020, 2:44pm

H there @ewpatton - using RegisterForShorts worked great - thank you!! Now to see what other similar data items I can poll e.g cadence data. If there’s anything that changes position within the data that’s going to be more challenging, so leave that for now eh

ewpatton · February 14, 2020, 3:40pm

Yes, unfortunately we don’t have a good solution right now for more arbitrarily structured BLE packets. You can use the bytes version for this but you end up needing to implement a bit of math to reconstruct the larger values from the individual bytes.

Anibal_de_la_Torre · June 15, 2020, 7:34am

Hello
In this thread I am looking for help. It is a similar problem to the one raised here.
Any help will be welcome.

Thank you so much for everything.

Martin_Wood · June 22, 2020, 8:12pm

I got bogged-down making sure I had all the right codes in the right places so set up a few functions to return the service and characteristics, like this :

Then to read battery level on HRM and heart rate the calls look like this now:

Hope this is somewhat useful as it's from a huge project and just trying to pull out the useful bits.

I wished when I was working on this that I could use an XML GATT specification to automatically map on to the data but it was very manual to parse this stuff.

This was the hardest bit - trying to extract speed, cadence, stride length from my power meter!

I had a working invterval timer for running workouts but once I got my Polar Vantage M I didn't need my app anymore

ABG · June 22, 2020, 9:43pm

I lost track of the original poster.

The downloaded blocks would be more useful if they were draggable, not clipped screen shots.
Draggable blocks save a whole lot of typing.
Here's how to make them ...

Please download and post each of those event block(s)/procedures here ...
(sample video)

Martin_Wood · June 22, 2020, 10:24pm

BLE_Snippets_01.aia (9.0 KB)

Here's some sample snippets I took from my overall project via the backpack and pasting into a new project - they are just to illustrate a few ideas and don't work as a project ( which also has grown out of all proportion and is totally unmanageable !! ). I hope this helps someone

mmoras · March 10, 2021, 12:57pm

Hello,
I have tried to read data from the Heart Rate and the battery simultaneously. I am cappable of reading well the HR data but the battery level doesen't refresh the data. This data only refresh when I turn off and turn on the bluetooth connection. Could you help me?

For reading the heart rate data I use a RegisterforShorts and ReadShorts. For reading the battery level I use RegisterForBytes and ReadBytes.

To obtain the data I use ShortsReceived while loop and BytesReceived while loop.

Martin_Wood · April 12, 2021, 7:17am

Hi Miquel - I found that I had to manually call the .ReadBytes for battery every so often on a timer / via a button as well. With one of my heart rate monitors the battery level was in increments of 10 i.e 90,80,70% etc so if you wanted to wait long enough you might get a battery level notification from the device and therefore your .BytesReceived event should trigger. I hope this helps. Cheers, Martin