[chbot] Lazy and Blue tooth

Mon Dec 11 04:06:53 GMT 2023

Unless you want to re-implement your code in another microcontroller 
with a built-in Bluetooth controller, you will need an external 
Bluetooth device. A few ways spring to mind:

* Various modules that set up a Bluetooth Serial Port Profile (SPP) 
connection with another Bluetooth device. For example, Bluetooth HC-05 
module for Arduino, from Surplustronics, Jaycar, etc. At the PC end, 
any Bluetooth module will work: either a USB dongle or your laptop's 
built-in Bluetooth module (which is often just a USB device anyway), or 
another HC-05 with a USB-serial dongle. The SPP should be usable with 
Android phones, but I think iOS does not support SPP. Your device will 
need a spare UART, or you can connect it to your 'engineering' UART. 
Any data sent from your device is streamed to the other end. This is 
simple to set up, but your software is responsible for formatting the 
data that you send so that the receiving end can decode it correctly, 
adding yet another ad-hoc protocol to the world. The default speed is 
9600 baud, but there are AT commands to change this as well as other 
parameters.

* General purpose Bluetoth modules from many suppliers (nRF, TI, etc) 
available from Digikey etc, but generally not available at Jaycar and 
such. Can do SPP; modern ones (BT 4.1 and above, I think) can do 
Bluetooth Low Energy (BLE) profiles including Generic ATTribute Profile 
(GATT, not to be confused with Generic Access Profile, GAP). They 
connect via serial or SPI, and your device communicates with them using 
the Bluetooth Host Controller Interface (HCI) API. HCI is well 
documented, but quite complex.

* My preferred method. I have used the Adafuit modules called 'BLE UART 
Friend': <https://www.adafruit.com/product/2479> with 3.3V UART 
interface <https://www.adafruit.com/product/2267> with USB interface. 
You can plug them in and set up a 2-way UART (note, it does not use 
SPP; see later), or you can enter command mode and use their 
well-documented AT-style commands to set up GATT services. These 
AT-commands are easier to use than HCI, but not as flexible.

I can use these modules on both embedded devices and PCs without having 
to learn multiple different operation-system specific APIs.

Not cheap (~US$15--20 last time I bought them)

GATT

GATT is intended to communicate attributes of one device to another: 
your BT headphones uses a GATT attribute to tell your phone what the 
battery level. They use other attributes to indicate that you have 
pressed the 'increase volume' button, and your phone uses yet other 
GATT attributes to adjust the volume. Updates can happen quite often 
(10-100 ms).

Attributes are organised in groups called 'services'. Bluetooth has 
many standardised GATT services. Some examples are: device information 
service for hardware and software version numbers, battery service for 
battery levels, current time service for ... the current time, 
electronic shelf label service for those eInk displays at the 
supermarket, location and navigation service for Bluetooth GPS  
devices, health-sensor service for sports watches, human interface 
device service for keyboard and mouses, and so on.

A device can support many services, and most do: device-information 
service and battery service are common. A smart-watch might support 
location and navigation, current-time and health-sensor services. 
Headphones include device information, battery level, audio control and 
human interface services. An electronic shelf label probably only 
supports shelf-label and perhaps device information service.

Each service defines a number of so-called 'characteristics': 
device-information service includes manufacturer, model number, serial 
number, hardware rev, firmware rev, and so on. Battery level service 
defines battery level (percent), battery voltage, energy, time until 
discharged or charged, etc).

Services and characterics within each service are identified by a 
128-bit UUID. If you can find suitable Bluetooth-defined services, then 
your phone will just understand them and a suitable app will do The 
Right Thing. If not, generate some random 128-bit UUIDs, plug them in 
to both ends, and a value set at one end is transferred to the other 
end as if by magic. Obviously, both ends have to Just-Know the same 
UUIDs, and you will need an app to uses these attributes.

Nordic ad-hoc-standard UART service

GATT provides no way to stream data from one device to another. 
However, Nordic have published a service (Nordic UART service or NUS) 
that defines two characteristics (TX data and RX data). You write to 
the TX data service on one end and read from the TX data service on the 
other; there is no flow control so you can lose data. If you use these 
UUIDs, there are many applications for Android and iOS that understand 
NUS.

The Adafruit BLE Friend devices use NUS rather than SPP for its serial 
transfer. This is transparent to you, if you just plug-in-and-use the 
devices. At the default 9600 baud, you will not lose any data, and BLE 
is quite robust. In this case, once again,l you need to have a way to 
parse the received data.

Automation IO service

This is a standard service for transferring digital and analog values. 
I have not used it so don't know the ins and outs. Analog values are 
16-bit values, digital values are 2-bit values (active, inactive, 
tristate, unknown), but it sounds useful for your application.
.

Debugging

nRF Connect is an app by nRF for Android and iPhone. After scanning for 
and connecting to a remote device, it tells you what services and 
characteristics are available and you can read and write the values. It 
knows the standard services and characteristics, and gives them 
sensible names. For user-defined services and characteristics, it gives 
the full 128-bit UUID.

Adafruit has an app that supports the BLE friend modules.

Documentation

* BLE GATT service documentation can be found 
<https://www.bluetooth.com/specifications/specs/?types=adopted> (look 
for 'services', not 'profiles').

* Adafruit BLE Friend documentation on the Adafruit website. They call 
it the Nordic UART 'profile'; this is wrong, it is the Nordic UART 
'service'. 'Profile' in Bluetooth parlance means something else.

Hope this helps

Stephen Irons

On Sun, Dec 10 2023 at 12:04:15 +1300, Marshland Engineering 
<marshland at marshland.co.nz> wrote:
> I have developed an EGT and CHT display for 2 strokes. Easy to see 
> when riding
> the bike. Uses an ATMEG328
> Never having use Bluetooth before, I not sure where to start in 
> getting the
> data to a phone or PC.
> 
> Basically All I want is to transmit 2 analog signals to a phone/PC.
> 
> Not sure whether to do it live or the ATMEG saves the info and 
> connects and
> downloads later.
> 
> A pointer in the right direction would be great.
> 
> Thanks Wallace.
> 
> 
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