Subscribe to notifications with btshell

[Copper Dr <co...@gmail.com>]

Hi all,

I'm trying to use btshell as a central and subscribe to notifications. I
have a device that lets me write to an attribute and sends the response
back as a notification. I do not see how to accomplish this task using
btshell. Am I over looking something?


Thanks,
Fred Angstadt

Re: Subscribe to notifications with btshell

[Christopher Collins <ch...@runtime.io>]

Hi Fred,

On Wed, Jan 02, 2019 at 06:50:00PM -0500, Copper Dr wrote:
> Hi all,
> 
> I'm trying to use btshell as a central and subscribe to notifications. I
> have a device that lets me write to an attribute and sends the response
> back as a notification. I do not see how to accomplish this task using
> btshell. Am I over looking something?

nimble / btshell does not have a user-friendly means of subscribing to a
characteristic.  So, you need to combine some knowledge of the Bluetooth
spec with some low-level operations.  In particular, you subscribe
notifications or indications by writing a special two-byte value to the
characteristic's CCCD (client characteristic configuration descriptor).
A CCCD is a descriptor with a UUID of 0x2902.  To subscribe, you write
one of the following values to this descriptor:

    * notifications: 0x01 0x00
    * indications:   0x02 0x00

In the below example, a btshell central connects to a bleprph peripheral
and subscribes to notifications for the
`da2e7828-fbce-4e01-ae9e-261174997c48` characteristic:

    # Connect to peripheral.
    connect peer_addr_type=public peer_addr=e3:d0:4b:9f:87:a3

    # Ensure connected.
    conn-rssi conn=1

    # Discover all services, characteristics, and descriptors.
    gatt-discover-full conn=1

    # Show discovered attributes.
    gatt-show

    # (output)
    016685 CONNECTION: handle=1
    016685     start=1 end=5 uuid=0x1800
    016686         def_handle=2 val_handle=3 properties=0x02 uuid=0x2a00
    016688         def_handle=4 val_handle=5 properties=0x02 uuid=0x2a01
    016690     start=6 end=9 uuid=0x1801
    016691         def_handle=7 val_handle=8 properties=0x20 uuid=0x2a05
    016692             dsc_handle=9 uuid=0x2902
    016694     start=10 end=22 uuid=0x1811
    016695         def_handle=11 val_handle=12 properties=0x02 uuid=0x2a47
    016696         def_handle=13 val_handle=14 properties=0x10 uuid=0x2a46
    016698             dsc_handle=15 uuid=0x2902
    016699         def_handle=16 val_handle=17 properties=0x02 uuid=0x2a48
    016701         def_handle=18 val_handle=19 properties=0x10 uuid=0x2a45
    016703             dsc_handle=20 uuid=0x2902
    016704         def_handle=21 val_handle=22 properties=0x08 uuid=0x2a44
    016706     start=23 end=26 uuid=8d53dc1d-1db7-4cd3-868b-8a527460aa84
    016708         def_handle=24 val_handle=25 properties=0x14 uuid=da2e7828-fbce-4e01-ae9e-261174997c48
    016710             dsc_handle=26 uuid=0x2902
    016711     start=27 end=65535 uuid=59462f12-9543-9999-12c8-58b459a2712d
    016713         def_handle=28 val_handle=29 properties=0x02 uuid=5c3a659e-897e-45e1-b016-007107c96df6
    016716         def_handle=30 val_handle=31 properties=0x0a uuid=5c3a659e-897e-45e1-b016-007107c96df7

    # Subscribe to notifications.
    gatt-write conn=1 attr=26 value=0x01:0x00

Chris