BLE Driver Python Concurrent Notifications

[carvalhn]

Hi Guys,

I am currently using the ble driver for windows (Python) to get notifications from two Gatt services (Same Service/charac/descriptors/etc basically same application running on both devices). I think I am getting notifications from both the devices but I am unable to distinguish or confirm that owing to the lack of a specific device handle. In the on_hvx function, I get gattc_event which only has an event id and status. Could you please help me with how to identify which device handle I am getting the notifications from?

Example:

I am getting this now:

Received handle value notification, handle: 0x000E, value: 0x035A0E39ABFEC3E45A0216000000000000000000

However, I would like to have:

Received handle value notification,Device Name/HAndle:HRM1 handle: 0x000E, value: 0x035A0E39

Received handle value notification,Device Name/HAndle:HRM2 handle: 0x000E, value: 0x035A0E39

A second issue I am facing in python is that using the script, everytime I have to plug/unplug the Dongle to run the script again if I stop/close the process. I am aware that its because of not closing something somewhere so I tried closing ble_driver.sd_rpc_close() on errors but that did not work too . Could you'll please help me with that?

A third and last thing is while receiving notifications, is there a way to write to a characteristic on the Gatt server? Could you'll please guide me on that. For example, during notifications from two servers, I may want to write values to a control point heart rate characteristic .

I would be very grateful if you'll could help me with the above.I am using the driver as suggested by Stian devzone.nordicsemi.com/.../

Thanks and Regards, Neil

[Stian Røed Hafskjold]

###Get connection handle

To know which handle you get the notifications from you can use ble_event.evt.gap_evt.conn_handle (documented here: developer.nordicsemi.com/.../a00124.html)

In order to check this quickly you can print the conn_handle inside the ble_evt_handler(ble_event) function in main.py

 elif evt_id == ble_driver.BLE_GATTC_EVT_HVX:
    print "handler", ble_event.evt.gap_evt.conn_handle
    on_hvx(ble_event.evt.gattc_evt)

Reset chip

The second issue you are facing regarding the script to not be able to re-initiate the scanner after disconnect, is a common issue. I have not found any other solution than to reset the chip using the jlink debugger. I am using linux (JLinkExe) and call this script at the top of main() in order to reset:

def reset_device():
    ble_driver.sd_rpc_close()
    cmd = "echo -e '\ndevice nrf51822\nw4 40000544 1\nsi 0\ntck0\nt0\nsleep 10\nt1\nexit\n' | JLinkExe"
    import subprocess
    pcmd = subprocess.Popen(cmd, stdout=subprocess.PIPE, shell='/bin/sh')
    pcmd.wait()
    pout = pcmd.communicate()
    print pout[0]

Write to characteristic

If you want to write to a characteristic (assuming that you have implemented a write characteristic in the HRM service), you first need to capture the descriptor handle of that characteristic and store it to a global variable, then initiate a write request to that handle.

In the on_descriptor_discovery_response() function you can add:

    global hrm_write_handle
    ....
        if descriptor.uuid.uuid == YOUR_WRITE_CHAR_UUID
            hrm_write_handle = descriptor.handle

Now you will have the handle stored in hrm_write_handle, and this function will execute a write on that characteristic:

def do_write(value):
    if hrm_write_handle == 0:
        print "Error. No LED handle has been found"
        return

    led_value = [value]
    led_array = util.list_to_uint8_array(led_value)

    write_params = ble_driver.ble_gattc_write_params_t()
    write_params.handle = hrm_write_handle
    write_params.len = 1
    write_params.p_value = led_array.cast()
    write_params.write_op = ble_driver.BLE_GATT_OP_WRITE_REQ
    write_params.offset = 0

    ble_driver.sd_ble_gattc_write(connection_handle, write_params)

You can of course pass the connection_handle variable to this function as well to choose which peripheral you want to send the write to. You can call this function from on_hvx() to test.

sd_ble_gattc_write is documented here: developer.nordicsemi.com/.../a00710.html

And the sequence diagram for that function is here: developer.nordicsemi.com/.../a00607.html

[carvalhn]

Thanks Stian :)