BLE does not read whole buffer data

[livius]

Hi,

current firmware wipy2:

os.uname()
(sysname='WiPy', nodename='WiPy', release='1.10.2.b1', version='v1.8.6-849-g0af003a4 on 2017-11-24', machine='WiPy with ESP32')

when i read adv = bt.get_adv()
adv data contain truncated data
e.g.
it reads

b'\x02\x01\x06\x1b\xffW\x01\x003"c\xe6\xb4\xd19\xdc\x83\xa6\x82\x98\xf3\x19w\xc3\x03\xd2lBk\xad\xc2'

which is:
0201061bff570100332263e6b4d139dc83a68298f31977c303d26c426badc2

but should be
'\x02\x01\x06\x1b\xffW\x01\x003"c\xe6\xb4\xd19\xdc\x83\xa6\x82\x98\xf3\x19w\xc3\x03\xd2lBk\xad\xc2\n\tMI Band 2\x03\x02\xe0\xfe'

0201061bff570100332263e6b4d139dc83a68298f31977c303d26c426badc20a094d492042616e6420320302e0fe

without correct buffer it is not possible to retrive name of device correctly

[livius]

@livius
I do not know much about sources but maybe someone know how to debug this
how can i send something to UART from e.g.
BTA_GATTC_GetAllChar or bta_gattc_get_db_with_opration
https://github.com/pycom/pycom-esp-idf/blob/master/components/bt/bluedroid/bta/gatt/bta_gattc_cache.c

[jmarcelino]

Hi @livius ,
The Bluetooth 4.1 spec only allows up to 31 bytes of LE advertising data (39 bytes maximum LL PDU packet composed of 2 bytes of header + 6 bytes of device address + 31 bytes of Advertising Data (AD)

Anything above that gets truncated in the stack.

[livius]

@jmarcelino
then why all scanners which i have got whole buffer?
Do you think that all are in BT4.2?

[jmarcelino]

@livius
I don know what devices you're trying to read from and what the scanner is but I think that it's unlikely they're all 4.2 devices.

Probably they combine advertising data and scan response somehow?

[livius]

@jmarcelino
I see that MI BAND 2 is BLE4.2 device.
and it follow spec
http://microchipdeveloper.com/wireless:ble-link-layer-packet-types

[jmarcelino]

@livius
It's not possible to send such a long advertising packet even in Bluetooth 4.2, you'd need Bluetooth 5 (with LE Advertising Extensions) for that.

If you look up that Microchip reference the Advertising Channel PDUs have a 0-37 bytes Payload and you need the device address on top (6 bytes) on that so the maximum actual data is 31 bytes.

If you are seeing longer packets it's probably because your BLE scanner combines the result of the advertising packet with the scan response.

[livius]

@jmarcelino
Thank you Jose i am new in BLE protocol and must dig more.
But this will be strange if BLE4.2 device will advertise in mixed protocol
I suppose that your last sentence is much more possible.

[livius]

@livius
It really combine SCAN_RSP packet.
And now i have question, how can i make active scan with wipy2.
Because i understand that 'start_scan' do passive scan only?

[jmarcelino]

@livius
The current scan is active: https://github.com/pycom/pycom-micropython-sigfox/blob/eca3b714a11c4b84e740a36b4096a20aac3c9be2/esp32/mods/modbt.c#L271

but we don't combine the scan response, only show advertising data.

[livius]

@jmarcelino
Is there possibility to got scan response data somehow then?

[jmarcelino]

@livius
That's not available yet, sorry

[livius]

@jmarcelino
thank you for that info.
I will dig into sources if something can be done self here.

[tudro]

@jmarcelino +1 to have the scan reponse available :) thank you

[livius]

@jmarcelino
I see that in public firmware it is now also avaiable :)

(mac=b'\xd2lBk\xad\xc2', addr_type=1, adv_type=0, rssi=-83, data=b'\x02\x01\x06\x1b\xffW\x01\x00\xef\xbf\xc9\xac\xe7\xd5\xd0.\xe5\xf0\x8cr[\x8b[w\x03\xd2lBk\xad\xc2\n\tMI Band 2\x03\x02\xe0\xfe\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00') {'NameShort': None, 'Flag': 6, 'Mac': b'd26c426badc2', 'NameComplete': b'4d492042616e642032', 'MfrData': b'570100efbfc9ace7d5d02ee5f08c725b8b5b7703d26c426badc2', 'Data': b'0201061bff570100efbfc9ace7d5d02ee5f08c725b8b5b7703d26c426badc20a094d492042616e6420320302e0fe00000000000000000000000000000000', 'Rssi': -83, 'Appearance': None}

'NameComplete': b'4d492042616e642032'

import binascii
a=b'4d492042616e642032'
c=binascii.unhexlify(a)
print(c)

give me proper result :)
'MI Band 2'