iOS Bluetooth dual-mode; connect BLE (GATT) to an already connected BR/EDR (A2DP/HFP) stereo headset simultaneously

14
votes

I'm developing a stereo headset with Bluetooth using the classic profiles (HFP, A2DP, AVRCP) as one would expect Ina stereo headset. However, I want to deploy a remote control app for iOS, and use it simultaneously to the other classic links, but the dual-mode chipset I'm working with in my design does not behave as I'd expect;

The headset is setup as a peripheral, letting the iOS device act as central. As such, the peripheral advertise its BLE services (with my specific 128-bit UUIDs) and all is good. I can browse the peripheral from any central, but only when I'm not connected with classic profiles (e.g., while not streaming audio).

My device does not seem to be able to advertise BLE, while connected with HFP and/or A2DP! However, I have seen demos of the same chipset acting as BLE central, scanning and connecting to other BLE peripherals, while simultaneously streaming audio via A2DP. However, in that setup the device was acting as A2DP sink connected to an iPhone, while scanning/connecting via BLE to a third unit acting as BLE peripheral. Hence, not point-to-point with both Classic and Smart Bluetooth to the same device.

Is there a dual-mode constraint that one cannot act as peripheral while supporting/connecting to Bluetooth Classic profiles? And that only Central-mode is supported in that case?

FYI, I'm using CSR's 8670 chipset.

UPDATE

New answer added. My apologies for not clarifying/cleaning-up my previous answer until now -- time flies!

3
iosbluetoothbluetooth-lowenergycore-bluetoothgatt
I think that if you have to communicate with the device over classic Bluetooth then you are in MFI territory. – Paulw11
But I'm not, really. My headset is a standard A2DP/HFP classic headset, and only connect to my iOS device for those adopted profiles, without any awareness of my iOS-app as such. But, the manufacturer of the Bluetooth chip that I'm using in the headset impl., (CSR8670) has some example code where they switch to what they call "vanilla" GATT, which is basically to simply add that GATT SDP-data to the device's service record. From what I've found, this is the way the BT 4.0 spec do things as well, but I'm not sure that Apple/iOS has implemented "GATT over L2CAP"... – Markus Millfjord
But yes, if all else fail, I'm looking at the MFi program for sure, but that would replace the BLE-stuff entirely and I'd just run Apple's iAP-profile in BT-classic mode instead. – Markus Millfjord
developer.bluetooth.org/TechnologyOverview/Pages/GATT.aspx – Markus Millfjord
Great that you found the solution! Please explain it in a short answer and accept it just for future documentation. – allprog

3 Answers

13
votes

Well, after digging down into specs and trying to understand things more clearly, I've found the answers I was looking for, even though I would have preferred to more optimistic answers... ;(

Nevertheless, let's get to it; the Bluetooth spec for 4.0 (BLE) says that;

Dual-mode gadgets cannot act as BLE peripheral and advertise its presence while still being connectable in "Classic" Bluetooth using BR/EDR.

Furthermore, CSR source-code examples for the CSR8670 dual-mode chipset I'm using all behave in the same say; BLE advertisements as peripheral are disabled when any classic BT-link is connected. Instead, the CSR source code promote that the device should act as the BLE central instead, allowing other BLE peripheral units to advertise and connect to it, all fully doable while streaming audio (acting as A2DP sink).

This does not suite my setup at all since;

  1. BLE centrals consume more power than BLE peripherals, and my device needs to conserve energy
  2. The dual-mode "combo"-problem of combining a BR/EDR-device with BLE peripheral functionality just moved to the phone instead, which will not work any better since we cannot expect Apple (or anyone else) to violate the BLE spec.

Instead, the recommended approach is let my stereo headset skip BLE entirely and use GATT over BR/EDR instead, also known as advertising via "vanilla", which makes sense really; I mean, I already have an ACL-link setup between two devices, why should I need to kick any sort of discovery mechanism?

Again, the Bluetooth SIG comes in handy;

https://developer.bluetooth.org/TechnologyOverview/Pages/GATT.aspx

GATT and ATT are not transport specific and can be used in both BR/EDR and LE. However, GATT and ATT are mandatory to implement in LE since it is used for discovering services

So, the SIG says "yes" to using BR/EDR as transport for GATT, but the question then instead becomes; how can I then get access to that connected BR/EDR device from within my iOS-app, where the typical scenario is to use CBCentral to scan, discover and connect to a CBPeripheral? The answer is simple; you cannot, since iOS 7.0 does not (yet?) support GATT for BR/EDR;

https://www.bluetooth.org/tpg/showDeclaration.cfm?3A000A5A005C5344535D5414403B0C0D0E2405022413010E57503F202A5A72

So, to sum things up; if you want to have a peer-to-peer setup between two dual-mode Bluetooth devices, using BOTH Bluetooth Classic profiles AND Bluetooth Smart services/characteristics, you should use GATT over BR/EDR, which is no-go for Apple-devices, but might be supported by Android (don't know, will port app to Android eventually though, but regardless it's not a big deal for Android since worst-case will imply a fallback to SPP and a simple byte-protocol to do the work that I need to get done).

That's that. Hope that I've helped someone ;) /Markus

8
votes

You learn as long as you live, I suppose, and this question's answer is NOT that it's not supported which I claimed earlier (based on what I thought I knew).

The short and clean answer to dual-mode and headset development on CSR-chipsets is simply that it was a constraint in the CSR bluetooth stack on earlier SDKs.

The Bluetooth SIG has never not supported dual-mode acting as Peripheral while being connected via classic links to the same device. On the contrary, it clearly spec. how such interoperability should be carried out -- but that doesn't always mean that all BT-stack implementations out there are capable of such functionality.

Hence; Using the latest devtools and the latest Bluetooth firmwares/stacks from CSR has resolved all issues and dual-mode is now fully, and actually quite nicely I might add, supported on the CSR8670/75 chipsets.

4
votes

I am also using the CSR8670 device. It have it working. Both a BLE peripheral and audio.

You need to be using ADK 4.0.0 by CSR.

Bluetooth 4.1 and Bluetooth 4.0 are different. For what you said about Bluetooth 4.0 is correct, but your chip can do Bluetooth 4.1.

4.1 allows such connections.

I also added "Dual mode" flags to the advertising packet.

There are a few conditions, such as a connection Interval of at least 90 miliseconds highly recommended so you dont mess with the audio.

Good luck!