Unclear how to connect a Peripheral to 2 Central devices

[th3-cr34t0r]

Is there a way to connect a peripheral device to 2 Central devices?

[lulf]

Yes, though the connecting is decided by the central. The peripheral must make sure it can support 2 connections in the HostResources when creating the stack, and advertise as before. On the peripheral you need to spawn one task for each connection that processes the connection (and gatt events if you're using GATT).

[th3-cr34t0r]

Thank you for the kind answer. So to summarize, create HostResources with max connections of 2, I create an advertisement task (creating advertiser, waiting for the advertiser to accept a connection), after connecting I spawn a gatt events handler task for the first connection, then advertisement task and gatt events handler task for the second one?

[lulf]

On the peripheral side, think of this as three tasks:

• A single advertiser tasks which advertises until it receives a connection handle and spawns another 'connection task' (or 'gatt events task' if you will).
• Two connection_task/gatt_events_task (with pool_size = 2 attribute if you want 2 connections) - this task just polls the events from the connection. This one is spawned from the advertiser task using the spawner from the embassy executor.

[th3-cr34t0r]

So I made some progress.

I had to set: .peripheral_count(2)? when building the softdeviceController in order to accept second connection, and update the memory for it.

Currently I have have an issue when the second device is connected, I get [server] unable to obtain CCCD slot;

[th3-cr34t0r]

Bottom line:

To fix connecting 2 devices to the peripheral one, you have to set .peripheral_count(2)? when creating the SDC. Furthermore, for the gatt server to be able to connect to 2 devices, you have to specify the connections_max and the cccd_table_size when creating the server:

#[gatt_server(cccd_table_size = 8, connections_max = 2)]
pub(crate) struct Server {
pub(crate) battery_service: BatteryService,
pub(crate) hid_service: HidService,
pub(crate) split_service: SplitService,
}

In my case, the cccd table size was 4 for 1 connection.

Then for each connection that gets accepted, you have to spawn a gatt events hander task (that is if using gatt).

Thanks for the help @lulf

[lulf]

We should definitely make this more intuitive in the future, and thanks for summarizing @th3-cr34t0r. It will be useful as a reference until there's a better api for it.