Where would I be sending my data using sd_ble_gatts_hvx?

[Mitch996]

OK. Weird question. I know, but that's only because I'm in the middle of a large program that I'm reading, believe me, I had done my fair share of homework and I understand the "send" and "receive" operation of GATT is a bit well, unique in that it sort "updates" and "pushes" "characteristics" using mostly notification and indication so conventional wisdom, as in 3 wire UART comm. doesn't really apply... all that well.

So I just copy pasted a bunch of codes that I don't really understand, I can "notify" my nrf51422 mcu with gatt packets, sent from my android phone using a program I coded well, a month ago, everything is fine, but then things got a bit awkward: when I try to hvx-push data FROM nrf51422 to my android phone, I don't really know where I'm sending the data. I have this little text box that will pop up once it receives data, any data, but it's not really doing anything when I execute the sd_ble_gatts_hvx() function, and the mcu program doesn't crash afterwards, everything is fine, with all the right parameters.

Please understand that I understand this could be tricky, because there could be an huge mistake in my android app and I haven't figure it out yet but let's start from something more basic shall we? Where will sd_ble_gatts_hvx send data to? Does it identify with a service? In that it will send data to that service, "shared" by all the characteristics, then that service will try to "relay" the data to the right characteristic, or does it send directly to a specific characteristic?

Please help me! Thanks!

[Keton]

sd_ble_gatts_hvx does BLE notifications to characteristic handle specified as one of its parameters. To receive them phone must be preconfigured. You have to write 0x1000 to CCCD descriptor of characteristic you want to receive notifications from. Otherwise android BLE stack will silently discard notification packets.

[Mitch996]

Hello, would

BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.read_perm); BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.write_perm);

Do the trick?

[Keton]

It depends on what you want to achieve. Those lines set CCCD permissions and should be set to open as per BLE spec. But that's just a very first step. You need to subscribe to notifications from a phone (or other BLE client device). So you need to perform write operations to CCCD as I've described above from your phone's app. And you need to catch write events in your nRF service code so you'll know when you can start sending notifications.

[Mitch996]

Hello Keton, Could you please tell me how do I write " 0x1000 to CCCD descriptor" to my target characteristic? Is there an android API somewhere? Alos, is this detailed in the BLE official documentation? Thank you in advance!

[Keton]

I'm not an Android developer but generic system agnostic procedure to do this is:

1. Scan for BLE device you want (apply filters ie. using manufacturer data or advertised service list)
2. Connect to it
3. Perform service discovery, find service containing your characteristic
4. perform secondary discovery to find CCCD handle of this characteristic
5. perform write operation of 0x01 on discovered CCCD handle
6. in Nordic firmware detect this write event and start sending notifications