Missing BLE_GATTS_EVT_HVN_TX_COMPLETE events

[Mark Dale]

Our BLE peripheral uses a custom GATT service similar to the NUS (Nordic UART service) used in the SDK example app_ble_uart. The peripheral receives data on the UART peripheral (not UARTE) from an external device at 115200 baud with full hardware handshaking.

Our PDU data length is configured as 251 and our MTU size as 247.
The peripheral repeatedly calls sd_ble_gatts_hvx with a 244 byte ATT data packet but we only receive 4 BLE_GATTS_EVT_HVN_TX_COMPLETE events.

You can see on the oscilloscope screenshot below that we have called sd_ble_gatts_hvx 11 times (SD_SEND) but have only received TX_COMPLETE 4 times. We keep track of the number of notifications available to us so we no longer try to send after calling sd_ble_gatts_hvx 7 times since the last TX_COMPLETE event.

We are outputting any errors returned by the sd_ble_gatts_hvx call using a spare GPIO line (SD_SEND_ERR) and you can see that there are none.

We have tried keeping the PDU data length configured as 251 and our MTU size as 247, but calling sd_ble_gatts_hvx with 200 byte data payload instead and this has resolved the issue.

So the main question is why do we stop receiving the BLE_GATTS_EVT_HVN_TX_COMPLETE event when we use the maximum payload size?

More detailed setup information:

BLE peripheral = nrf52 dev kit (PCA10040).
BLE central = custom hardware using nrf52832.

We are using the S132 SoftDevice v 7.0.1.
We are not using the nRF SDK driver or HAL layers – we’re interfacing with the peripherals directly, but the chip header files we’re using are taken from SDK version 15.2.

BLE connection parameters:

Connection interval 7.5ms
Event length 7.5ms
Slave latency 0.
Connection supervisory timeout 1s.
Extended event length enabled.

MTU size 247
PDU data length 251
BLE notifications queue size (hvn_tx_queue_size) 7

Happy to provide more information if needed.

[Einar Thorsrud]

Hi,

There is not a one-to-one mapping between the number of Tx packets and the number of BLE_GATTS_EVT_HVN_TX_COMPLETE. This is because the event comes with a count field, which can be more than 1. I suspect that is the case here.

[Mark Dale]

Hi Einar, thanks for your response. 

We take the count into account when tracking the number of available buffers.

Here is our event handler:

__STATIC_INLINE void fnOnNotificationComplete(const uint16_t conn_handle, ble_evt_t const * p_ble_evt)
{
    ble_gatts_evt_hvn_tx_complete_t const * p_evt_complete = &p_ble_evt->evt.gatts_evt.params.hvn_tx_complete;
    guNotifyBuffersAvailable += p_evt_complete->count; // increase available buffers for notifications by the number of completed
}

And we then decrement guNotifyBuffersAvailable each time we call 'sd_ble_gatts_hvx'.

[Mark Dale]

To clarify, you should already be able to build and run the central side from the project I uploaded previously as there are no I/O changes required for deployment on the development kit.

When peeking the event length on calling sd_ble_evt_get() I've observed that the event length is always 16 bytes more than the maximum MTU size...
and I've confirmed that my application works with an MTU size of 231 bytes (247 - 16) but not 232.

I was using the 'BLE_EVT_LEN_MAX' macro defined in the SDK to calculate the evt_buffer length and this sets the event buffer size to be 247. 

If I set this event buffer size to 263 (247 + 16) then I am then able to set the ATT data payload to 244 and my application works successfully.

Why does the event buffer size need to be 16 bytes larger than the event size?

Thanks,
Mark

[Einar Thorsrud]

Hi Mark,

Can you upload the files again? There is a bug in this forum which makes uploading a file with the same name as another file in the same thread causes it to get overwritten. So all three files are the same.

Can you also upload the full projects, so that I can see what you have done in more detail (and if I need to debug on my side)?

[Mark Dale]

Hi Einar,

I've renamed them for you and re-attached them here.

Central:

/cfs-file/__key/communityserver-discussions-components-files/4/deployment-_5F00_c.hex

Peripheral (sends 200 bytes ATT payload):

/cfs-file/__key/communityserver-discussions-components-files/4/deployment_5F00_200.hex

Peripheral (sends 244 bytes ATT payload):

/cfs-file/__key/communityserver-discussions-components-files/4/deployment_5F00_244.hex

We would rather not provide the full projects on a public forum. Please can you advise an alternate way to send you the code?

Thanks,

Mark

[Einar Thorsrud]

Hi Mark,

Can you make a private case to upload the project and refer to this case? Then we can continue in the private case and update here afterwards.

Einar

[Mark Dale]

Yes, okay.