Calling sd_ble_gatts_hvx in radio notification

[FormerMember]

Hey everyone, I'm trying to send a value as a notification packet from GATT Server (slave) to GATT client (master) at 7.5 ms connection interval. I have set up the radio notification IRQ (SW IRQ) to fire before the radio becomes active at low IRQ priority. In this routine I call the sd_ble_gatts_hvx() routine. Without the sd_ble_gatts_hvx call, I get 7.5 ms interval. But with the call I get 15 ms routine.

void RADIO_NOTIFICATION_IRQHandler(void){
      //sd_ble_gatts_hvx call to send a notification
}

Is there a way send a GATT notification packet from the RADIO_NOTIFICATION_IRQHandler while still having 7.5 ms connection interval with S110 SD?

Edit: Adding that I have checked with different 'radio notification distances', from 800 us to 3260 us.

Edit 2: This behaviour is consistent even with 10 ms, 15 ms or 250 ms connection interval. I.e. the interval at which the radio notification event happens doubles when sd_ble_gatts_hvx is called in the radio notification IRQ handler.

Edit 3: Added a minimal example that has the problem mentioned in the comments.main_notify_radio_evt.c

Edit 4: Added a sniffer trace of how the notifications are sent.Sniffer_Trace.png

Edit 5: Added a small flow diagram to explain a comment. (It'll be nice to add pics to comments) Setup: nrf51 DK with S110 v8.0.0 in connection with nrf51 DK S120 v2.1.0. Compiled with GCC in Linux. Thanks!

[Susheel Nuguru]

Hi Prithvi, I have just used your radio init and radio irq handler (exactly as it is) I confirm that after connection param update is done, the connection interval is 7.5ms and after the connection param update, I have checked that notification is sent at the same interval and received on peer at the same interval

I believe that the image you see is self explanatory.

[FormerMember]

Write command and notification are similar in the sense that they are unacknowledged at the GATT layer. What is being used depends on which device (link layer master or slave) is GATT client and GATT server. For this discussion lets consider both as goodies.

Now there are two cases of goodies being sent, whether its LL master sending them or the slave. In case the master is sending goodies to slave TX_COMPLETE is sent immediately after the event. In case the slave is sending goodies to master, the slave has to wait till after the next conn event to get the TX_COMPLETE.

In BLE it is always the master sending the first packet and slave responding in a connection event. With this we can see that for a slave to get a LL ack from the master with the Sequence Number, it has to wait one conn event. In case of master the slave has to respond back immediately, thus getting an ack and then giving TX_COMPLETE event to the application. I've made a small diagram to explain this and is added as an attachment to the question.