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]

Hi Hung,

There are few setups that I've tested:

1. S110 slave and S120 master on PCA10028 both with hardcoded max and min conn interval of 7.5 ms
2. S110 with 7.5 ms max and min conn interval and S120 with 7.5 ms min conn interval and 30 ms max conn interval. In this I call sd_ble_gap_conn_param_update(m_conn_handle,0); right after connection establishment. I wait for BLE_GAP_EVT_CONN_PARAM_UPDATE before sending notification packets as shown in the second comment to the original question.
3. Same as above with an Android phone as master instead of S120.

In all the cases the notification packets are received at an interval of 15 ms. Please note that I check that I wait for BLE_EVT_TX_COMPLETE event before calling sd_ble_gatts_hvx in the active signal radio notification handler.

This application cannot work if the notification are only sent at 15 ms interval. Could you please look into this.