APP TWI schedule/perform function is getting blocked/struck randomly when in use with BLE UART profile

Question

I am trying to build a device to stream the 9 axis sensor data from the I2C based sensor and then transmit it using BLE UART profile. It is sampled every 3 seconds using app timer with the callback functionality. After successively being able to read and transmit the value using BLE UART, randomly TWI schedule/perform gets blocked. Can you guys suggest the possibility of where it could go wrong ?

Einar Thorsrud · Accepted Answer

This is likely a matter of interrupt priorities. Maintaining the BLE connection have highest priority, and therefore BLE connection events will delay other tasks. The Interrupt model and processor availability chapter in the SoftDevice spec covers this in detail, and particularly BLE peripheral connection processor usage pattern s is relevant (assuming your device is a BLE peripheral). 
 You could increase the priority of your app timer where you schedule TWI transfers to APP_HIGH and see if that improves the situation. However, you cannot be confident to sample your sensor in exact intervals, as BLE must have priority in order to not miss connection events. 
 The way I read your question I do not think it is relevant in this case. However, if you just need a short interval where you know with confidence that you will have the CPU and other resources exclusively for the application, you can use the Timeslot API , which grants timeslots where the application have exclusive access to the chip. (Note that the length of these timeslots are short, as it is scheduled between connection events.)