In 'standard' mode (without considering USB for mass-storage), two interrupts are used:
1. Systick, that calls most of the handlers,
2. Timer2 (higher priority) that is used to access the MEMS with a fixed frequency. The timer2 handler used to manage the I2C communication with the audio device too (version 3.0 to 3.7). Bu tthis call has been moved into the audio handler in 3.8.
If you don't care about fixed values for the MEMS, you can (dynamically, without recompiling CircleOS) disable the Timer2 interrupt. If you still need to keep the MEMS features, you will have to call sometimes the MEMS handler... (that is by default the Timer2 interrupt handler).