11.5.3 Preempting Low-Priority Interrupts
Low-priority interrupts can be preempted by high-priority interrupts. While in the low-priority ISR, if a high-priority interrupt arrives, the high-priority interrupt request is generated and the low-priority ISR is suspended, while the high-priority ISR is executed.
After the high-priority ISR is complete and if any other high-priority interrupt requests are not active, the execution returns to the preempted low-priority ISR.