11.5.2 Serving a High-Priority Interrupt While a Low-Priority Interrupt Is Pending

A high priority interrupt request will always take precedence over any interrupt of a lower priority. The high-priority interrupt is acknowledged first, then the low-priority interrupt is acknowledged. Upon a return from the high-priority ISR (by executing the RETFIE instruction), the low-priority interrupt is serviced.

If any other high-priority interrupts are pending and enabled, they are serviced before servicing the pending low-priority interrupt. If no other high-priority interrupt requests are active, the low-priority interrupt is serviced.