Re:add user interrupts on future device

Interesting topic, event scheduling. I've done a similar thing. see Events - using interrupt event queue and cyclic event queues, no stacks or context switches or pre-emption, events all run to completion - real-time system - low overhead from scheduler - typically < 2us scheduler latency (interrupts)(@50MHz).

Having ISRs send interrupts to other ISRs which send interrupts to other ISRs which send ... - adds no value. There is one processor and there is one processor clock, it doesn't matter whether you spend your time (clocks) in an ISR or in a thread - it's the same time.

The question is whether you want a RTOS or a single thread system (+ISRs). With the RTOS you gain pre-emption but lose with stacks/context switches.

Like rocksoft says, you should always design your system to have many quick events, not a few slow events, so that it responds well. Then the difference between an RTOS and a non-RTOS begins to blur.

Both are good - it depends on your application ...