Callbacks

Please enable Callbacks, so that timer values can be changed, while the counters are running.

While in an interrupt routine, I cannot even stop and restart a timer with a new value. This makes it impossible to synchronize the timers to another event..

The issue names no files or tests; start by locating the timer, counter, and interrupt-handling entry points in the Arduino DueTimer implementation. Define what callback support must allow while counters run, including changing timer values and stopping or restarting a timer from an interrupt routine; done means those operations can synchronize timers to another event.

Written by the indexing model from the issue text.