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Please correct me if I miss something here.
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There is no technical reason why you can't use a secondary Avalon-MM Slave port for the control activity, and that may very well be a better starting point for you, since you only have to learn one interface (Avalon-MM, but two different directions).
The main reason (I see) for using custom instructions is the higher performance you should be able to achieve.
(as dsl noted in post# 8 of this thread, NIOS IORD_ and IOWR_ macros [and the underlying hardware interactions] are relatively slow).
In the end, for this component the control aspect is a relatively small portion of where your development time is going to be spent, so it is definitely something you can easily revisit later for performance or other reasons.
Honored Contributor
